473e3b93146799286ee94134cc8d2258791f07a3
[chickadee.git] / doc / api.texi
1 @menu
2 * Kernel:: The fundamental components.
3 * Math:: Linear algebra, spatial partitioning, and more.
4 * Graphics:: Eye candy.
5 * Scripting:: Bringing the game world to life.
6 @end menu
7
8 @node Kernel
9 @section Kernel
10
11 At the very core of Chickadee, in the @code{(chickadee game-loop)}
12 module, lies an event loop. This loop, or ``kernel'', is responsible
13 for ensuring that the game is updated at the desired interval,
14 rendering the current state of the game world, and handling errors if
15 they occur. The kernel implements what is known as a ``fixed
16 timestep'' game loop, meaning that the game simulation will be
17 advanced by a fixed interval of time and will never vary from frame to
18 frame, unlike some other styles of game loops. The appropriately
19 named @code{run-game*} and @code{abort-game} procedures are the entry
20 and exit points to the Chickadee game loop kernel.
21
22 On its own, the kernel does not do very much at all. In order to
23 actually respond to input events, update game state, or render output,
24 the programmer must provide an engine. But don’t worry, you don’t
25 have to start from scratch! Chickadee comes with a simple engine that
26 uses SDL to create a graphical window and handle input devices, and
27 OpenGL to handle rendering. This default engine is enough for most
28 users to get started writing games quickly. More advanced users may
29 want to write a custom engine that uses a different I/O system.
30 Perhaps you are writing a text adventure or roguelike that reads from
31 and writes to a terminal instead of a graphical window. The game loop
32 kernel makes no assumptions.
33
34 @deffn {Procedure} run-game [#:update] [#:render] [#:time] [#:error] @
35 [#:update-hz 60]
36
37 Start the game loop. This procedure will not return until
38 @code{abort-game} is called.
39
40 The core game loop is generic and requires four additional procedures
41 to operate:
42
43 @itemize
44 @item
45 @var{update}: Called @var{update-hz} times per second to advance the
46 game simulation. This procedure is called with a single argument: The
47 amount of time that has passed since the last update, in milliseconds.
48 @item
49 @var{render}: Called each iteration of the loop to render the game to
50 the desired output device. This procedure is called with a single
51 argument: A value in the range [0, 1] which represents how much time
52 has past since the last game state update relative to the upcoming
53 game state update, as a percentage. Because the game state is updated
54 independent of rendering, it is often the case that rendering is
55 occuring between two updates. If the game is rendered as it was
56 during the last update, a strange side-effect will occur that makes
57 animation appear rough or ``choppy''. To counter this, the
58 @var{alpha} value can be used to perfrom a linear interpolation of a
59 moving object between its current position and its previous position.
60 This odd trick has the pleasing result of making the animation look
61 smooth again, but requires keeping track of previous state.
62 @item
63 @var{time}: Called to get the current time in milliseconds. This
64 procedure is called with no arguments.
65 @item
66 @var{error}: Called when an error from the @var{update} or
67 @var{render} procedures reaches the game loop. This procedure is
68 called with three arguments: The call stack, the error key, and the
69 error arguments. If no error handler is provided, the default
70 behavior is to simply re-throw the error.
71 @end itemize
72
73 @end deffn
74
75 @deffn {Procedure} abort-game
76 Stop the currently running Chickadee game loop.
77 @end deffn
78
79 Since most users will want to write 2D/3D games with hardware
80 accelerated graphics rendering, controlled via keyboard, mouse, or
81 game controller, Chickadee comes with an easy to use engine just for
82 this purpose in the @code{(chickadee)} module: @code{run-game}.
83
84 @deffn {Procedure} run-game [#:window-title "Chickadee!"] @
85 [#:window-width 640] [#:window-height 480] @
86 [#:window-fullscreen? @code{#f}] [#:update-hz 60] @
87 [#:load] [#:update] [#:draw] [#:quit] @
88 [#:key-press] [#:key-release] [#:text-input] @
89 [#:mouse-press] [#:mouse-release] [#:mouse-move] @
90 [#:controller-add] [#:controller-remove] [#:controller-press] @
91 [#:controller-release] [#:controller-move] [#:error]
92
93 Run the Chickadee game loop using the SDL engine in OpenGL mode.
94
95 A new graphical window will be opened with @var{window-width} x
96 @var{window-height} as its dimensions, @var{window-title} as its
97 title, and in fullscreen mode if @var{window-fullscreen?} is
98 @code{#t}.
99
100 @itemize
101 @item
102 @var{load}: Called with zero arguments when the game window has opened
103 but before the game loop has started. Can be used to perform
104 initialization that requires an open window and OpenGL context such as
105 loading textures.
106
107 @item
108 @var{update}: Called @var{update-hz} times per second with one
109 argument: The amount of time to advance the game simulation.
110
111 @item
112 @var{draw}: Called each time a frame should be rendered with a single
113 argument known as the @code{alpha} value. See the documentation for
114 @code{run-game} for an explanation of this value.
115
116 @item
117 @var{quit}: Called with zero arguments when the user tries to close
118 the game window. The default behavior is to exit the game.
119
120 @item
121 @var{key-press}: Called with four arguments when a key is pressed on
122 the keyboard:
123
124 @enumerate
125 @item
126 @var{key}: The symbolic name of the ``virtual'' key that was pressed.
127 For example: @code{backspace}. It's called a virtual key because the
128 operating system may map a physical keyboard key to another key
129 entirely, such as how the author likes to bind the ``caps lock'' key
130 to mean ``control''.
131
132 @item
133 @var{scancode}: The symbolic name of the physical key that was
134 pressed.
135
136 @item
137 @var{modifiers}: A list of the symbolic names of modifier keys that
138 were being held down when the key was pressed. Possible values
139 include @code{ctrl}, @code{alt}, and @code{shift}.
140
141 @item
142 @var{repeat?}: @code{#t} if this is a repeated press of the same key.
143
144 @end enumerate
145
146 @item
147 @var{key-release}: Called with three arguments when a key is released
148 on the keyboard:
149
150 @enumerate
151 @item
152 @var{key}: The symbolic name of the ``virtual'' key that was released.
153
154 @item
155 @var{scancode}: The symbolic name of the physical key that was
156 released.
157
158 @item
159 @var{modifiers}: A list of the symbolic names of modifier keys that
160 were being held down when the key was released.
161
162 @end enumerate
163
164 @item
165 @var{text-input}: Called with a single argument, a string of text,
166 when printable text is typed on the keyboard.
167
168 @item
169 @var{mouse-press}: Called with four arguments when a mouse button is
170 pressed:
171 @enumerate
172
173 @item
174 @var{button}: The symbolic name of the button that was pressed, such
175 as @code{left}, @code{middle}, or @code{right}.
176
177 @item
178 @var{clicks}: The number of times the button has been clicked in a row.
179
180 @item
181 @var{x}: The x coordinate of the mouse cursor.
182
183 @item
184 @var{y}: The y coordinate of the mouse cursor.
185
186 @end enumerate
187
188 @item
189 @var{mouse-release}: Called with three arguments when a mouse button
190 is released:
191
192 @enumerate
193
194 @item
195 @var{button}: The symbolic name of the button that was released.
196
197 @item
198 @var{x}: The x coordinate of the mouse cursor.
199
200 @item
201 @var{y}: The y coordinate of the mouse cursor.
202
203 @end enumerate
204
205 @item
206 @var{mouse-move}: Called with five arguments when the mouse is moved:
207
208 @enumerate
209
210 @item
211 @var{x}: The x coordinate of the mouse cursor.
212
213 @item
214 @var{y}: The y coordinate of the mouse cursor.
215
216 @item
217 @var{dx}: The amount the mouse has moved along the x axis since the
218 last mouse move event.
219
220 @item
221 @var{dy}: The amount the mouse has moved along the y axis since the
222 last mouse move event.
223
224 @item
225 @var{buttons}: A list of the buttons that were pressed down when the
226 mouse was moved.
227
228 @end enumerate
229
230 @item
231 @var{controller-add}: Called with a single argument, an SDL game
232 controller object, when a game controller is connected.
233
234 @item
235 @var{controller-remove}: Called with a single argument, an SDL game
236 controller object, when a game controller is disconnected.
237
238 @item
239 @var{controller-press}: Called with two arguments when a button on a
240 game controller is pressed:
241
242 @enumerate
243
244 @item
245 @var{controller}: The controller that triggered the event.
246
247 @item
248 @var{button}: The symbolic name of the button that was pressed.
249 Possible buttons are:
250
251 @itemize
252 @item
253 @code{a}
254 @item
255 @code{b}
256 @item
257 @code{x}
258 @item
259 @code{y}
260 @item
261 @code{back}
262 @item
263 @code{guide}
264 @item
265 @code{start}
266 @item
267 @code{left-stick}
268 @item
269 @code{right-stick}
270 @item
271 @code{left-shoulder}
272 @item
273 @code{right-shoulder}
274 @item
275 @code{dpad-up}
276 @item
277 @code{dpad-down}
278 @item
279 @code{dpad-left}
280 @item
281 @code{dpad-right}
282
283 @end itemize
284
285 @end enumerate
286
287 @item
288 @var{controller-release}: Called with two arguments when a button on a
289 game controller is released:
290
291 @enumerate
292
293 @item
294 @var{controller}: The controller that triggered the event.
295
296 @item
297 @var{button}: The symbolic name of the button that was released.
298
299 @end enumerate
300
301 @item
302 @var{controller-move}: Called with three arguments when an analog
303 stick or trigger on a game controller is moved:
304
305 @enumerate
306
307 @item
308 @var{controller}: The controller that triggered the event.
309
310 @item
311 @var{axis}: The symbolic name of the axis that was moved. Possible
312 values are:
313
314 @itemize
315 @item
316 @code{left-x}
317 @item
318 @code{left-y}
319 @item
320 @code{right-x}
321 @item
322 @code{right-y}
323 @item
324 @code{trigger-left}
325 @item
326 @code{trigger-right}
327 @end itemize
328
329 @end enumerate
330
331 @item
332 @var{error}: Called with three arguments when an error occurs:
333
334 @enumerate
335
336 @item
337 @var{stack}: The call stack at the point of error.
338
339 @item
340 @var{key}: The exception key.
341
342 @item
343 @var{args}: The arguments thrown with the exception.
344
345 @end enumerate
346
347 The default behavior is to re-throw the error.
348
349 @end itemize
350
351 @end deffn
352
353 @node Math
354 @section Math
355
356 Chickadee contains data types and procedures for performing the most
357 common computations in video game simulations such as linear algebra
358 with vectors and matrices and axis-aligned bounding box collision
359 detection.
360
361 @menu
362 * Basics:: Commonly used, miscellaneous things.
363 * Vectors:: Euclidean vectors.
364 * Rectangles:: Axis-aligned bounding boxes.
365 * Grid:: Spatial partitioning for bounding boxes.
366 * Matrices:: Transformation matrices.
367 * Quaternions:: Rotations about an arbitrary axis.
368 * Easings:: Easing functions for interesting animations.
369 * Bezier Curves:: Cubic Bezier curves and paths in 2D space.
370 * Path Finding:: Generic A* path finding.
371 @end menu
372
373 @node Basics
374 @subsection Basics
375
376 @defvar pi
377 An essential constant for all trigonometry. @code{@U{03C0}} is the ratio
378 of a circle's circumferences to its diameter. Since @code{@U{03C0}} is an
379 irrational number, the @var{pi} in Chickadee is a mere floating point
380 approximation that is ``good enough.''
381 @end defvar
382
383 @defvar pi/2
384 Half of @var{pi}.
385 @end defvar
386
387 @deffn {Procedure} cotan @var{z}
388 Return the cotangent of @var{z}.
389 @end deffn
390
391 @node Vectors
392 @subsection Vectors
393
394 Unlike Scheme's vector data type, which is a sequence of arbitrary
395 Scheme objects, Chickadee's @code{(chickadee math vector)} module
396 provides vectors in the linear algebra sense: Sequences of numbers
397 specialized for particular coordinate spaces. As of now, Chickadee
398 provides 2D and 3D vectors, with 4D vector support coming in a future
399 release.
400
401 Here's a quick example of adding two vectors:
402
403 @example
404 (define v (vec2+ (vec2 1 2) (vec2 3 4)))
405 @end example
406
407 Since vectors are used so frequently, the reader macro @code{#v} is
408 used to cut down on typing:
409
410 @example
411 (define v (vec2+ #v(1 2) #v(3 4)))
412 @end example
413
414 @subsubsection A Note About Performance
415
416 A lot of time has been spent making Chickadee's vector operations
417 perform relatively efficiently in critical code paths where excessive
418 garbage generation will cause major performance issues. The general
419 rule is that procedures ending with @code{!} perform an in-place
420 modification of one of the arguments in order to avoid allocating a
421 new vector. These procedures are also inlined by Guile's compiler in
422 order to take advantage of optimizations relating to floating point
423 math operations. The downside is that since these are not pure
424 functions, they do not compose well and create more verbose code.
425
426 @subsubsection 2D Vectors
427
428 @deffn {Procedure} vec2 @var{x} @var{y}
429 Return a new 2D vector with coordinates (@var{x}, @var{y}).
430 @end deffn
431
432 @deffn {Procedure} vec2/polar @var{r} @var{theta}
433 Return a new 2D vector containing the Cartesian representation of the
434 polar coordinate (@var{r}, @var{theta}). The angle @var{theta} is
435 measured in radians.
