path: root/doc/api.texi

@menu
* Kernel::                      The fundamental components.
* Math::                        Linear algebra and more.
* Graphics::                    Eye candy.
* Scripting::                   Bringing the game world to life.
@end menu

@node Kernel
@section Kernel

At the very core of Chickadee, in the @code{(chickadee)} module, lies
an event loop.  This loop, or ``kernel'', is responsible for ensuring
that the game is updated at the desired interval, rendering the
current state of the game world, and handling errors if they occur.
The kernel implements what is known as a ``fixed timestep'' game loop,
meaning that the game simulation will be advanced by a fixed interval
of time and will never vary from frame to frame, unlike some other
styles of game loops.  The appropriately named @code{run-game} and
@code{abort-game} procedures are the entry and exit points to the
Chickadee game loop kernel.

On its own, the kernel does not do very much at all.  In order to
actually respond to input events, update game state, or render output,
the programmer must provide an engine.  But don’t worry, you don’t
have to start from scratch!  Chickadee comes with a simple engine that
uses SDL to create a graphical window and handle input devices, and
OpenGL to handle rendering.  This default engine is enough for most
users to get started writing games quickly.  More advanced users may
want to write a custom engine that uses a different I/O system.
Perhaps you are writing a text adventure or roguelike that reads from
and writes to a terminal instead of a graphical window.  The game loop
kernel makes no assumptions.

@deffn {Procedure} run-game [#:update] [#:render] [#:time] [#:error] @
       [#:update-hz 60]

Start the game loop.  This procedure will not return until
@code{abort-game} is called.

The core game loop is generic and requires four additional procedures
to operate:

@itemize
@item
@var{update}: Called @var{update-hz} times per second to advance the
game simulation.  This procedure is called with a single argument: The
amount of time that has passed since the last update, in milliseconds.
@item
@var{render}: Called each iteration of the loop to render the game to
the desired output device.  This procedure is called with a single
argument: A value in the range [0, 1] which represents how much time
has past since the last game state update relative to the upcoming
game state update, as a percentage.  Because the game state is updated
independent of rendering, it is often the case that rendering is
occuring between two updates.  If the game is rendered as it was
during the last update, a strange side-effect will occur that makes
animation appear rough or ``choppy''.  To counter this, the
@var{alpha} value can be used to perfrom a linear interpolation of a
moving object between its current position and its previous position.
This odd trick has the pleasing result of making the animation look
smooth again, but requires keeping track of previous state.
@item
@var{time}: Called to get the current time in milliseconds.  This
procedure is called with no arguments.
@item
@var{error}: Called when an error from the @var{update} or
@var{render} procedures reaches the game loop.  This procedure is
called with three arguments: The call stack, the error key, and the
error arguments.  If no error handler is provided, the default
behavior is to simply re-throw the error.
@end itemize

@end deffn

@deffn {Procedure} abort-game
Stop the currently running Chickadee game loop.
@end deffn

Since most users will want to write 2D/3D games with hardware
accelerated graphics rendering, controlled via keyboard, mouse, or
game controller, Chickadee comes with an easy to use engine just for
this purpose in the @code{(chickadee sdl)} module:
@code{run-game/sdl}.

@deffn {Procedure} run-game/sdl [#:window-title "Chickadee!"] @
       [#:window-width 640] [#:window-height 480] @
       [#:window-fullscreen? @code{#f}] [#:update-hz 60] @
       [#:load] [#:update] [#:draw] [#:quit] @
       [#:key-press] [#:key-release] [#:text-input] @
       [#:mouse-press] [#:mouse-release] [#:mouse-move] @
       [#:controller-add] [#:controller-remove] [#:controller-press] @
       [#:controller-release] [#:controller-move] [#:error]

Run the Chickadee game loop using the SDL engine.

A new graphical window will be opened with @var{window-width} x
@var{window-height} as its dimensions, @var{window-title} as its
title, and in fullscreen mode if @var{window-fullscreen?} is
@code{#t}.

@itemize
@item
@var{load}: Called with zero arguments when the game window has opened
but before the game loop has started.  Can be used to perform
initialization that requires an open window and OpenGL context such as
loading textures.

@item
@var{update}: Called @var{update-hz} times per second with one
argument: The amount of time to advance the game simulation.

@item
@var{draw}: Called each time a frame should be rendered with a single
argument known as the @code{alpha} value.  See the documentation for
@code{run-game} for an explanation of this value.

@item
@var{quit}: Called with zero arguments when the user tries to close
the game window.  The default behavior is to exit the game.

@item
@var{key-press}: Called with four arguments when a key is pressed on
the keyboard:

@enumerate
@item
@var{key}: The symbolic name of the ``virtual'' key that was pressed.
For example: @code{backspace}.  It's called a virtual key because the
operating system may map a physical keyboard key to another key
entirely, such as how the author likes to bind the ``caps lock'' key
to mean ``control''.

@item
@var{scancode}: The symbolic name of the physical key that was
pressed.

@item
@var{modifiers}: A list of the symbolic names of modifier keys that
were being held down when the key was pressed.  Possible values
include @code{ctrl}, @code{alt}, and @code{shift}.

@item
@var{repeat?}: @code{#t} if this is a repeated press of the same key.

@end enumerate

@item
@var{key-release}: Called with three arguments when a key is released
on the keyboard:

@enumerate
@item
@var{key}: The symbolic name of the ``virtual'' key that was released.

@item
@var{scancode}: The symbolic name of the physical key that was
released.

@item
@var{modifiers}: A list of the symbolic names of modifier keys that
were being held down when the key was released.

@end enumerate

@item
@var{text-input}: Called with a single argument, a string of text,
when printable text is typed on the keyboard.

@item
@var{mouse-press}: Called with four arguments when a mouse button is
pressed:
@enumerate

@item
@var{button}: The symbolic name of the button that was pressed, such
as @code{left}, @code{middle}, or @code{right}.

@item
@var{clicks}: The number of times the button has been clicked in a row.

@item
@var{x}: The x coordinate of the mouse cursor.

@item
@var{y}: The y coordinate of the mouse cursor.

@end enumerate

@item
@var{mouse-release}: Called with three arguments when a mouse button
is released:

@enumerate

@item
@var{button}: The symbolic name of the button that was released.

@item
@var{x}: The x coordinate of the mouse cursor.

@item
@var{y}: The y coordinate of the mouse cursor.

@end enumerate

@item
@var{mouse-move}: Called with five arguments when the mouse is moved:

@enumerate

@item
@var{x}: The x coordinate of the mouse cursor.

@item
@var{y}: The y coordinate of the mouse cursor.

@item
@var{dx}: The amount the mouse has moved along the x axis since the
last mouse move event.

@item
@var{dy}: The amount the mouse has moved along the y axis since the
last mouse move event.

@item
@var{buttons}: A list of the buttons that were pressed down when the
mouse was moved.

@end enumerate

@item
@var{controller-add}: Called with a single argument, an SDL game
controller object, when a game controller is connected.

@item
@var{controller-remove}: Called with a single argument, an SDL game
controller object, when a game controller is disconnected.

@item
@var{controller-press}: Called with two arguments when a button on a
game controller is pressed:

@enumerate

@item
@var{controller}: The controller that triggered the event.

@item
@var{button}: The symbolic name of the button that was pressed.
Possible buttons are:

@itemize
@item
@code{a}
@item
@code{b}
@item
@code{x}
@item
@code{y}
@item
@code{back}
@item
@code{guide}
@item
@code{start}
@item
@code{left-stick}
@item
@code{right-stick}
@item
@code{left-shoulder}
@item
@code{right-shoulder}
@item
@code{dpad-up}
@item
@code{dpad-down}
@item
@code{dpad-left}
@item
@code{dpad-right}

@end itemize

@end enumerate

@item
@var{controller-release}: Called with two arguments when a button on a
game controller is released:

@enumerate

@item
@var{controller}: The controller that triggered the event.

@item
@var{button}: The symbolic name of the button that was released.

@end enumerate

@item
@var{controller-move}: Called with three arguments when an analog
stick or trigger on a game controller is moved:

@enumerate

@item
@var{controller}: The controller that triggered the event.

@item
@var{axis}: The symbolic name of the axis that was moved.  Possible
values are:

@itemize
@item
@code{left-x}
@item
@code{left-y}
@item
@code{right-x}
@item
@code{right-y}
@item
@code{trigger-left}
@item
@code{trigger-right}
@end itemize

@end enumerate

@item
@var{error}: Called with three arguments when an error occurs:

@enumerate

@item
@var{stack}: The call stack at the point of error.