436 @end deffn
437
438 @deffn {Procedure} vec2? @var{obj}
439 Return @code{#t} if @var{obj} is a 2D vector.
440 @end deffn
441
442 @deffn {Procedure} vec2-x @var{v}
443 Return the X coordinate of the 2D vector @var{v}.
444 @end deffn
445
446 @deffn {Procedure} vec2-y @var{v}
447 Return the Y coordinate of the 2D vector @var{v}.
448 @end deffn
449
450 @deffn {Procedure} vec2-copy @var{v}
451 Return a fresh copy of the 2D vector @var{v}.
452 @end deffn
453
454 @deffn {Procedure} vec2-magnitude @var{v}
455 Return the magnitude of the 2D vector @var{v}.
456 @end deffn
457
458 @deffn {Procedure} vec2-dot-product @var{v1} @var{v2}
459 Return the dot product of the 2D vectors @var{v1} and @var{v2}.
460 @end deffn
461
462 @deffn {Procedure} vec2-normalize @var{v}
463 Return the normalized form of the 2D vector @var{v}.
464 @end deffn
465
466 @deffn {Procedure} vec2+ @var{v} @var{x}
467 Add @var{x}, either a 2D vector or a scalar (i.e. a real number), to
468 the 2D vector @var{v} and return a new vector containing the sum.
469 @end deffn
470
471 @deffn {Procedure} vec2- @var{v} @var{x}
472 Subtract @var{x}, either a 2D vector or a scalar, from the 2D vector
473 @var{v} and return a new vector containing the difference.
474 @end deffn
475
476 @deffn {Procedure} vec2* @var{v} @var{x}
477 Multiply the 2D vector @var{v} by @var{x}, a 2D vector or a scalar,
478 and return a new vector containing the product.
479 @end deffn
480
481 @deffn {Procedure} set-vec2-x! @var{v} @var{x}
482 Set the X coordinate of the 2D vector @var{v} to @var{x}.
483 @end deffn
484
485 @deffn {Procedure} set-vec2-y! @var{v} @var{y}
486 Set the Y coordinate of the 2D vector @var{v} to @var{y}.
487 @end deffn
488
489 @deffn {Procedure} set-vec2! @var{v} @var{x} @var{y}
490 Set the X and Y coordinates of the 2D vector @var{v} to @var{x} and
491 @var{y}, respectively.
492 @end deffn
493
494 @deffn {Procedure} vec2-copy! @var{source} @var{target}
495 Copy the 2D vector @var{source} into the 2D vector @var{target}.
496 @end deffn
497
498 @deffn {Procedure} vec2-add! @var{v} @var{x}
499 Perform an in-place modification of the 2D vector @var{v} by adding
500 @var{x}, a 2D vector or a scalar.
501 @end deffn
502
503 @deffn {Procedure} vec2-sub! @var{v} @var{x}
504 Perform an in-place modification of the 2D vector @var{v} by
505 subtracting @var{x}, a 2D vector or a scalar.
506 @end deffn
507
508 @deffn {Procedure} vec2-mult! @var{v} @var{x}
509 Perform an in-place modification of the 2D vector @var{v} by
510 multiplying it by @var{x}, a 2D vector or a scalar.
511 @end deffn
512
513 @subsubsection 3D Vectors
514
515 @deffn {Procedure} vec3 @var{x} @var{y}
516 Return a new 2D vector with coordinates (@var{x}, @var{y}).
517 @end deffn
518
519 @deffn {Procedure} vec3? @var{obj}
520 Return @code{#t} if @var{obj} is a 3D vector.
521 @end deffn
522
523 @deffn {Procedure} vec3-x @var{v}
524 Return the X coordinate of the 3D vector @var{v}.
525 @end deffn
526
527 @deffn {Procedure} vec3-y @var{v}
528 Return the Y coordinate of the 3D vector @var{v}.
529 @end deffn
530
531 @deffn {Procedure} vec3-z @var{v}
532 Return the Z coordinate of the 3D vector @var{v}.
533 @end deffn
534
535 @deffn {Procedure} vec3-copy @var{v}
536 Return a fresh copy of the 3D vector @var{v}.
537 @end deffn
538
539 @deffn {Procedure} vec3-magnitude @var{v}
540 Return the magnitude of the 3D vector @var{v}.
541 @end deffn
542
543 @deffn {Procedure} vec3-dot-product @var{v1} @var{v2}
544 Return the dot product of the 3D vectors @var{v1} and @var{v2}.
545 @end deffn
546
547 @deffn {Procedure} vec3-normalize @var{v}
548 Return the normalized form of the 3D vector @var{v}.
549 @end deffn
550
551 @deffn {Procedure} vec3+ @var{v} @var{x}
552 Add @var{x}, either a 3D vector or a scalar (i.e. a real number), to
553 the 3D vector @var{v} and return a new vector containing the sum.
554 @end deffn
555
556 @deffn {Procedure} vec3- @var{v} @var{x}
557 Subtract @var{x}, either a 3D vector or a scalar, from the 3D vector
558 @var{v} and return a new vector containing the difference.
559 @end deffn
560
561 @deffn {Procedure} vec3* @var{v} @var{x}
562 Multiply the 3D vector @var{v} by @var{x}, a 3D vector or a scalar,
563 and return a new vector containing the product.
564 @end deffn
565
566 @deffn {Procedure} set-vec3-x! @var{v} @var{x}
567 Set the X coordinate of the 3D vector @var{v} to @var{x}.
568 @end deffn
569
570 @deffn {Procedure} set-vec3-y! @var{v} @var{y}
571 Set the Y coordinate of the 3D vector @var{v} to @var{y}.
572 @end deffn
573
574 @deffn {Procedure} set-vec3-z! @var{v} @var{z}
575 Set the Z coordinate of the 3D vector @var{v} to @var{z}.
576 @end deffn
577
578 @deffn {Procedure} set-vec3! @var{v} @var{x} @var{y} @var{z}
579 Set the X, Y, and Z coordinates of the 3D vector @var{v} to @var{x},
580 @var{y}, and @var{z}, respectively.
581 @end deffn
582
583 @deffn {Procedure} vec3-copy! @var{source} @var{target}
584 Copy the 3D vector @var{source} into the 3D vector @var{target}.
585 @end deffn
586
587 @deffn {Procedure} vec3-add! @var{v} @var{x}
588 Perform an in-place modification of the 3D vector @var{v} by adding
589 @var{x}, a 3D vector or a scalar.
590 @end deffn
591
592 @deffn {Procedure} vec3-sub! @var{v} @var{x}
593 Perform an in-place modification of the 3D vector @var{v} by
594 subtracting @var{x}, a 3D vector or a scalar.
595 @end deffn
596
597 @deffn {Procedure} vec3-mult! @var{v} @var{x}
598 Perform an in-place modification of the 3D vector @var{v} by
599 multiplying it by @var{x}, a 3D vector or a scalar.
600 @end deffn
601
602 @node Rectangles
603 @subsection Rectangles
604
605 The @code{(chickadee math rect)} module provides an API for
606 manipulating axis-aligned bounding boxes (AABBs). AABBs are often
607 used for collision detection in games. Common use-cases are defining
608 ``hitboxes'' in platformers or using them for the ``broad phase'' of a
609 collision detection algorithm that uses a more complex (and thus
610 slower) method of determining the actual collisions.
611
612 Like some of the other math modules, there exists a collection of
613 functions that do in-place modification of rectangles for use in
614 performance critical code paths.
615
616 @deffn {Procedure} rect @var{x} @var{y} @var{width} @var{height}
617 @deffnx {Procedure} make-rect @var{x} @var{y} @var{width} @var{height}
618 Create a new rectangle that is @var{width} by @var{height} in size and
619 whose bottom-left corner is located at (@var{x}, @var{y}).
620 @end deffn
621
622 @deffn {Procedure} rect? @var{obj}
623 Return @code{#t} if @var{obj} is a rectangle.
624 @end deffn
625
626 @deffn {Procedure} rect-within? @var{rect1} @var{rect2}
627 Return @code{#t} if @var{rect2} is completely within @var{rect1}.
628 @end deffn
629
630 @deffn {Procedure} rect-intersects? @var{rect1} @var{rect2}
631 Return @code{#t} if @var{rect2} overlaps @var{rect1}.
632 @end deffn
633
634 @deffn {Procedure} rect-contains? @var{rect} @var{x} @var{y}
635 Return @code{#t} if the coordinates (@var{x}, @var{y}) are within
636 @var{rect}.
637 @end deffn
638
639 @deffn {Procedure} rect-contains-vec2? @var{rect} @var{v}
640 Return @code{#t} if the 2D vector @var{v} is within the bounds of
641 @var{rect}.
642 @end deffn
643
644 @deffn {Procedure} rect-x @var{rect}
645 Return the X coordinate of the lower-left corner of @var{rect}.
646 @end deffn
647
648 @deffn {Procedure} rect-y @var{rect}
649 Return the Y coordinate of the lower-left corner of @var{rect}.
650 @end deffn
651
652 @deffn {Procedure} rect-left @var{rect}
653 Return the left-most X coordinate of @var{rect}.
654 @end deffn
655
656 @deffn {Procedure} rect-right @var{rect}
657 Return the right-most X coordinate of @var{rect}.
658 @end deffn
659
660 @deffn {Procedure} rect-bottom @var{rect}
661 Return the bottom-most Y coordinate of @var{rect}.
662 @end deffn
663
664 @deffn {Procedure} rect-top @var{rect}
665 Return the top-most Y coordinate of @var{rect}.
666 @end deffn
667
668 @deffn {Procedure} rect-center-x @var{rect}
669 Return the X coordinate of the center of @var{rect}.
670 @end deffn
671
672 @deffn {Procedure} rect-center-y @var{rect}
673 Return the Y coordinate of the center of @var{rect}.
674 @end deffn
675
676 @deffn {Procedure} rect-width @var{rect}
677 Return the width of @var{rect}.
678 @end deffn
679
680 @deffn {Procedure} rect-height @var{rect}
681 Return the height of @var{rect}.
682 @end deffn
683
684 @deffn {Procedure} rect-area @var{rect}
685 Return the surface area covered by @var{rect}.
686 @end deffn
687
688 @deffn {Procedure} rect-clamp-x @var{rect} @var{x}
689 Restrict @var{x} to the portion of the X axis covered by @var{rect}.
690 @end deffn
691
692 @deffn {Procedure} rect-clamp-y @var{rect} @var{y}
693 Restrict @var{y} to the portion of the Y axis covered by @var{rect}.
694 @end deffn
695
696 @deffn {Procedure} rect-clamp @var{rect1} @var{rect2}
697 Return a new rect that adjusts the location of @var{rect1} so that it
698 is completely within @var{rect2}. An exception is thrown in the case
699 that @var{rect1} cannot fit completely within @var{rect2}.
700 @end deffn
701
702 @deffn {Procedure} rect-move @var{rect} @var{x} @var{y}
703 Return a new rectangle based on @var{rect} but moved to the
704 coordinates (@var{x}, @var{y}).
705 @end deffn
706
707 @deffn {Procedure} rect-move-vec2 @var{rect} @var{v}
708 Return a new rectangle based on @var{rect} but moved to the
709 coordinates in the 2D vector @var{v}.
710 @end deffn
711
712 @deffn {Procedure} rect-move-by @var{rect} @var{x} @var{y}
713 Return a new rectangle based on @var{rect} but moved by (@var{x},
714 @var{y}) units relative to its current location.
715 @end deffn
716
717 @deffn {Procedure} rect-move-by-vec2 @var{rect} @var{v}
718 Return a new rectangle based on @var{rect} but moved by the 2D vector
719 @var{v} relative to its current location.
720 @end deffn
721
722 @deffn {Procedure} rect-inflate @var{rect} @var{width} @var{height}
723 Return a new rectangle based on @var{rect}, but expanded by
724 @var{width} units on the X axis and @var{height} units on the Y axis,
725 while keeping the rectangle centered on the same point.
726 @end deffn
727
728 @deffn {Procedure} rect-union @var{rect1} @var{rect2}
729 Return a new rectangle that completely covers the area of @var{rect1}
730 and @var{rect2}.
731 @end deffn
732
733 @deffn {Procedure} rect-clip @var{rect1} @var{rect2}
734 Return a new rectangle that is the overlapping region of @var{rect1}
735 and @var{rect2}. If the two rectangles do not overlap, a rectangle of
736 0 width and 0 height is returned.
737 @end deffn
738
739 @deffn {Procedure} set-rect-x! @var{rect} @var{x}
740 Set the left X coordinate of @var{rect} to @var{x}.
741 @end deffn
742
743 @deffn {Procedure} set-rect-y! @var{rect} @var{y}
744 Set the bottom Y coordinate of @var{rect} to @var{y}.
745 @end deffn
746
747 @deffn {Procedure} set-rect-width! @var{rect} @var{width}
748 Set the width of @var{rect} to @var{width}.
749 @end deffn
750
751 @deffn {Procedure} set-rect-height! @var{rect} @var{height}
752 Set the height of @var{rect} to @var{height}.
753 @end deffn
754
755 @deffn {Procedure} rect-move! @var{rect} @var{x} @var{y}
756 Move @var{rect} to (@var{x}, @var{y}) in-place.
757 @end deffn
758
759 @deffn {Procedure} rect-move-vec2! @var{rect} @var{v}
760 Move @var{rect} to the 2D vector @var{v} in-place.
761 @end deffn
762
763 @deffn {Procedure} rect-move-by! @var{rect} @var{x} @var{y}
764 Move @var{rect} by (@var{x}, @var{y}) in-place.
765 @end deffn
766
767 @deffn {Procedure} rect-move-by-vec2! @var{rect} @var{v}
768 Move @var{rect} by the 2D vector @var{v} in-place.
769 @end deffn
770
771 @deffn {Procedure} rect-inflate! @var{rect} @var{width} @var{height}
772 Expand @var{rect} by @var{width} and @var{height} in-place.
773 @end deffn
774
775 @deffn {Procedure} rect-union! @var{rect1} @var{rect2}
776 Modify @var{rect1} in-place to completely cover the area of both
777 @var{rect1} and @var{rect2}.