@item
@var{key}: The exception key.

@item
@var{args}: The arguments thrown with the exception.

@end enumerate

The default behavior is to re-throw the error.

@end itemize

@end deffn

@node Math
@section Math

Chickadee contains data types and procedures for performing the most
common computations in video game simulations such as linear algebra
with vectors and matrices and axis-aligned bounding box collision
detection.

@menu
* Basics::                      Commonly used, miscellaneous things.
* Vectors::                     Euclidean vectors.
* Matrices::                    Transformation matrices.
* Quaternions::                 Rotations.
* Rectangles::                  Axis-aligned bounding boxes.
* Easings::                     Easing functions for interesting animations.
@end menu

@node Basics
@subsection Basics

@defvar pi
An essential constant for all trigonometry.  @code{@U{03C0}} is the ratio
of a circle's circumferences to its diameter.  Since @code{@U{03C0}} is an
irrational number, the @var{pi} in Chickadee is a mere floating point
approximation that is ``good enough.''
@end defvar

@defvar pi/2
Half of @var{pi}.
@end defvar

@deffn {Procedure} cotan @var{z}
Return the cotangent of @var{z}.
@end deffn

@node Vectors
@subsection Vectors

@node Matrices
@subsection Matrices

@node Quaternions
@subsection Quaternions

In game development, the quaternion is most often used to represent
rotations.  Why not use a matrix for that, you may ask.  Unlike
matrices, quaternions can be interpolated (animated) and produce a
meaningful result.  When interpolating two quaternions, there is a
smooth transition from one rotation to another, whereas interpolating
two matrices would yield garbage.

@deffn {Procedure} quaternion @var{x} @var{y} @var{z} @var{w}
Return a new quaternion with values @var{x}, @var{y}, @var{z}, and
@var{w}.
@end deffn

@deffn {Procedure} quaternion? @var{obj}
Return @code{#t} if @var{obj} is a quaternion.
@end deffn

@deffn {Procedure} quaternion-w @var{q}
Return the W component of the quaternion @var{q}.
@end deffn

@deffn {Procedure} quaternion-x @var{q}
Return the X component of the quaternion @var{q}.
@end deffn

@deffn {Procedure} quaternion-y @var{q}
Return the Y component of the quaternion @var{q}.
@end deffn

@deffn {Procedure} quaternion-z @var{q}
Return the Z component of the quaternion @var{q}.
@end deffn

@deffn {Procedure} make-identity-quaternion
Return the identity quaternion.
@end deffn

@node Rectangles
@subsection Rectangles

@node Easings
@subsection Easings

@deffn {Procedure} linear @var{t}
@end deffn

@deffn {Procedure} smoothstep @var{t}
@end deffn

@deffn {Procedure} ease-in-quad @var{t}
@end deffn

@deffn {Procedure} ease-out-quad @var{t}
@end deffn

@deffn {Procedure} ease-in-out-quad @var{t}
@end deffn

@deffn {Procedure} ease-in-cubic @var{t}
@end deffn

@deffn {Procedure} ease-out-cubic @var{t}
@end deffn

@deffn {Procedure} ease-in-out-cubic @var{t}
@end deffn

@deffn {Procedure} ease-in-quart @var{t}
@end deffn

@deffn {Procedure} ease-out-quart @var{t}
@end deffn

@deffn {Procedure} ease-in-out-quart @var{t}
@end deffn

@deffn {Procedure} ease-in-quint @var{t}
@end deffn

@deffn {Procedure} ease-out-quint @var{t}
@end deffn

@deffn {Procedure} ease-in-out-quint @var{t}
@end deffn

@deffn {Procedure} ease-in-sine @var{t}
@end deffn

@deffn {Procedure} ease-out-sine @var{t}
@end deffn

@deffn {Procedure} ease-in-out-sine @var{t}
@end deffn

@node Graphics
@section Graphics

Chickadee aims to make hardware-accelerated graphics rendering as
simple and efficient as possible by providing high-level APIs that
interact with the low-level OpenGL API under the hood.  Anyone that
has worked with OpenGL directly knows that it has a steep learning
curve and a lot of effort is needed to render even a single triangle.
The Chickadee rendering engine attempts to make it easy to do common
tasks like rendering a sprite while also providing all of the building
blocks to implement additional rendering techniques.