778 @end deffn
779
780 @deffn {Procedure} rect-clip! @var{rect1} @var{rect2}
781 Modify @var{rect1} in-place to be the overlapping region of
782 @var{rect1} and @var{rect2}.
783 @end deffn
784
785 @deffn {Procedure} rect-clamp! @var{rect1} @var{rect2}
786 Adjust the location of @var{rect1} in-place so that its bounds are
787 completely within @var{rect2}. An exception is thrown in the case
788 that @var{rect1} cannot fit completely within @var{rect2}.
789 @end deffn
790
791 @deffn {Procedure} vec2-clamp-to-rect! @var{v} @var{rect}
792 Restrict the coordinates of the 2D vector @var{v} so that they are
793 within the bounds of @var{rect}. @var{v} is modified in-place.
794 @end deffn
795
796 @node Grid
797 @subsection Grid
798
799 The @code{(chickadee math grid)} module provides a simple spatial
800 partitioning system for axis-aligned bounding boxes
801 (@pxref{Rectangles}) in 2D space. The grid divides the world into
802 tiles and keeps track of which rectangles occupy which tiles. When
803 there are lots of moving objects in the game world that need collision
804 detection, the grid greatly speeds up the process. Instead of
805 checking collisions of each object against every other object (an
806 O(n^2) operation), the grid quickly narrows down which objects could
807 possibly be colliding and only performs collision testing against a
808 small set of objects.
809
810 In addition to checking for collisions, the grid also handles the
811 resolution of collisions. Exactly how each collision is resolved is
812 user-defined. A player bumping into a wall may slide against it. An
813 enemy colliding with a projectile shot by the player may get pushed
814 back in the opposite direction. Two players colliding may not need
815 resolution at all and will just pass through each other. The way this
816 works is that each time an object (A) is moved within the grid, the
817 grid looks for an object (B) that may possibly be colliding with A. A
818 user-defined procedure known as a ``filter'' is then called with both
819 A and B. If the filter returns @code{#f}, it means that even if A and
820 B are colliding, no collision resolution is needed. In this case the
821 grid won't waste time checking if they really do collide because it
822 doesn't matter. If A and B are collidable, then the filter returns a
823 procedure that implements the resolution technique. The grid will
824 then perform a collision test. If A and B are colliding, the resolver
825 procedure is called. It's the resolvers job to adjust the objects
826 such that they are no longer colliding. The grid module comes with a
827 very simple resolution procedure, @code{slide}, that adjusts object A
828 by the smallest amount so that it no longer overlaps with B. By using
829 this filtering technique, a game can resolve collisions between
830 different objects in different ways.
831
832 @deffn {Procedure} make-grid [@var{cell-size} 64]
833 Return a new grid partitioned into @var{cell-size} tiles.
834 @end deffn
835
836 @deffn {Procedure} grid? @var{obj}
837 Return @code{#t} if @var{obj} is a grid.
838 @end deffn
839
840 @deffn {Procedure} cell? @var{obj}
841 Return @code{#t} if @var{obj} is a grid cell.
842 @end deffn
843
844 @deffn {Procedure} cell-count @var{cell}
845 Return the number of items in @var{cell}.
846 @end deffn
847
848 @deffn {Procedure} grid-cell-size @var{grid}
849 Return the cell size of @var{grid}.
850 @end deffn
851
852 @deffn {Procedure} grid-cell-count @var{grid}
853 Return the number of cells currently in @var{grid}.
854 @end deffn
855
856 @deffn {Procedure} grid-item-count @var{grid}
857 Return the number of items in @var{grid}.
858 @end deffn
859
860 @deffn {Procedure} grid-add @var{grid} @var{item} @var{x} @var{y} @
861 @var{width} @var{height}
862
863 Add @var{item} to @var{grid} represented by the axis-aligned bounding
864 box whose lower-left corner is at (@var{x}, @var{y}) and is
865 @var{width} x @var{height} in size.
866 @end deffn
867
868 @deffn {Procedure} grid-remove @var{grid} @var{item}
869 Return @var{item} from @var{grid}.
870 @end deffn
871
872 @deffn {Procedure} grid-clear @var{grid}
873 Remove all items from @var{grid}.
874 @end deffn
875
876 @deffn {Procedure} grid-move @var{grid} @var{item} @var{position} @var{filter}
877 Attempt to move @var{item} in @var{grid} to @var{position} (a 2D
878 vector) and check for collisions. For each collision, @var{filter}
879 will be called with two arguments: @var{item} and the item it collided
880 with. If a collision occurs, @var{position} may be modified to
881 resolve the colliding objects.
882 @end deffn
883
884 @deffn {Procedure} for-each-cell @var{proc} @var{grid} [@var{rect}]
885 Call @var{proc} with each cell in @var{grid} that intersects
886 @var{rect}, or every cell if @var{rect} is @code{#f}.
887 @end deffn
888
889 @deffn {Procedure} for-each-item @var{proc} @var{grid}
890 Call @var{proc} for each item in @var{grid}.
891 @end deffn
892
893 @deffn {Procedure} slide @var{item} @var{item-rect} @
894 @var{other} @var{other-rect} @var{goal}
895
896 Resolve the collision that occurs between @var{item} and @var{other}
897 when moving @var{item-rect} to @var{goal} by sliding @var{item-rect}
898 the minimum amount needed to make it no longer overlap
899 @var{other-rect}.
900 @end deffn
901
902 @node Matrices
903 @subsection Matrices
904
905 The @code{(chickadee math matrix)} module provides an interface for
906 working with the most common type of matrices in game development: 4x4
907 transformation matrices.
908
909 @subsubsection Another Note About Performance
910
911 Much like the vector API, the matrix API is commonly used in
912 performance critical code paths. In order to reduce the amount of
913 garbage generated and improve matrix multiplication performance, there
914 are many procedures that perform in-place modifications of matrix
915 objects.
916
917 @subsubsection Matrix Operations
918
919 @deffn {Procedure} make-matrix4 @var{aa} @var{ab} @var{ac} @var{ad} @
920 @var{ba} @var{bb} @var{bc} @var{bd} @
921 @var{ca} @var{cb} @var{cc} @var{cd} @
922 @var{da} @var{db} @var{dc} @var{dd}
923
924 Return a new 4x4 matrix initialized with the given 16 values in
925 column-major format.
926 @end deffn
927
928 @deffn {Procedure} make-null-matrix4
929 Return a new 4x4 matrix with all values initialized to 0.
930 @end deffn
931
932 @deffn {Procedure} make-identity-matrix4
933 Return a new 4x4 identity matrix. Any matrix multiplied by the
934 identity matrix yields the original matrix. This procedure is
935 equivalent to the following code:
936
937 @example
938 (make-matrix4 1 0 0 0
939 0 1 0 0
940 0 0 1 0
941 0 0 0 1)
942 @end example
943
944 @end deffn
945
946 @deffn {Procedure} matrix4? @var{obj}
947 Return @code{#t} if @var{obj} is a 4x4 matrix.
948 @end deffn
949
950 @deffn {Procedure} matrix4* . @var{matrices}
951 Return a new 4x4 matrix containing the product of multiplying all of
952 the given @var{matrices}.
953
954 Note: Remember that matrix multiplication is @strong{not} commutative!
955 @end deffn
956
957 @deffn {Procedure} orthographic-projection @var{left} @var{right} @
958 @var{top} @var{bottom} @
959 @var{near} @var{far}
960
961 Return a new 4x4 matrix that represents an orthographic (2D)
962 projection for the horizontal clipping plane @var{top} and
963 @var{bottom}, the vertical clipping plane @var{top} and @var{bottom},
964 and the depth clipping plane @var{near} and @var{far}.
965 @end deffn
966
967 @deffn {Procedure} perspective-projection @var{fov} @
968 @var{aspect-ratio} @
969 @var{near} @var{far}
970
971 Return a new 4x4 matrix that represents a perspective (3D) projection
972 with a field of vision of @var{fov} radians, an aspect ratio of
973 @var{aspect-ratio}, and a depth clipping plane defined by @var{near}
974 and @var{far}.
975 @end deffn
976
977 @deffn {Procedure} matrix4-translate @var{x}
978 Return a new 4x4 matrix that represents a translation by @var{x}, a 2D
979 vector, a 3D vector, or a rectangle (in which case the bottom-left
980 corner of the rectangle is used).
981 @end deffn
982
983 @deffn {Procedure} matrix4-scale @var{s}
984 Return a new 4x4 matrix that represents a scaling along the X, Y, and
985 Z axes by the scaling factor @var{s}, a real number.
986 @end deffn
987
988 @deffn {Procedure} matrix4-rotate @var{q}
989 Return a new 4x4 matrix that represents a rotation about an arbitrary
990 axis defined by the quaternion @var{q}.
991 @end deffn
992
993 @deffn {Procedure} matrix4-rotate-z @var{theta}
994 Return a new 4x4 matrix that represents a rotation about the Z axis by
995 @var{theta} radians.
996 @end deffn
997
998 @deffn {Procedure} matrix4-identity! @var{matrix}
999 Modify @var{matrix} in-place to contain the identity matrix.
1000 @end deffn
1001
1002 @deffn {Procedure} matrix4-mult! @var{dest} @var{a} @var{b}
1003 Multiply the 4x4 matrix @var{a} by the 4x4 matrix @var{b} and store
1004 the result in the 4x4 matrix @var{dest}.
1005 @end deffn
1006
1007 @deffn {Procedure} matrix4-translate! @var{matrix} @var{x}
1008 Modify @var{matrix} in-place to contain a translation by @var{x}, a 2D
1009 vector, a 3D vector, or a rectangle (in which case the bottom-left
1010 corner of the rectangle is used).
1011 @end deffn
1012
1013 @deffn {Procedure} matrix4-scale! @var{matrix} @var{s}
1014 Modify @var{matrix} in-place to contain a scaling along the X, Y, and
1015 Z axes by the scaling factor @var{s}, a real number.
1016 @end deffn
1017
1018 @deffn {Procedure} matrix4-rotate! @var{matrix} @var{q}
1019 Modify @var{matrix} in-place to contain a rotation about an arbitrary
1020 axis defined by the quaternion @var{q}.
1021 @end deffn
1022
1023 @deffn {Procedure} matrix4-rotate-z! @var{matrix} @var{theta}
1024 Modify @var{matrix} in-place to contain a rotation about the Z axis by
1025 @var{theta} radians.
1026 @end deffn
1027
1028 @deffn {Procedure} matrix4-2d-transform! @var{matrix} [#:origin] @
1029 [#:position] [#:rotation] @
1030 [#:scale] [#:skew]
1031
1032 Modify @var{matrix} in-place to contain the transformation described
1033 by @var{position}, a 2D vector or rectangle, @var{rotation}, a scalar
1034 representing a rotation about the Z axis, @var{scale}, a 2D vector,
1035 and @var{skew}, a 2D vector. The transformation happens with respect
1036 to @var{origin}, a 2D vector. If an argument is not provided, that
1037 particular transformation will not be included in the result.
1038 @end deffn
1039
1040 @deffn {Procedure} transform! @var{matrix} @var{v}
1041 Modify the 2D vector @var{v} in-place by multiplying it by the 4x4
1042 matrix @var{matrix}.
1043 @end deffn
1044
1045 @node Quaternions
1046 @subsection Quaternions
1047
1048 In game development, the quaternion is most often used to represent
1049 rotations. Why not use a matrix for that, you may ask. Unlike
1050 matrices, quaternions can be interpolated (animated) and produce a
1051 meaningful result. When interpolating two quaternions, there is a
1052 smooth transition from one rotation to another, whereas interpolating
1053 two matrices would yield garbage.
1054
1055 @deffn {Procedure} quaternion @var{x} @var{y} @var{z} @var{w}
1056 Return a new quaternion with values @var{x}, @var{y}, @var{z}, and
1057 @var{w}.
1058 @end deffn
1059
1060 @deffn {Procedure} quaternion? @var{obj}
1061 Return @code{#t} if @var{obj} is a quaternion.
1062 @end deffn
1063
1064 @deffn {Procedure} quaternion-w @var{q}
1065 Return the W component of the quaternion @var{q}.
1066 @end deffn
1067
1068 @deffn {Procedure} quaternion-x @var{q}
1069 Return the X component of the quaternion @var{q}.
1070 @end deffn
1071
1072 @deffn {Procedure} quaternion-y @var{q}
1073 Return the Y component of the quaternion @var{q}.
1074 @end deffn
1075
1076 @deffn {Procedure} quaternion-z @var{q}
1077 Return the Z component of the quaternion @var{q}.
1078 @end deffn
1079
1080 @deffn {Procedure} make-identity-quaternion
1081 Return the identity quaternion.
1082 @end deffn
1083
1084 @node Easings
1085 @subsection Easings
1086
1087 Easing functions are essential for animation. Each easing function
1088 provides a different path to go from an initial value to a final
1089 value. These functions make an excellent companion to the
1090 @code{tween} procedure (@pxref{Tweening}). Experiment with them to
1091 figure out which function makes an animation look the best.