@menu
* Rendering Engine::            Rendering state management.
* Textures::                    2D images.
* Sprites::                     Draw 2D images.
* Tile Maps::                   Draw 2D tile maps.
* Lines and Shapes::            Draw line segments and polygons.
* Fonts::                       Drawing text.
* Blending and Depth Testing::  Control how pixels are combined.
* Vertex Arrays::               Create 2D/3D models.
* Shaders::                     Create custom GPU programs.
* Framebuffers::                Render to texture.
* Viewports::                   Restrict rendering to
@end menu

@node Rendering Engine
@subsection Rendering Engine

Chickadee defines rendering using a metaphor familiar to Scheme
programmers: procedure application.  A shader (@pxref{Shaders}) is
like a procedure for the GPU to apply.  Shaders are passed arguments:
A vertex array containing the geometry to render (@pxref{Vertex
Arrays}) and zero or more keyword arguments that the shader
understands.  Similar to how Scheme has @code{apply} for calling
procedures, Chickadee provides @code{gpu-apply} for calling shaders.

Additionally, there is some dynamic state that effects how
@code{gpu-apply} will behave.  Things like the current viewport,
framebuffer, and blend mode are stored as dynamic state because it
would be tedious to have to have to specify them each time
@code{gpu-apply} is called.

The following procedures and syntax can be found in the
@code{(chickadee render)} module.

@deffn {Syntax} gpu-apply @var{shader} @var{vertex-array} @
       [#:uniform-key @var{uniform-value} ...]
@deffnx {Syntax} gpu-apply* @var{shader} @var{vertex-array} @
        @var{count} [#:uniform-key @var{uniform-value} ...]

Render @var{vertex-array} using @var{shader} with the uniform values
specified in the following keyword arguments.

While @code{gpu-apply} will draw every vertex in @var{vertex-array},
@code{gpu-apply*} will only draw @var{count} vertices.

@end deffn

@deffn {Procedure} current-viewport
Return the currently bound viewport (@pxref{Viewports}).
@end deffn

@deffn {Procedure} current-framebuffer
Return the currently bound framebuffer (@pxref{Framebuffers}).
@end deffn

@deffn {Procedure} current-blend-mode
Return the currently bound blend mode (@pxref{Blending and Depth
Testing}).
@end deffn

@deffn {Procedure} current-depth-test
Return @code{#t} if depth testing is currently enabled (@pxref{Blending and Depth Testing}).
@end deffn

@deffn {Procedure} current-texture
Return the currently bound texture (@pxref{Textures}).
@end deffn

@deffn {Procedure} current-projection
Return the currently bound projection matrix (@pxref{Matrices}).
@end deffn

@deffn {Syntax} with-viewport @var{viewport} @var{body} ...
Evaluate @var{body} with the current viewport bound to @var{viewport} (@pxref{Viewports}).
@end deffn

@deffn {Syntax} with-framebuffer @var{framebuffer} @var{body} ...
Evaluate @var{body} with the current framebuffer bound to
@var{framebuffer} (@pxref{Framebuffers}).
@end deffn

@deffn {Syntax} with-blend-mode @var{blend-mode} @var{body} ...
Evaluate @var{body} with the current blend mode bound to
@var{blend-mode} (@pxref{Blending and Depth Testing}).
@end deffn

@deffn {Syntax} with-depth-test @var{depth-test?} @var{body} ...
Evaluate @var{body} with the depth-test disabled if @var{depth-test?}
is @code{#f}, or enabled otherwise (@pxref{Blending and Depth
Testing}).
@end deffn

@deffn {Syntax} with-texture @var{texture} @var{body} ...
Evaluate @var{body} with the current texture bound to @var{texture}
(@pxref{Textures}).
@end deffn

@deffn {Syntax} with-projection @var{projection} @var{body} ...
Evaluate @var{body} with the current projection matrix bound to
@var{projection} (@pxref{Matrices}).
@end deffn

@node Textures
@subsection Textures

@deffn {Procedure} load-image @var{file} [#:min-filter nearest] @
       [#:mag-filter nearest] [#:wrap-s repeat] [#:wrap-t repeat]

Load the image data from @var{file} and return a new texture object.

@var{min-filter} and @var{mag-filter} describe the method that should
be used for minification and magnification when rendering,
respectively.  Possible values are @code{nearest} and @code{linear}.