1092
1093 Pro tip: @code{smoothstep} provides nice results most of the time and
1094 creates smoother animation than using @code{linear}.
1095
1096 @deffn {Procedure} linear @var{t}
1097 @end deffn
1098
1099 @deffn {Procedure} smoothstep @var{t}
1100 @end deffn
1101
1102 @deffn {Procedure} ease-in-quad @var{t}
1103 @end deffn
1104
1105 @deffn {Procedure} ease-out-quad @var{t}
1106 @end deffn
1107
1108 @deffn {Procedure} ease-in-out-quad @var{t}
1109 @end deffn
1110
1111 @deffn {Procedure} ease-in-cubic @var{t}
1112 @end deffn
1113
1114 @deffn {Procedure} ease-out-cubic @var{t}
1115 @end deffn
1116
1117 @deffn {Procedure} ease-in-out-cubic @var{t}
1118 @end deffn
1119
1120 @deffn {Procedure} ease-in-quart @var{t}
1121 @end deffn
1122
1123 @deffn {Procedure} ease-out-quart @var{t}
1124 @end deffn
1125
1126 @deffn {Procedure} ease-in-out-quart @var{t}
1127 @end deffn
1128
1129 @deffn {Procedure} ease-in-quint @var{t}
1130 @end deffn
1131
1132 @deffn {Procedure} ease-out-quint @var{t}
1133 @end deffn
1134
1135 @deffn {Procedure} ease-in-out-quint @var{t}
1136 @end deffn
1137
1138 @deffn {Procedure} ease-in-sine @var{t}
1139 @end deffn
1140
1141 @deffn {Procedure} ease-out-sine @var{t}
1142 @end deffn
1143
1144 @deffn {Procedure} ease-in-out-sine @var{t}
1145 @end deffn
1146
1147 @node Bezier Curves
1148 @subsection Bezier Curves
1149
1150 The @code{(chickadee math bezier)} module provides an API for
1151 describing cubic Bezier curves in 2D space. These curves are notably
1152 used in font description, vector graphics programs, and when it comes
1153 to games: path building. With Bezier curves, it's somewhat easy to
1154 create a smooth looking path for an enemy to move along, for example.
1155 Bezier curves become particularly interesting when they are chained
1156 together to form a Bezier ``path'', where the end point of one curve
1157 becomes the starting point of the next.
1158
1159 Currently, the rendering of Bezier curves is rather crude and provided
1160 mostly for visualizing and debugging curves that would be unseen in
1161 the final game. See @xref{Lines and Shapes} for more information.
1162
1163 @deffn {Procedure} make-bezier-curve @var{p0} @var{p1} @var{p2} @var{p3}
1164 Return a new Bezier curve object whose starting point is @var{p0},
1165 ending point is @var{p3}, and control points are @var{p1} and
1166 @var{p2}. All points are 2D vectors.
1167 @end deffn
1168
1169 @deffn {Procedure} bezier-curve? @var{obj}
1170 Return @code{#t} if @var{obj} is a Bezier curve.
1171 @end deffn
1172
1173 @deffn {Procedure} bezier-curve-p0 @var{bezier}
1174 Return the starting point of @var{bezier}.
1175 @end deffn
1176
1177 @deffn {Procedure} bezier-curve-p1 @var{bezier}
1178 Return the first control point of @var{bezier}.
1179 @end deffn
1180
1181 @deffn {Procedure} bezier-curve-p2 @var{bezier}
1182 Return the second control point of @var{bezier}.
1183 @end deffn
1184
1185 @deffn {Procedure} bezier-curve-p3 @var{bezier}
1186 Return the end point of @var{bezier}.
1187 @end deffn
1188
1189 @deffn {Procedure} bezier-path . @var{control-points}
1190 Return a list of connected bezier curves defined by
1191 @var{control-points}. The first curve is defined by the first 4
1192 arguments and every additional curve thereafter requires 3 additional
1193 arguments.
1194 @end deffn
1195
1196 @deffn {Procedure} bezier-curve-point-at @var{bezier} @var{t}
1197 Return the coordinates for @var{bezier} at @var{t} (a value in the
1198 range [0, 1] representing how far from the start of the curve to
1199 check) as a 2D vector.
1200 @end deffn
1201
1202 @deffn {Procedure} bezier-curve-point-at! @var{dest} @var{bezier} @var{t}
1203 Modify the 2D vector @var{dest} in-place to contain the coordinates
1204 for @var{bezier} at @var{t}.
1205 @end deffn
1206
1207 @node Path Finding
1208 @subsection Path Finding
1209
1210 Most game worlds have maps. Often, these games have a need to move
1211 non-player characters around in an unscripted fashion. For example,
1212 in a real-time strategy game, the player may command one of their
1213 units to attack something in the enemy base. To do so, the unit must
1214 calculate the shortest route to get there. It wouldn't be a very fun
1215 game if units didn't know how to transport themselves efficiently.
1216 This is where path finding algorithms come in handy. The
1217 @code{(chickadee math path-finding)} module provides a generic
1218 implementation of the popular A* path finding algorithm. Just add a
1219 map implementation!
1220
1221 The example below defines a very simple town map and finds the
1222 quickest way to get from the town common to the school.
1223
1224 @example
1225 (define world-map
1226 '((town-common . (town-hall library))
1227 (town-hall . (town-common school))
1228 (library . (town-common cafe))
1229 (school . (town-hall cafe))
1230 (cafe . (library school))))
1231 (define (neighbors building)
1232 (assq-ref town-map building))
1233 (define (cost a b) 1)
1234 (define (distance a b) 1)
1235 (define pf (make-path-finder))
1236 (a* pf 'town-common 'school neighbors cost distance)
1237 @end example
1238
1239 In this case, the @code{a*} procedure will return the list
1240 @code{(town-common town-hall school)}, which is indeed the shortest
1241 route. (The other possible route is @code{(town-common library cafe
1242 school)}.)
1243
1244 The @code{a*} procedure does not know anything about about any kind of
1245 map and therefore must be told how to look up neighboring nodes, which
1246 is what the @code{neighbors} procedure in the example does. To
1247 simulate different types of terrain, a cost procedure is used. In
1248 this example, it is just as easy to move between any two nodes because
1249 @code{cost} always returns 1. In a real game, perhaps moving from
1250 from a field to a rocky hill would cost a lot more than moving from
1251 one field to another. Finally, a heuristic is used to calculate an
1252 approximate distance between two nodes on the map. In this simple
1253 association list based graph it is tough to calculate a distance
1254 between nodes, so the @code{distance} procedure isn't helpful and
1255 always returns 1. In a real game with a tile-based map, for example,
1256 the heuristic could be a quick Manhattan distance calculation based on
1257 the coordinates of the two map tiles. Choose an appropriate heuristic
1258 for optimal path finding!
1259
1260 @deffn {Procedure} make-path-finder
1261 Return a new path finder object.
1262 @end deffn
1263
1264 @deffn {Procedure} path-finder? @var{obj}
1265 Return @code{#t} if @var{obj} is a path finder.
1266 @end deffn
1267
1268 @deffn {Procedure} a* @var{path-finder} @var{start} @var{goal} @
1269 @var{neighbors} @var{cost} @var{distance}
1270
1271 Return a list of nodes forming a path from @var{start} to @var{goal}
1272 using @var{path-finder} to hold state. @var{neighbors} is a procedure
1273 that accepts a node and returns a list of nodes that neighbor it.
1274 @var{cost} is a procedure that accepts two neighboring nodes and
1275 returns the cost of moving from the first to the second as a real
1276 number. @var{distance} is a procedure that accepts two nodes and
1277 returns an approximate distance between them.
1278 @end deffn
1279
1280 @node Graphics
1281 @section Graphics
1282
1283 Chickadee aims to make hardware-accelerated graphics rendering as
1284 simple and efficient as possible by providing high-level APIs that
1285 interact with the low-level OpenGL API under the hood. Anyone that
1286 has worked with OpenGL directly knows that it has a steep learning
1287 curve and a lot of effort is needed to render even a single triangle.
1288 The Chickadee rendering engine attempts to make it easy to do common
1289 tasks like rendering a sprite while also providing all of the building
1290 blocks to implement additional rendering techniques.
1291
1292 @menu
1293 * Textures:: 2D images.
1294 * Sprites:: Draw 2D images.
1295 * Tile Maps:: Draw 2D tile maps.
1296 * Lines and Shapes:: Draw line segments and polygons.
1297 * Fonts:: Drawing text.
1298 * Particles:: Pretty little flying pieces!
1299 * Blending:: Control how pixels are combined.
1300 * Framebuffers:: Render to texture.
1301 * Viewports:: Restrict rendering to a particular area.
1302 * Rendering Engine:: Rendering state management.
1303 * Buffers:: Send data to the GPU.
1304 * Shaders:: Create custom GPU programs.
1305 @end menu
1306
1307 @node Textures
1308 @subsection Textures
1309
1310 @deffn {Procedure} load-image @var{file} [#:min-filter nearest] @
1311 [#:mag-filter nearest] [#:wrap-s repeat] [#:wrap-t repeat]
1312
1313 Load the image data from @var{file} and return a new texture object.
1314
1315 @var{min-filter} and @var{mag-filter} describe the method that should
1316 be used for minification and magnification when rendering,
1317 respectively. Possible values are @code{nearest} and @code{linear}.
1318
1319 @var{wrap-s} and @var{wrap-t} describe how to interpret texture
1320 coordinates that are greater than @code{1.0}. Possible values are
1321 @code{repeat}, @code{clamp}, @code{clamp-to-border}, and
1322 @code{clamp-to-edge}.
1323
1324 @end deffn
1325
1326 @node Sprites
1327 @subsection Sprites
1328
1329 For those who are new to this game, a sprite is a 2D rectangular
1330 bitmap that is rendered to the screen. For 2D games, sprites are the
1331 most essential graphical abstraction. They are used for drawing maps,
1332 players, NPCs, items, particles, text, etc. In Chickadee, bitmaps are
1333 stored in textures (@pxref{Textures}) and can be used to draw sprites
1334 via the @code{draw-sprite} procedure.
1335
1336 @deffn {Procedure} draw-sprite @var{texture} @var{position} @
1337 [#:tint white] [#:origin] [#:scale] [#:rotation] [#:blend-mode alpha] @
1338 [#:rect]
1339
1340 Draw @var{texture} at @var{position}.
1341
1342 Optionally, other transformations may be applied to the sprite.
1343 @var{rotation} specifies the angle to rotate the sprite, in radians.
1344 @var{scale} specifies the scaling factor as a 2D vector. All
1345 transformations are applied relative to @var{origin}, a 2D vector,
1346 which defaults to the lower-left corner.
1347
1348 @var{tint} specifies the color to multiply against all the sprite's
1349 pixels. By default white is used, which does no tinting at all.
1350
1351 Alpha blending is used by default but the blending method can be
1352 changed by specifying @var{blend-mode}.
1353
1354 The area drawn to is as big as the texture, by default. To draw to an
1355 arbitrary section of the screen, specify @var{rect}.
1356 @end deffn
1357
1358 It's not uncommon to need to draw hundreds or thousands of sprites
1359 each frame. However, GPUs (graphics processing units) are tricky
1360 beasts that prefer to be sent few, large chunks of data to render
1361 rather than many, small chunks. Using @code{draw-sprite} on its own
1362 will involve at least one GPU call @emph{per sprite}. This is fine
1363 for rendering a few dozen sprites, but will become a serious
1364 bottleneck when rendering hundreds or thousands of sprites. To deal
1365 with this, a technique known as ``sprite batching'' is used. Instead
1366 of drawing each sprite immediately, the sprite batch will build up a
1367 large of buffer of sprites to draw and send them to the GPU all at
1368 once. There is one caveat, however. Batching only works if the
1369 sprites being drawn share a common texture. A good strategy for
1370 reducing the number of different textures is to stuff many bitmaps
1371 into a single image file and create a ``texture atlas''
1372 (@pxref{Textures}) to access the sub-images within.
1373
1374 @deffn {Procedure} make-sprite-batch @var{texture} [#:capacity 256]
1375 Create a new sprite batch for @var{texture} with initial space for
1376 @var{capacity} sprites. Sprite batches automatically resize when they
1377 are full to accomodate as many sprites as necessary.
1378 @end deffn
1379
1380 @deffn {Procedure} sprite-batch? @var{obj}
1381 Return @code{#t} if @var{obj} is a sprite batch.
1382 @end deffn
1383
1384 @deffn {Procedure} sprite-batch-texture @var{batch}
1385 Return the texture for @var{batch}.
1386 @end deffn
1387
1388 @deffn {Procedure} set-sprite-batch-texture! @var{batch} @var{texture}
1389 Set texture for @var{batch} to @var{texture}.
1390 @end deffn
1391
1392 @deffn {Procedure} sprite-batch-add! @var{batch} @var{position} @@
1393 [#:origin] [#:scale] [:rotation] @@
1394 [#:tint @code{white}] [#:texture-region]
1395
1396 Add sprite located at @var{position} to @var{batch}.
1397
1398 To render a subsection of the batch's texture, a texture object whose
1399 parent is the batch texture may be specified as @var{texture-region}.