@var{wrap-s} and @var{wrap-t} describe how to interpret texture
coordinates that are greater than @code{1.0}.  Possible values are
@code{repeat}, @code{clamp}, @code{clamp-to-border}, and
@code{clamp-to-edge}.

@end deffn

@node Sprites
@subsection Sprites

For those who are new to this game, a sprite is a 2D rectangular
bitmap that is rendered to the screen.  For 2D games, sprites are the
most essential graphical abstraction.  They are used for drawing maps,
players, NPCs, items, particles, text, etc.  In Chickadee, bitmaps are
stored in textures (@pxref{Textures}) and can be used to draw sprites
via the @code{draw-sprite} procedure.

@deffn {Procedure} draw-sprite @var{texture} @var{position} @
       [#:origin] [#:scale] [#:rotation] [#:blend-mode alpha] @
       [#:rect] [#:shader]

Draw @var{texture} at @var{position}.

Optionally, other transformations may be applied to the sprite.
@var{rotation} specifies the angle to rotate the sprite, in radians.
@var{scale} specifies the scaling factor as a 2D vector.  All
transformations are applied relative to @var{origin}, a 2D vector,
which defaults to the lower-left corner.

Alpha blending is used by default but the blending method can be
changed by specifying @var{blend-mode}.

The area drawn to is as big as the texture, by default.  To draw to an
arbitrary section of the screen, specify @var{rect}.

Finally, advanced users may specify @var{shader} to change the way the
sprite is rendered entirely.
@end deffn

It's not uncommon to need to draw hundreds or thousands of sprites
each frame.  However, GPUs (graphics processing units) are tricky
beasts that prefer to be sent few, large chunks of data to render
rather than many, small chunks.  Using @code{draw-sprite} on its own
will involve at least one GPU call @emph{per sprite}, which will
quickly lead to poor performance.  To deal with this, a technique
known as ``sprite batching'' can be used.  Instead of drawing each
sprite immediately, the sprite batch will build up a large of buffer
of sprites to draw and defer rendering until the last possible moment.
Batching isn't a panacea, though.  Batching only works if the sprites
being drawn share as much in common as possible.  Every time you draw
a sprite with a different texture or blend mode, the batch will be
sent off to the GPU.  Therefore, batching is most useful if you
minimize such changes.  A good strategy for reducing texture changes
is to stuff many bitmaps into a single image file and create a
``texture atlas'' (@pxref{Textures}) to access the sub-images within.

Taking advantage of sprite batching in Chickadee is easy, just wrap
the code that is calling @code{draw-sprite} a lot in the
@code{with-batched-sprites} form.

@deffn {Syntax} with-batched-sprites @var{body} @dots{}
Use batched rendering for all @code{draw-sprite} calls within
@var{body}.
@end deffn

With a basic sprite abstraction in place, it's possible to build other
abstractions on top of it.  One such example is the ``nine patch''.  A
nine patch is a sprite that can be rendered at various sizes without
becoming distorted.  This is achieved by diving up the sprite into
nine regions:

@itemize
@item
the center, which can be scaled horizontally and vertically
@item
the four corners, which can never be scaled
@item
the left and right sides, which can be scaled vertically
@item
the top and bottom sides, which can be scaled horizontally
@end itemize

The one caveat is that the bitmap regions must be designed in such a
way so that they are not distorted when stretched along the affected
axes.  For example, that means that the top and bottom sides could
have varying colored pixels vertically, but not horizontally.

The most common application of this technique is for graphical user
interface widgets like buttons and dialog boxes.  By using a nine
patch, they can be rendered at any size without unappealing scaling
artifacts.

@deffn {Procedure} draw-nine-patch @var{texture} @var{rect} @
       [#:margin 0] [#:top-margin margin] [#:bottom-margin margin] @
       [#:left-margin margin] [#:right-margin margin] @
       [#:origin] [#:scale] [#:rotation] [#:blend-mode alpha] @
       [#:shader]

Draw a nine patch sprite.  A nine patch sprite renders @var{texture}
as a @var{width} x @var{height} rectangle whose stretchable areas are
defined by the given margin measurements @var{top-margin},
@var{bottom-margin}, @var{left-margin}, and @var{right-margin}. The
@var{margin} argument may be used to configure all four margins at
once.