1400
1401 See @code{draw-sprite} for information about the other arguments.
1402 @end deffn
1403
1404 @deffn {Procedure} sprite-batch-clear! @var{batch}
1405 Reset size of @var{batch} to 0.
1406 @end deffn
1407
1408 @deffn {Procedure} draw-sprite-batch @var{batch} [#:blend-mode @code{alpha}]
1409 Render @var{batch} using @var{blend-mode}. Alpha blending is used by
1410 default.
1411 @end deffn
1412
1413 With a basic sprite abstraction in place, it's possible to build other
1414 abstractions on top of it. One such example is the ``nine patch''. A
1415 nine patch is a sprite that can be rendered at various sizes without
1416 becoming distorted. This is achieved by dividing up the sprite into
1417 nine regions:
1418
1419 @itemize
1420 @item
1421 the center, which can be scaled horizontally and vertically
1422 @item
1423 the four corners, which can never be scaled
1424 @item
1425 the left and right sides, which can be scaled vertically
1426 @item
1427 the top and bottom sides, which can be scaled horizontally
1428 @end itemize
1429
1430 The one caveat is that the bitmap regions must be designed in such a
1431 way so that they are not distorted when stretched along the affected
1432 axes. For example, that means that the top and bottom sides could
1433 have varying colored pixels vertically, but not horizontally.
1434
1435 The most common application of this technique is for graphical user
1436 interface widgets like buttons and dialog boxes. By using a nine
1437 patch, they can be rendered at any size without unappealing scaling
1438 artifacts.
1439
1440 @deffn {Procedure} draw-nine-patch @var{texture} @var{rect} @
1441 [#:margin 0] [#:top-margin margin] [#:bottom-margin margin] @
1442 [#:left-margin margin] [#:right-margin margin] @
1443 [#:origin] [#:scale] [#:rotation] [#:blend-mode alpha] @
1444 [#:tint white]
1445
1446 Draw a nine patch sprite. A nine patch sprite renders @var{texture}
1447 as a @var{width} x @var{height} rectangle whose stretchable areas are
1448 defined by the given margin measurements @var{top-margin},
1449 @var{bottom-margin}, @var{left-margin}, and @var{right-margin}. The
1450 @var{margin} argument may be used to configure all four margins at
1451 once.
1452
1453 Refer to @code{draw-sprite} (@pxref{Sprites}) for information about
1454 the other arguments.
1455 @end deffn
1456
1457 @node Tile Maps
1458 @subsection Tile Maps
1459
1460 A tile map is a scene created by composing lots of small sprites,
1461 called ``tiles'', into a larger image. One program for editing such
1462 maps is called @url{http://mapeditor.org,Tiled}. Chickadee has native
1463 support for loading and rendering Tiled maps in the @code{(chickadee
1464 render tiled)} module.
1465
1466 @deffn {Procedure} load-tile-map @var{file-name}
1467 Load the Tiled formatted map in @var{file-name} and return a new tile
1468 map object.
1469 @end deffn
1470
1471 @deffn {Procedure} draw-tile-map @var{tile-map} [#:layers] [#:region] @
1472 [#:origin] [#:position] [#:scale] [#:rotation]
1473
1474 Draw the layers of @var{tile-map}. By default, all layers are drawn.
1475 To draw a subset of the available layers, pass a list of layer ids
1476 using the @var{layers} keyword argument.
1477
1478 Refer to @code{draw-sprite} (@pxref{Sprites}) for information about
1479 the other arguments.
1480 @end deffn
1481
1482 @node Lines and Shapes
1483 @subsection Lines and Shapes
1484
1485 Sprites are fun, but sometimes simple, untextured lines and polygons
1486 are desired. That's where the @code{(chickadee render shapes)} module
1487 comes in!
1488
1489 @deffn {Procedure} draw-line @var{start} @var{end} @
1490 [#:thickness 0.5] [#:feather 1.0] [#:cap round] [#:color] @
1491 [#:shader]
1492
1493 Draw a line segment from @var{start} to @var{end}. The line will be
1494 @var{thickness} pixels thick with an antialiased border @var{feather}
1495 pixels wide. The line will be colored @var{color}. @var{cap}
1496 specifies the type of end cap that should be used to terminate the
1497 lines, either @code{none}, @code{butt}, @code{square}, @code{round},
1498 @code{triangle-in}, or @code{triangle-out}. Advanced users may use
1499 the @var{shader} argument to override the built-in line segment
1500 shader.
1501 @end deffn
1502
1503 @deffn {Procedure} draw-bezier-curve @var{bezier} [#:segments 32] @
1504 [#:control-points?] [#:tangents?] @
1505 [#:control-point-size 8] @
1506 [#:control-point-color yellow] @
1507 [#:tangent-color yellow] @
1508 [#:thickness 0.5] [#:feather 1.0] @
1509 [#:matrix]
1510
1511 Draw the curve defined by @var{bezier} using a resolution of N
1512 @var{segments}. When @var{control-points?} is @code{#t}, the control
1513 points are rendered as squares of size @var{control-point-size} pixels
1514 and a color of @var{control-point-color}. When @var{tangents?} is
1515 @code{#t}, the tangent lines from terminal point to control point are
1516 rendered using the color @var{tangent-color}.
1517
1518 All line segments rendered use @code{draw-line}, and thus the
1519 arguments @var{thickness} and @var{feather} have the same effect as in
1520 that procedure.
1521
1522 A custom @var{matrix} may be passed for applications that require more
1523 control over the final output.
1524 @end deffn
1525
1526 @deffn {Procedure} draw-bezier-path @var{path} [#:segments 32] @
1527 [#:control-points?] [#:tangents?] @
1528 [#:control-point-size 8] @
1529 [#:control-point-color yellow] @
1530 [#:tangent-color yellow] @
1531 [#:thickness 0.5] [#:feather 1.0] @
1532 [#:matrix]
1533
1534 Render @var{path}, a list of bezier curves. See the documentation for
1535 @code{draw-bezier-curve} for an explanation of all the keyword
1536 arguments.
1537 @end deffn
1538
1539 @node Fonts
1540 @subsection Fonts
1541
1542 Printing text to the screen is quite easy:
1543
1544 @example
1545 (draw-text "Hello, world" (vec2 100.0 100.0))
1546 @end example
1547
1548 Chickadee loads and renders bitmap fonts in the
1549 @url{http://www.angelcode.com/products/bmfont/doc/file_format.html,
1550 Angel Code format}. A default font named ``Good Neighbors'' is
1551 built-in to Chickadee and is used for all text rendering operations
1552 where a font is not specified, as is the case in the above example.
1553
1554 The following procedures can be found in the @code{(chickadee render
1555 font)} module:
1556
1557 @deffn {Procedure} load-font @var{file}
1558 Load the Angel Code font (in either XML or FNT format) in @var{file}
1559 and return a new font object.
1560 @end deffn
1561
1562 @deffn {Procedure} font? @var{obj}
1563 Return @code{#t} if @var{obj} is a font object.
1564 @end deffn
1565
1566 @deffn {Procedure} font-face @var{font}
1567 Return the name of @var{font}.
1568 @end deffn
1569
1570 @deffn {Procedure} font-line-height @var{font}
1571 Return the line height of @var{font}.
1572 @end deffn
1573
1574 @deffn {Procedure} font-line-height @var{font}
1575 Return the line height of @var{font}.
1576 @end deffn
1577
1578 @deffn {Procedure} font-bold? @var{font}
1579 Return @code{#t} if @var{font} is a bold font.
1580 @end deffn
1581
1582 @deffn {Procedure} font-italic? @var{font}
1583 Return @code{#t} if @var{font} is an italicized font.
1584 @end deffn
1585
1586 @deffn {Procedure} draw-text @var{text} @var{position}
1587 [#:font] [#:origin] [#:scale] [#:rotation] [#:blend-mode]
1588 [#:start 0] [#:end @code{(string-length text)}]
1589
1590 Draw the string @var{text} with the first character starting at
1591 @var{position} using @var{font}. If @var{font} is not specified, a
1592 built-in font is used.
1593
1594 @example
1595 (draw-text font "Hello, world!" (vec2 128.0 128.0))
1596 @end example
1597
1598 To render a substring of @var{text}, use the @var{start} and @var{end}
1599 arguments.
1600
1601 Refer to @code{draw-sprite} (@pxref{Sprites}) for information about
1602 the other arguments.
1603 @end deffn
1604
1605 @node Particles
1606 @subsection Particles
1607
1608 Effects like smoke, fire, sparks, etc. are often achieved by animating
1609 lots of little, short-lived sprites known as ``particles''. In fact,
1610 all of these effects, and more, can be accomplished by turning a few
1611 configuration knobs in a ``particle system''. A particle system takes
1612 care of managing the many miniscule moving morsels so the developer
1613 can quickly produce an effect and move on with their life. The
1614 @code{(chickadee render particles)} module provides an API for
1615 manipulating particle systems.
1616
1617 Below is an example of a very simple particle system that utilizes
1618 nearly all of the default configuration settings:
1619
1620 @example
1621 (use-modules (chickadee render particles))
1622 (define texture (load-image "particle.png"))
1623 (define particles (make-particles 2000 #:texture texture))
1624 @end example
1625
1626 In order to put particles into a particle system, a particle
1627 ``emitter'' is needed. Emitters know where to spawn new particles,
1628 how many of them to spawn, and for how long they should do it.
1629
1630 Below is an example of an emitter that spawns 16 particles per frame
1631 at the coordinates @code{(320, 240)}:
1632
1633 @example
1634 (use-modules (chickadee math vector))
1635 (define emitter (make-particle-emitter (vec2 320.0 240.0) 16))
1636 (add-particle-emitter particles emitter)
1637 @end example
1638
1639 To see all of the tweakable knobs and switches, read on!
1640
1641 @deffn {Procedure} make-particles @var{capacity} [#:blend-mode @code{alpha}] @
1642 [#:color white] [#:end-color transparent] [#:texture] @
1643 [#:animation-rows 1] [#:animation-columns 1] [#:width] [#:height] @
1644 [#:speed-range (vec2 0.1 1.0)] [#:acceleration-range (vec2 0.0 0.1)] @
1645 [#:direction-range (vec2 0 (* 2 pi))] [#:lifetime 30] [#:sort]
1646
1647 Return a new particle system that may contain up to @var{capacity}
1648 particles. Achieving the desired particle effect involves tweaking
1649 the following keyword arguments as needed:
1650
1651 - @var{blend-mode}: Pixel blending mode. @code{alpha} by default.
1652 (@pxref{Blending} for more about blend modes).
1653
1654 - @var{start-color}: The tint color of the particle at the beginning of its
1655 life. White by default.
1656
1657 - @var{end-color}: The tint color of the particle at the end of of its
1658 life. Completely transparent by default for a fade-out effect. The
1659 color in the middle of a particle's life will be an interpolation of
1660 @var{start-color} and @var{end-color}.
1661
1662 - @var{texture}: The texture applied to the particles. The texture
1663 may be subdivided into many animation frames.
1664
1665 - @var{animation-rows}: How many animation frame rows there are in the
1666 texture. Default is 1.
1667
1668 - @var{animation-columns}: How many animation frame columns there are
1669 in the texture. Default is 1.
1670
1671 - @var{width}: The width of each particle. By default, the width of
1672 an animation frame (in pixels) is used.
1673
1674 - @var{height}: The height of each particle. By default, the height
1675 of an animation frame (in pixels) is used.
1676
1677 - @var{speed-range}: A 2D vector containing the min and max particle
1678 speed. Each particle will have a speed chosen at random from this
1679 range. By default, speed ranges from 0.1 to 1.0.
1680
1681 - @var{acceleration-range}: A 2D vector containing the min and max
1682 particle acceleration. Each particle will have an acceleration chosen
1683 at random from this range. By default, acceleration ranges from 0.0
1684 to 0.1.
1685
1686 - @var{direction-range}: A 2D vector containing the min and max
1687 particle direction as an angle in radians. Each particle will have a
1688 direction chosen at random from this range. By default, the range
1689 covers all possible angles.
1690
1691 - @var{lifetime}: How long each particle lives, measured in
1692 updates. 30 by default.
1693
1694 - @var{sort}: @code{youngest} if youngest particle should be drawn
1695 last or @code{oldest} for the reverse. By default, no sorting is
1696 applied at all.
1697 @end deffn
1698
1699 @deffn {Procedure} particles? @var{obj}
1700 Return @code{#t} if @var{obj} is a particle system.
1701 @end deffn
1702
1703 @deffn {Procedure} update-particles @var{particles}
1704 Advance the simulation of @var{particles}.
1705 @end deffn
1706
1707 @deffn {Procedure} draw-particles @var{particles}
1708 Render @var{particles}.
1709 @end deffn
1710
1711 @deffn {Procedure} draw-particles* @var{particles} @var{matrix}
1712 Render @var{particles} with @var{matrix} applied.
1713 @end deffn
1714
1715 @deffn {Procedure} make-particle-emitter @var{spawn-area} @
1716 @var{rate} [@var{duration}]
1717
1718 Return a new particle emitter that spawns @var{rate} particles per
1719 frame within @var{spawn-area} (a rectangle or 2D vector) for
1720 @var{duration} frames. If @var{duration} is not specified, the
1721 emitter will spawn particles indefinitely.
1722 @end deffn
1723
1724 @deffn {Procedure} particle-emitter? @var{obj}
1725 Return @code{#t} if @var{obj} is a particle emitter.
1726 @end deffn
1727
1728 @deffn {Procedure} particle-emitter-spawn-area @var{emitter}
1729 Return the spawn area for @var{emitter}.
1730 @end deffn
1731
1732 @deffn {Procedure} particle-emitter-rate @var{emitter}
1733 Return the number of particles that @var{emitter} will spawn per
1734 frame.
1735 @end deffn
1736
1737 @deffn {Procedure} particle-emitter-life @var{emitter}
1738 Return the number of frames remaining in @var{emitter}'s lifespan.
1739 @end deffn
1740
1741 @deffn {Procedure} particle-emitter-done? @var{emitter}
1742 Return @code{#t} if @var{emitter} has finished spawning particlces.
1743 @end deffn
1744
1745 @deffn {Procedure} add-particle-emitter @var{particles} @var{emitter}
1746 Add @var{emitter} to @var{particles}.
1747 @end deffn
1748
1749 @deffn {Procedure} remove-particle-emitter @var{particles} @var{emitter}
1750 Remove @var{emitter} to @var{particles}
1751 @end deffn
1752
1753 @node Blending
1754 @subsection Blending
1755
1756 Rendering a scene often involves drawing layers of objects that
1757 overlap each other. Blending determines how two overlapping pixels
1758 are combined in the final image that is rendered to the screen.
1759 Chickadee provides the following blend modes:
1760
1761 @itemize
1762
1763 @item @code{replace}
1764 Use the latest color, ignoring all others.
1765
1766 @item @code{alpha}
1767 Blend pixels according to the values of their alpha channels. This is
1768 the most commonly used blend mode and thus is Chickadee's default
1769 mode.
1770
1771 @item @code{add}
1772 Add all pixel color values together. The more colors blended
1773 together, the more white the final color becomes.
1774
1775 @item @code{subtract}
1776 Subtract all pixel color values. The more colors blended together,
1777 the more black the final color becomes.
1778
1779 @item @code{multiply}
1780
1781 @item @code{darken}
1782
1783 @item @code{lighten}
1784
1785 @item @code{screen}
1786
1787 @end itemize
1788
1789 @node Framebuffers
1790 @subsection Framebuffers
1791
1792 A framebuffer is a chunk of memory that the GPU can render things
1793 onto. By default, the framebuffer that is used for rendering is the
1794 one belonging to the game window, but custom framebuffers can be used
1795 as well. A common use-case for custom framebuffers is applying
1796 post-processing effects: The entire scene is rendered to a
1797 framebuffer, and then the contents of that framebuffer are applied to
1798 a post-processing shader and rendered to the game window. The
1799 post-processing shader could do any number of things: scaling,
1800 antialiasing, motion blur, etc.
1801
1802 @deffn {Procedure} make-framebuffer @var{width} @var{height} [#:min-filter 'linear] [#:mag-filter 'linear] [#:wrap-s 'repeat] [#:wrap-t 'repeat]
1803
1804 Create a new framebuffer that is @var{width} pixels wide and @var{height} pixels high.
1805
1806 @var{min-filter} and @var{mag-filter} determine the scaling algorithm
1807 applied to the framebuffer when rendering. By default, linear scaling
1808 is used in both cases. To perform no smoothing at all, use
1809 @code{nearest} for simple nearest neighbor scaling. This is typically
1810 the best choice for pixel art games.
1811 @end deffn
1812
1813 @deffn {Procedure} framebuffer? @var{obj}
1814 Return @code{#t} if @var{obj} is a framebuffer.
1815 @end deffn
1816
1817 @deffn {Procedure} framebuffer-texture @var{fb}
1818 Return the texture backing the framebuffer @var{fb}.
1819 @end deffn
1820
1821 @deffn {Procedure} framebuffer-viewport @var{fb}
1822 Return the default viewport (@pxref{Viewports}) used by the
1823 framebuffer @var{fb}.
1824 @end deffn
1825
1826 @deffn {Procedure} null-framebuffer
1827 The default framebuffer.
1828 @end deffn
1829
1830 @node Viewports
1831 @subsection Viewports
1832
1833 A viewport represents a subset of the screen (or framebuffer). When
1834 rendering a frame, the resulting image will only appear within that
1835 viewport. These aren't often needed, and Chickadee's default viewport
1836 occupies the entire screen, but there are certain situations where
1837 they are useful. For example, a split-screen multiplayer game may
1838 render to two different viewports, each occupying a different half of
1839 the screen. For information about how to set the current viewport,
1840 see @code{with-viewport} in @ref{Rendering Engine}.
1841
1842 The @code{(chickadee render viewport)} module provides the following
1843 API:
1844
1845 @deffn {Procedure} make-viewport @var{x} @var{y} @var{width} @var{height} @
1846 [#:clear-color] [#:clear-flags]
1847
1848 Create a viewport that covers an area of the window starting from
1849 coordinates (@var{x}, @var{y}) and spanning @var{width} @code{x}
1850 @var{height} pixels. Fill the viewport with @var{clear-color} when
1851 clearing the screen. Clear the buffers denoted by the list of symbols
1852 in @var{clear-flags}.
1853
1854 Possible values for @var{clear-flags} are @var{color-buffer},
1855 @var{depth-buffer}, @var{accum-buffer}, and @var{stencil-buffer}.
1856 @end deffn
1857
1858 @deffn {Procedure} viewport? @var{obj}
1859 Return @code{#t} if @var{obj} is a viewport.
1860 @end deffn
1861
1862 @deffn {Procedure} viewport-x @var{viewport}
1863 Return the left edge of @var{viewport}.
1864 @end deffn
1865
1866 @deffn {Procedure} viewport-y @var{viewport}
1867 Return the bottom edge of @var{viewport}.
1868 @end deffn
1869
1870 @deffn {Procedure} viewport-width @var{viewport}
1871 Return the width of @var{viewport}.
1872 @end deffn
1873
1874 @deffn {Procedure} viewport-height @var{viewport}
1875 Return the height of @var{viewport}.
1876 @end deffn
1877
1878 @deffn {Procedure} viewport-clear-color @var{viewport}
1879 Return the clear color for @var{viewport}.
1880 @end deffn
1881
1882 @deffn {Procedure} viewport-clear-flags @var{viewport}
1883 Return the list of clear flags for @var{viewport}.
1884 @end deffn
1885
1886 @node Rendering Engine
1887 @subsection Rendering Engine
1888
1889 Chickadee defines rendering using a metaphor familiar to Scheme
1890 programmers: procedure application. A shader (@pxref{Shaders}) is
1891 like a procedure for the GPU to apply. Shaders are passed arguments:
1892 A vertex array containing the geometry to render (@pxref{Buffers}) and
1893 zero or more keyword arguments that the shader understands. Similar
1894 to how Scheme has @code{apply} for calling procedures, Chickadee
1895 provides @code{gpu-apply} for calling shaders.
1896
1897 Additionally, there is some dynamic state that effects how
1898 @code{gpu-apply} will behave. Things like the current viewport,
1899 framebuffer, and blend mode are stored as dynamic state because it
1900 would be tedious to have to have to specify them each time
1901 @code{gpu-apply} is called.
1902
1903 The following procedures and syntax can be found in the
1904 @code{(chickadee render)} module.
1905
1906 @deffn {Syntax} gpu-apply @var{shader} @var{vertex-array} @
1907 [#:uniform-key @var{uniform-value} ...]
1908 @deffnx {Syntax} gpu-apply* @var{shader} @var{vertex-array} @
1909 @var{count} [#:uniform-key @var{uniform-value} ...]
1910
1911 Render @var{vertex-array} using @var{shader} with the uniform values
1912 specified in the following keyword arguments.
1913
1914 While @code{gpu-apply} will draw every vertex in @var{vertex-array},
1915 @code{gpu-apply*} will only draw @var{count} vertices.
1916 @end deffn
1917
1918 @deffn {Syntax} gpu-apply/instanced @var{shader} @var{vertex-array} @
1919 @var{n} [#:uniform-key @var{uniform-value} ...]
1920 @deffnx {Syntax} gpu-apply/instanced @var{shader} @var{vertex-array} @
1921 @var{count} @var{n} [#:uniform-key @var{uniform-value} ...]
1922
1923 Render @var{vertex-array} @var{n} times using @var{shader} with the
1924 uniform values specified in the following keyword arguments.
1925
1926 Instanced rendering is very beneficial for rendering the same object
1927 many times with only small differences for each one. For example, the
1928 particle effects described in @ref{Particles} use instanced rendering.
1929
1930 While @code{gpu-apply/instanced} will draw every vertex in
1931 @var{vertex-array}, @code{gpu-apply*} will only draw @var{count}
1932 vertices.
1933 @end deffn
1934
1935 @deffn {Procedure} current-viewport
1936 Return the currently bound viewport (@pxref{Viewports}).
1937 @end deffn
1938
1939 @deffn {Procedure} current-framebuffer
1940 Return the currently bound framebuffer (@pxref{Framebuffers}).
1941 @end deffn
1942
1943 @deffn {Procedure} current-blend-mode
1944 Return the currently bound blend mode (@pxref{Blending}).
1945 @end deffn
1946
1947 @deffn {Procedure} current-depth-test
1948 Return @code{#t} if depth testing is currently enabled (@pxref{Blending}).
1949 @end deffn
1950
1951 @deffn {Procedure} current-texture
1952 Return the currently bound texture (@pxref{Textures}).
1953 @end deffn
1954
1955 @deffn {Procedure} current-projection
1956 Return the currently bound projection matrix (@pxref{Matrices}).
1957 @end deffn
1958
1959 @deffn {Syntax} with-viewport @var{viewport} @var{body} ...
1960 Evaluate @var{body} with the current viewport bound to @var{viewport} (@pxref{Viewports}).
1961 @end deffn
1962
1963 @deffn {Syntax} with-framebuffer @var{framebuffer} @var{body} ...
1964 Evaluate @var{body} with the current framebuffer bound to
1965 @var{framebuffer} (@pxref{Framebuffers}).
1966 @end deffn
1967
1968 @deffn {Syntax} with-blend-mode @var{blend-mode} @var{body} ...
1969 Evaluate @var{body} with the current blend mode bound to
1970 @var{blend-mode} (@pxref{Blending}).
1971 @end deffn
1972
1973 @deffn {Syntax} with-depth-test @var{depth-test?} @var{body} ...
1974 Evaluate @var{body} with the depth-test disabled if @var{depth-test?}
1975 is @code{#f}, or enabled otherwise (@pxref{Blending}).
1976 @end deffn
1977
1978 @deffn {Syntax} with-texture @var{texture} @var{body} ...
1979 Evaluate @var{body} with the current texture bound to @var{texture}
1980 (@pxref{Textures}).
1981 @end deffn
1982
1983 @deffn {Syntax} with-projection @var{projection} @var{body} ...
1984 Evaluate @var{body} with the current projection matrix bound to
1985 @var{projection} (@pxref{Matrices}).
1986 @end deffn
1987
1988 @node Buffers
1989 @subsection Buffers
1990
1991 Alright, let's brush aside all of those pretty high level abstractions
1992 and discuss what is going on under the hood. The GPU exists as a
1993 discrete piece of hardware separate from the CPU. In order to make it
1994 draw things, we must ship lots of data out of our memory space and
1995 into the GPU. The @code{(chickadee render buffer}) module provides an
1996 API for manipulating GPU buffers.
1997
1998 In OpenGL terminology, a chunk of data allocated on the GPU is a
1999 ``vertex buffer object'' or VBO. For example, here is a bytevector
2000 that could be transformed into a GPU buffer that packs together vertex
2001 position and texture coordinates:
2002
2003 @example
2004 (use-modules (chickadee render buffer) (srfi srfi-4))
2005 (define data
2006 (f32vector -8.0 -8.0 ; 2D vertex
2007 0.0 0.0 ; 2D texture coordinate
2008 8.0 -8.0 ; 2D vertex
2009 1.0 0.0 ; 2D texture coordinate
2010 8.0 8.0 ; 2D vertex
2011 1.0 1.0 ; 2D texture coordinate
2012 -8.0 8.0 ; 2D vertex
2013 0.0 1.0)) ; 2D texture coordinate
2014 @end example
2015
2016 This data represents a textured 16x16 square centered on the
2017 origin. To send this data to the GPU, the @code{make-buffer} procedure
2018 is needed:
2019
2020 @example
2021 (define buffer (make-buffer data #:stride 16))
2022 @end example
2023
2024 The @code{#:stride} keyword argument indicates how many bytes make up
2025 each element of the buffer. In this case, there are 4 floats per
2026 element: 2 for the vertex, and 2 for the texture coordinate. A 32-bit
2027 float is 4 bytes in length, so the buffer's stride is 16.
2028
2029 Within a VBO, one or more ``attributes'', as OpenGL calls them, may be
2030 present. Attributes are subregions within the buffer that have a
2031 particular data type. In this case, there are two attributes packed
2032 into the buffer. To provided a typed view into a buffer, the
2033 @code{make-typed-buffer} procedure is needed:
2034
2035 @example
2036 (define vertices
2037 (make-typed-buffer #:buffer buffer
2038 #:type 'vec2
2039 #:component-type 'float
2040 #:length 4))
2041 (define texcoords
2042 (make-typed-buffer #:buffer buffer
2043 #:type 'vec2
2044 #:component-type 'float
2045 #:length 4
2046 #:offset 8))
2047 @end example
2048
2049 To render a square, the GPU needs to draw two triangles, which means
2050 we need 6 vertices in total. However, the above buffer only contains
2051 data for 4 vertices. This is becase there are only 4 unique vertices
2052 for a square, but 2 of them must be repeated for each triangle. To
2053 work with deduplicated vertex data, an ``index buffer'' must be
2054 created.
2055
2056 @example
2057 (define index-buffer
2058 (make-buffer (u32vector 0 3 2 0 2 1)
2059 #:target 'index)
2060 (define indices
2061 (make-typed-buffer #:type 'scalar
2062 #:component-type 'unsigned-int
2063 #:buffer index-buffer))
2064 @end example
2065
2066 Note the use of the @code{#:target} keyword argument. It is required
2067 because the GPU treats index data in a special way and must be told
2068 which data is index data.
2069
2070 Now that the typed buffers representing each attribute have been
2071 created, all that's left is to bind them all together in a ``vertex
2072 array object'', or VAO. Vertex arrays associate each typed buffer
2073 with an attribute index on the GPU. The indices that are chosen must
2074 correspond with the indices that the shader (@pxref{Shaders}) expects
2075 for each attribute.
2076
2077 @example
2078 (define vertex-array
2079 (make-vertex-array #:indices indices
2080 #:attributes `((0 . ,vertices)
2081 (1 . ,texcoords))))
2082 @end example
2083
2084 With the vertex array created, the GPU is now fully aware of how to
2085 interpret the data that it has been given in the original buffer.
2086 Actually rendering this square is left as an exercise to the reader.
2087 See the @ref{Shaders} section and the @code{gpu-apply} procedure in
2088 @ref{Rendering Engine} for the remaining pieces of a successful draw
2089 call. Additionally, consider reading the source code for sprites,
2090 shapes, or particles to see GPU buffers in action.
2091
2092 Without further ado, the API reference:
2093
2094 @deffn {Procedure} make-buffer @var{data} [#:name "anonymous"] @
2095 [#:length] [#:offset 0] [#:stride 0] [#:target @code{vertex}] @
2096 [#:usage @code{static}]
2097
2098 Upload @var{data}, a bytevector, to the GPU. By default, the entire
2099 bytevector is uploaded. A subset of the data may be uploaded by
2100 specifying the @var{offset}, the index of the first byte to be
2101 uploaded, and @var{length}, the number of bytes to upload.
2102
2103 If @var{data} is @code{#f}, allocate @var{length} bytes of fresh GPU
2104 memory instead.
2105
2106 @var{target} and @var{usage} are hints that tell the GPU how the
2107 buffer is intended to be used.
2108
2109 @var{target} may be:
2110
2111 @itemize
2112 @item @code{vertex}
2113 Vertex attribute data.
2114
2115 @item @code{index}
2116 Index buffer data.
2117
2118 @end itemize
2119
2120 @var{usage} may be:
2121
2122 @itemize
2123 @item @code{static}
2124 The buffer data will not be modified after creation.
2125
2126 @item @code{stream}
2127 The buffer data will be modified frequently.
2128
2129 @end itemize
2130
2131 @var{name} is simply an arbitrary string for debugging purposes that
2132 is never sent to the GPU.
2133 @end deffn
2134
2135 @deffn {Procedure} buffer? @var{obj}
2136 Return @code{#t} if @var{obj} is a GPU buffer.
2137 @end deffn
2138
2139 @deffn {Procedure} index-buffer? @var{buffer}
2140 Return @code{#t} if @var{buffer} is an index buffer.
2141 @end deffn
2142
2143 @defvar null-buffer
2144 Represents the absence of a buffer.
2145 @end defvar
2146
2147 @deffn {Procedure} buffer-name @var{buffer}
2148 Return the name of @var{buffer}.
2149 @end deffn
2150
2151 @deffn {Procedure} buffer-length @var{buffer}
2152 Return the length of @var{buffer}.
2153 @end deffn
2154
2155 @deffn {Procedure} buffer-stride @var{buffer}
2156 Return the amount of space, in bytes, between each element in
2157 @var{buffer}.
2158 @end deffn
2159
2160 @deffn {Procedure} buffer-target @var{buffer}
2161 Return the the intended usage of @var{buffer}, either @code{vertex} or
2162 @code{index}.
2163 @end deffn
2164
2165 @deffn {Procedure} buffer-usage @var{buffer}
2166 Return the intended usage of @var{buffer}, either @code{static} for
2167 buffer data that will not change once sent to the GPU, or
2168 @code{stream} for buffer data that will be frequently updated from the
2169 client-side.
2170 @end deffn
2171
2172 @deffn {Procedure} buffer-data @var{buffer}
2173 Return a bytevector containing all the data within @var{buffer}. If
2174 @var{buffer} has not been mapped (see @code{with-mapped-buffer}) then
2175 this procedure will return @code{#f}.
2176 @end deffn
2177
2178 @deffn {Syntax} with-mapped-buffer @var{buffer} @var{body} @dots{}
2179 Evaluate @var{body} in the context of @var{buffer} having its data
2180 synced from GPU memory to RAM. In this context, @code{buffer-data}
2181 will return a bytevector of all the data stored in @var{buffer}. When
2182 program execution exits this form, the data (including any
2183 modifications) is synced back to the GPU.
2184
2185 This form is useful for streaming buffers that need to update their
2186 contents dynamically, such as a sprite batch.
2187 @end deffn
2188
2189 @deffn {Procedure} make-typed-buffer #:buffer #:type @
2190 #:component-type #:length [#:offset 0] [#:divisor] @
2191 [#:name "anonymous"]
2192
2193 Return a new typed buffer view for @var{buffer} starting at byte index
2194 @var{offset} of @var{length} elements, where each element is of
2195 @var{type} and composed of @var{component-type} values.
2196
2197 Valid values for @var{type} are:
2198
2199 @itemize
2200 @item @code{scalar}
2201 single number
2202
2203 @item @code{vec2}
2204 2D vector
2205
2206 @item @code{vec3}
2207 3D vector
2208
2209 @item @code{vec4}
2210 4D vector
2211
2212 @item @code{mat2}
2213 2x2 matrix
2214
2215 @item @code{mat3}
2216 3x3 matrix
2217
2218 @item @code{mat4}
2219 4x4 matrix
2220 @end itemize
2221
2222 Valid values for @var{component-type} are:
2223
2224 @itemize
2225
2226 @item @code{byte}
2227 @item @code{unsigned-byte}
2228 @item @code{short}
2229 @item @code{unsigned-short}
2230 @item @code{int}
2231 @item @code{unsigned-int}
2232 @item @code{float}
2233 @item @code{double}
2234
2235 @end itemize
2236
2237 @var{divisor} is only needed for instanced rendering applications (see
2238 @code{gpu-apply/instanced} in @ref{Rendering Engine}) and represents
2239 how many instances each vertex element applies to. A divisor of 0
2240 means that a single element is used for every instance and is used for
2241 the data being instanced. A divisor of 1 means that each element is
2242 used for 1 instance. A divisor of 2 means that each element is used
2243 for 2 instances, and so on.
2244 @end deffn
2245
2246 @deffn {Procedure} typed-buffer? @var{obj}
2247 Return @code{#t} if @var{obj} is a typed buffer.
2248 @end deffn
2249
2250 @deffn {Procedure} typed-buffer->buffer @var{typed-buffer}
2251 Return the buffer that @var{typed-buffer} is using.
2252 @end deffn
2253
2254 @deffn {Procedure} typed-buffer-name @var{typed-buffer}
2255 Return the name of @var{typed-buffer}.
2256 @end deffn
2257
2258 @deffn {Procedure} typed-buffer-offset @var{typed-buffer}
2259 Return the byte offset of @var{typed-buffer}.
2260 @end deffn
2261
2262 @deffn {Procedure} typed-buffer-type @var{typed-buffer}
2263 Return the data type of @var{typed-buffer}.
2264 @end deffn
2265
2266 @deffn {Procedure} typed-buffer-component-type @var{typed-buffer}
2267 Return the component data type of @var{typed-buffer}
2268 @end deffn
2269
2270 @deffn {Procedure} typed-buffer-divisor @var{typed-buffer}
2271 Return the instance divisor for @var{typed-buffer}.
2272 @end deffn
2273
2274 @deffn {Syntax} with-mapped-typed-buffer @var{typed-buffer} @var{body} @dots{}
2275
2276 Evaluate @var{body} in the context of @var{typed-buffer} having its
2277 data synced from GPU memory to RAM. See @code{with-mapped-buffer} for
2278 more information.
2279 @end deffn
2280
2281 @deffn {Procedure} make-vertex-array #:indices #:attributes @
2282 [#:mode @code{triangles}]
2283
2284 Return a new vertex array using the index data within the typed buffer
2285 @var{indices} and the vertex attribute data within @var{attributes}.
2286
2287 @var{attributes} is an alist mapping shader attribute indices to typed
2288 buffers containing vertex data:
2289
2290 @example
2291 `((1 . ,typed-buffer-a)
2292 (2 . ,typed-buffer-b)
2293 ...)
2294 @end example
2295
2296 By default, the vertex array is interpreted as containing a series of
2297 triangles. If another primtive type is desired, the @var{mode}
2298 keyword argument may be overridden. The following values are
2299 supported:
2300
2301 @itemize
2302 @item @code{points}
2303 @item @code{lines}
2304 @item @code{line-loop}
2305 @item @code{line-strip}
2306 @item @code{triangles}
2307 @item @code{triangle-strip}
2308 @item @code{triangle-fan}
2309 @end itemize
2310
2311 @end deffn
2312
2313 @defvar null-vertex-array
2314 Represents the absence of a vertex array.
2315 @end defvar
2316
2317 @deffn {Procedure} vertex-array? @var{obj}
2318 Return @code{#t} if @var{obj} is a vertex array.
2319 @end deffn
2320
2321 @deffn {Procedure} vertex-array-indices @var{vertex-array}
2322 Return the typed buffer containing index data for @var{vertex-array}.
2323 @end deffn
2324
2325 @deffn {Procedure} vertex-array-attributes @var{vertex-array}
2326 Return the attribute index -> typed buffer mapping of vertex attribute
2327 data for @var{vertex-array}.
2328 @end deffn
2329
2330 @deffn {Procedure} vertex-array-mode @var{vertex-array}
2331 Return the primitive rendering mode for @var{vertex-array}.
2332 @end deffn
2333
2334 @node Shaders
2335 @subsection Shaders
2336
2337 Shaders are programs that the GPU can evaluate that allow the
2338 programmer to completely customized the final output of a GPU draw
2339 call. The @code{(chickadee render shader)} module provides an API for
2340 building custom shaders.
2341
2342 Shaders are written in the OpenGL Shading Language, or GLSL for short.
2343 Chickadee aspires to provide a domain specific language for writing
2344 shaders in Scheme, but we are not there yet.
2345
2346 Shader programs consist of two components: A vertex shader and a
2347 fragment shader. A vertex shader receives vertex data (position
2348 coordinates, texture coordinates, normals, etc.) and transforms them
2349 as desired, whereas a fragment shader controls the color of each
2350 pixel.
2351
2352 Sample vertex shader:
2353
2354 @example
2355 @verbatim
2356 #version 130
2357
2358 in vec2 position;
2359 in vec2 tex;
2360 out vec2 fragTex;
2361 uniform mat4 mvp;
2362
2363 void main(void) {
2364 fragTex = tex;
2365 gl_Position = mvp * vec4(position.xy, 0.0, 1.0);
2366 }
2367 @end verbatim
2368 @end example
2369
2370 Sample fragment shader:
2371
2372 @example
2373 @verbatim
2374 #version 130
2375
2376 in vec2 fragTex;
2377 uniform sampler2D colorTexture;
2378
2379 void main (void) {
2380 gl_FragColor = texture2D(colorTexture, fragTex);
2381 }
2382 @end verbatim
2383 @end example
2384
2385 This manual will not cover GLSL features and syntax as there is lots
2386 of information already available about this topic.
2387
2388 One way to think about rendering with shaders, and the metaphor
2389 Chickadee uses, is to think about it as a function call: The shader is
2390 a function, and it is applied to some ``attributes'' (positional
2391 arguments), and some ``uniforms'' (keyword arguments).
2392
2393 @example
2394 (define my-shader (load-shader "vert.glsl" "frag.glsl"))
2395 (define vertices (make-vertex-array ...))
2396 (gpu-apply my-shader vertices #:color red)
2397 @end example
2398
2399 @xref{Rendering Engine} for more details about the @code{gpu-apply}
2400 procedure.
2401
2402 Shaders are incredibly powerful tools, and there's more information
2403 about them than we could ever fit into this manual, so we highly
2404 recommend searching the web for more information and examples. What
2405 we can say, though, is how to use our API:
2406
2407 @deffn {Procedure} strings->shader @var{vertex-source} @var{fragment-source}
2408 Compile @var{vertex-source}, the GLSL code for the vertex shader, and
2409 @var{fragment-source}, the GLSL code for the fragment shader, into a
2410 GPU shader program.
2411 @end deffn
2412
2413 @deffn {Procedure} load-shader @var{vertex-source-file} @
2414 @var{fragment-source-file}
2415
2416 Compile the GLSL source code within @var{vertex-source-file} and
2417 @var{fragment-source-file} into a GPU shader program.
2418 @end deffn
2419
2420 @deffn {Procedure} make-shader @var{vertex-port} @var{fragment-port}
2421 Read GLSL source from @var{vertex-port} and @var{fragment-port} and
2422 compile them into a GPU shader program.
2423 @end deffn
2424
2425 @deffn {Procedure} shader? @var{obj}
2426 Return @code{#t} if @var{obj} is a shader.
2427 @end deffn
2428
2429 @defvar null-shader
2430 Represents the absence shader program.
2431 @end defvar
2432
2433 @deffn {Procedure} shader-uniform @var{shader} @var{name}
2434 Return the metadata for the uniform @var{name} in @var{shader}.
2435 @end deffn
2436
2437 @deffn {Procedure} shader-uniforms @var{shader}
2438 Return a hash table of uniforms for @var{shader}.
2439 @end deffn
2440
2441 @deffn {Procedure} shader-attributes @var{shader}
2442 Return a hash table of attributes for @var{shader}.
2443 @end deffn
2444
2445 @deffn {Procedure} uniform? @var{obj}
2446 Return @code{#t} if @var{obj} is a uniform.
2447 @end deffn
2448
2449 @deffn {Procedure} uniform-name @var{uniform}
2450 Return the variable name of @var{uniform}.
2451 @end deffn
2452
2453 @deffn {Procedure} uniform-type @var{uniform}
2454 Return the data type of @var{uniform}.
2455 @end deffn
2456
2457 @deffn {Procedure} uniform-value @var{uniform}
2458 Return the current value of @var{uniform}.
2459 @end deffn
2460
2461 @deffn {Procedure} uniform-default-value @var{uniform}
2462 Return the default value of @var{uniform}.
2463 @end deffn
2464
2465 @deffn {Procedure} attribute? @var{obj}
2466 Return @code{#t} if @var{obj} is an attribute.
2467 @end deffn
2468
2469 @deffn {Procedure} attribute-name @var{attribute}
2470 Return the variable name of @var{attribute}.
2471 @end deffn
2472
2473 @deffn {Procedure} attribute-location @var{attribute}
2474 Return the binding location of @var{attribute}.
2475 @end deffn
2476
2477 @deffn {Procedure} attribute-type @var{attribute}
2478 Return the data type of @var{attribute}.
2479 @end deffn
2480
2481 @node Scripting
2482 @section Scripting
2483
2484 Game logic is a web of asynchronous events that are carefully
2485 coordinated to bring the game world to life. In order to make an
2486 enemy follow and attack the player, or move an NPC back and forth in
2487 front of the item shop, or do both at the same time, a scripting
2488 system is a necessity. Chickadee comes with an asynchronous
2489 programming system in the @code{(chickadee scripting)} module.
2490 Lightweight, cooperative threads known as ``scripts'' allow the
2491 programmer to write asynchronous code as if it were synchronous, and
2492 allow many such ``threads'' to run concurrently.
2493
2494 But before we dig deeper into scripts, let's discuss the simple act
2495 of scheduling tasks.
2496
2497 @menu
2498 * Agendas:: Scheduling tasks.
2499 * Scripts:: Cooperative multitasking.
2500 * Tweening:: Animations.
2501 * Channels:: Publish data to listeners.
2502 @end menu
2503
2504 @node Agendas
2505 @subsection Agendas
2506
2507 To schedule a task to be performed later, an ``agenda'' is used.
2508 There is a default, global agenda that is ready to be used, or
2509 additional agendas may be created for different purposes. The
2510 following example prints the text ``hello'' when the agenda has
2511 advanced to time unit 10.
2512
2513 @example
2514 (at 10 (display "hello\n"))
2515 @end example
2516
2517 Most of the time it is more convenient to schedule tasks relative to
2518 the current time. This is where @code{after} comes in handy:
2519
2520 @example
2521 (after 10 (display "hello\n"))
2522 @end example
2523
2524 Time units in the agenda are in no way connected to real time. It's
2525 up to the programmer to decide what agenda time means. A simple and
2526 effective approach is to map each call of the update hook
2527 (@pxref{Kernel}) to 1 unit of agenda time, like so:
2528
2529 @example
2530 (add-hook! update-hook (lambda (dt) (update-agenda 1)))
2531 @end example
2532
2533 It is important to call @code{update-agenda} periodically, otherwise
2534 no tasks will ever be run!
2535
2536 In addition to using the global agenda, it is useful to have multiple
2537 agendas for different purposes. For example, the game world can use a
2538 different agenda than the user interface, so that pausing the game is
2539 a simple matter of not updating the world's agenda while continuing to
2540 update the user interface's agenda. The current agenda is dynamically
2541 scoped and can be changed using the @code{with-agenda} special form:
2542
2543 @example
2544 (define game-world-agenda (make-agenda))
2545
2546 (with-agenda game-world-agenda
2547 (at 60 (spawn-goblin))
2548 (at 120 (spawn-goblin))
2549 (at 240 (spawn-goblin-king)))
2550 @end example
2551
2552 @deffn {Procedure} make-agenda
2553 Return a new task scheduler.
2554 @end deffn
2555
2556 @deffn {Procedure} agenda? @var{obj}
2557 Return @code{#t} if @var{obj} is an agenda.
2558 @end deffn
2559
2560 @deffn {Procedure} current-agenda
2561 @deffnx {Procedure} current-agenda @var{agenda}
2562 When called with no arguments, return the current agenda. When called
2563 with one argument, set the current agenda to @var{agenda}.
2564 @end deffn
2565
2566 @deffn {Syntax} with-agenda @var{agenda} @var{body} @dots{}
2567 Evaluate @var{body} with the current agenda set to @var{agenda}.
2568 @end deffn
2569
2570 @deffn {Procedure} agenda-time
2571 Return the current agenda time.
2572 @end deffn
2573
2574 @deffn {Procedure} update-agenda @var{dt}
2575 Advance the current agenda by @var{dt}.
2576 @end deffn
2577
2578 @deffn {Procedure} schedule-at @var{time} @var{thunk}
2579 Schedule @var{thunk}, a procedure of zero arguments, to be run at
2580 @var{time}.
2581 @end deffn
2582
2583 @deffn {Procedure} schedule-after @var{delay} @var{thunk}
2584 Schedule @var{thunk}, a procedure of zero arguments, to be run after
2585 @var{delay}.
2586 @end deffn
2587
2588 @deffn {Procedure} schedule-every @var{interval} @var{thunk} [@var{n}]
2589 Schedule @var{thunk}, a procedure of zero arguments, to be run every
2590 @var{interval} amount of time. Repeat this @var{n} times, or
2591 indefinitely if not specified.
2592 @end deffn
2593
2594 @deffn {Syntax} at @var{time} @var{body} @dots{}
2595 Schedule @var{body} to be evaluated at @var{time}.
2596 @end deffn
2597
2598 @deffn {Syntax} after @var{delay} @var{body} @dots{}
2599 Schedule @var{body} to be evaluated after @var{delay}.
2600 @end deffn
2601
2602 @deffn {Syntax} every @var{interval} @var{body} @dots{}
2603 @deffnx {Syntax} every (@var{interval} @var{n}) @var{body} @dots{}
2604 Schedule @var{body} to be evaluated every @var{interval} amount of
2605 time. Repeat this @var{n} times, or indefinitely if not specified.
2606 @end deffn
2607
2608 @node Scripts
2609 @subsection Scripts
2610
2611 Now that we can schedule tasks, let's take things to the next level.
2612 It sure would be great if we could make procedures that described a
2613 series of actions that happened over time, especially if we could do
2614 so without contorting our code into a nest of callback procedures.
2615 This is where scripts come in. With scripts we can write code in a
2616 linear way, in a manner that appears to be synchronous, but with the
2617 ability to suspend periodically in order to let other scripts have a
2618 turn and prevent blocking the game loop. Building on top of the
2619 scheduling that agendas provide, here is a script that models a child
2620 trying to get their mother's attention:
2621
2622 @example
2623 (script
2624 (while #t
2625 (display "mom!")
2626 (newline)
2627 (sleep 60))) ; where 60 = 1 second of real time
2628 @end example
2629
2630 This code runs in an endless loop, but the @code{sleep} procedure
2631 suspends the script and schedules it to be run later by the agenda.
2632 So, after each iteration of the loop, control is returned back to the
2633 game loop and the program is not stuck spinning in a loop that will
2634 never exit. Pretty neat, eh?
2635
2636 Scripts can suspend to any capable handler, not just the agenda.
2637 The @code{yield} procedure will suspend the current script and pass
2638 its ``continuation'' to a handler procedure. This handler procedure
2639 could do anything. Perhaps the handler stashes the continuation
2640 somewhere where it will be resumed when the user presses a specific
2641 key on the keyboard, or maybe it will be resumed when the player picks
2642 up an item off of the dungeon floor; the sky is the limit.
2643
2644 Sometimes it is necessary to abruptly terminate a script after it has
2645 been started. For example, when an enemy is defeated their AI routine
2646 needs to be shut down. When a script is spawned, a handle to that
2647 script is returned that can be used to cancel it when desired.
2648
2649 @example
2650 (define script (script (while #t (display "hey\n") (sleep 60))))
2651 ;; sometime later
2652 (cancel-script script)
2653 @end example
2654
2655 @deffn {Procedure} spawn-script @var{thunk}
2656 Apply @var{thunk} as a script and return a handle to it.
2657 @end deffn
2658
2659 @deffn {Syntax} script @var{body} @dots{}
2660 Evaluate @var{body} as a script and return a handle to it.
2661 @end deffn
2662
2663 @deffn {Procedure} script? @var{obj}
2664 Return @code{#t} if @var{obj} is a script handle.
2665 @end deffn
2666
2667 @deffn {Procedure} script-cancelled? @var{obj}
2668 Return @code{#t} if @var{obj} has been cancelled.
2669 @end deffn
2670
2671 @deffn {Procedure} script-running? @var{obj}
2672 Return @code{#t} if @var{obj} has not yet terminated or been
2673 cancelled.
2674 @end deffn
2675
2676 @deffn {Procedure} script-complete? @var{obj}
2677 Return @code{#t} if @var{obj} has terminated.
2678 @end deffn
2679
2680 @deffn {Procedure} cancel-script @var{co}
2681 Prevent further execution of the script @var{co}.
2682 @end deffn
2683
2684 @deffn {Procedure} yield @var{handler}
2685 Suspend the current script and pass its continuation to the
2686 procedure @var{handler}.
2687 @end deffn
2688
2689 @deffn {Procedure} sleep @var{duration}
2690 Wait @var{duration} before resuming the current script.
2691 @end deffn
2692
2693 @deffn {Syntax} forever @var{body} @dots{}
2694 Evaluate @var{body} in an endless loop.
2695 @end deffn
2696
2697 @node Tweening
2698 @subsection Tweening
2699
2700 Tweening is the process of transitioning something from an initial
2701 state to a final state over a pre-determined period of time. In other
2702 words, tweening is a way to create animation. The @code{tween}
2703 procedure can be used within any script like so:
2704
2705 @example
2706 (define x 0)
2707 (script
2708 ;; 0 to 100 in 60 ticks of the agenda.
2709 (tween 60 0 100 (lambda (y) (set! x y))))
2710 @end example
2711
2712 @deffn {Procedure} tween @var{duration} @var{start} @var{end} @var{proc} [#:step 1 #:ease @code{smoothstep} #:interpolate @code{lerp}]
2713 Transition a value from @var{start} to @var{end} over @var{duration},
2714 sending each succesive value to @var{proc}. @var{step} controls the
2715 amount of time between each update of the animation.
2716
2717 To control how the animation goes from the initial to final state, an
2718 ``easing'' procedure may be specified. By default, the
2719 @code{smoothstep} easing is used, which is a more pleasing default
2720 than a simplistic linear function. @xref{Easings} for a complete list
2721 of available easing procedures.
2722
2723 The @var{interpolate} procedure computes the values in between
2724 @var{start} and @var{end}. By default, linear interpolation (``lerp''
2725 for short) is used.
2726 @end deffn
2727
2728 @node Channels
2729 @subsection Channels
2730
2731 Channels are a tool for communicating amongst different scripts. One
2732 script can write a value to the channel and another can read from it.
2733 Reading or writing to a channel suspends that script until there is
2734 someone on the other end of the line to complete the transaction.
2735
2736 Here's a simplistic example:
2737
2738 @example
2739 (define c (make-channel))
2740
2741 (script
2742 (forever
2743 (let ((item (channel-get c)))
2744 (pk 'got item))))
2745
2746 (script
2747 (channel-put c 'sword)
2748 (channel-put c 'shield)
2749 (channel-put c 'potion))
2750 @end example
2751
2752 @deffn {Procedure} make-channel
2753 Return a new channel
2754 @end deffn
2755
2756 @deffn {Procedure} channel? @var{obj}
2757 Return @code{#t} if @var{obj} is a channel.
2758 @end deffn
2759
2760 @deffn {Procedure} channel-get @var{channel}
2761 Retrieve a value from @var{channel}. The current script suspends
2762 until a value is available.
2763 @end deffn
2764
2765 @deffn {Procedure} channel-put @var{channel} @var{data}
2766 Send @var{data} to @var{channel}. The current script suspends until
2767 another script is available to retrieve the value.
2768 @end deffn
2769
2770 A low-level API also exists for using channels outside of a script via
2771 callback procedures:
2772
2773 @deffn {Procedure} channel-get! @var{channel} @var{proc}
2774 Asynchronously retrieve a value from @var{channel} and call @var{proc}
2775 with that value.
2776 @end deffn
2777
2778 @deffn {Procedure} channel-put! @var{channel} @var{data} [@var{thunk}]
2779 Asynchronously send @var{data} to @var{channel} and call @var{thunk}
2780 after it has been received.
2781 @end deffn