Refer to @code{draw-sprite} (@pxref{Sprites}) for information about
the other arguments.
@end deffn

@node Tile Maps
@subsection Tile Maps

A tile map is a scene created by composing lots of small sprites,
called ``tiles'', into a larger image.  One program for editing such
maps is called @url{http://mapeditor.org,Tiled}.  Chickadee has native
support for loading and rendering Tiled maps in the @code{(chickadee
render tiled)} module.

@deffn {Procedure} load-tile-map @var{file-name}
Load the Tiled formatted map in @var{file-name} and return a new tile
map object.
@end deffn

@deffn {Procedure} draw-tile-map @var{tile-map} [#:layers] [#:region] @
       [#:origin] [#:position] [#:scale] [#:rotation]

Draw the layers of @var{tile-map}.  By default, all layers are drawn.
To draw a subset of the available layers, pass a list of layer ids
using the @var{layers} keyword argument.

Refer to @code{draw-sprite} (@pxref{Sprites}) for information about
the other arguments.
@end deffn

@node Lines and Shapes
@subsection Lines and Shapes

Sprites are fun, but sometimes simple, untextured lines and polygons
are desired.  That's where the @code{(chickadee render shapes)} module
comes in!

@deffn {Procedure} draw-line @var{start} @var{end} @
       [#:thickness 0.5] [#:feather 1.0] [#:cap round] [#:color] @
       [#:shader]

Draw a line segment from @var{start} to @var{end}. The line will be
@var{thickness} pixels thick with an antialiased border @var{feather}
pixels wide.  The line will be colored @var{color}. @var{cap}
specifies the type of end cap that should be used to terminate the
lines, either @code{none}, @code{butt}, @code{square}, @code{round},
@code{triangle-in}, or @code{triangle-out}.  Advanced users may use
the @var{shader} argument to override the built-in line segment
shader.
@end deffn

@node Fonts
@subsection Fonts

Unlike the traditional TrueType font format that many are accustomed
to, Chickadee loads and renders bitmap fonts in the
@url{http://www.angelcode.com/products/bmfont/doc/file_format.html,
Angel Code format}.  But why use this seemingly obscure format?  It's
easy to find TTFs but not easy to find FNTs (the canonical file
extension used for Angel Code fonts) and bitmap fonts don't scale
well.  The reason is efficiency.

If all of the glyphs of a font are pre-rendered and packed into an
image file then it becomes possible to use a texture atlas
(@pxref{Textures}) and a sprite batch (@pxref{Sprites}) when
rendering, which is a more efficient way to render fonts than using,
say, @url{https://www.libsdl.org/projects/SDL_ttf/, SDL_ttf} or other
solutions that involve using the FreeType library directly.

Now what about scaling?  In libraries that use TTF fonts, one must
choose the size that the glyphs will be rasterized at up front.  To
use @code{n} sizes of the same font, one must load @code{n} variants
of that font.  If the size of the text is dynamic, some kind of
texture scaling algorithm must be used and the text will inevitably
look blurry.  At first glance, using bitmap fonts seem to have an even
worse issue.  Instead of just loading the same font @code{n} times at
different sizes, one would need to generate @code{n} image files for
each font size needed.  This is where the ``signed distance field''
rendering technique comes in.  Introduced by
@url{http://www.valvesoftware.com/.../2007/SIGGRAPH2007_AlphaTestedMagnification.pdf,
Valve} in 2007, signed distance field fonts can be efficiently stored
in a bitmap and be rendered at arbitrary scale factors with good
results.

While Chickadee does not yet offer a tool for converting TTF fonts
into FNT fonts, tools such as
@url{https://github.com/libgdx/libgdx/wiki/Hiero, Hiero} may be used
in the meantime.

The following procedures can be found in the @code{(chickadee render
font)} module.

@deffn {Procedure} load-font @var{file}
Load the Angel Code formatted XML document in @var{file} and return a
new font object.
@end deffn

@deffn {Procedure} font? @var{obj}
Return @code{#t} if @var{obj} is a font object.
@end deffn

@deffn {Procedure} font-face @var{font}
Return the name of @var{font}.
@end deffn

@deffn {Procedure} font-line-height @var{font}
Return the line height of @var{font}.
@end deffn

@deffn {Procedure} font-line-height @var{font}
Return the line height of @var{font}.
@end deffn

@deffn {Procedure} font-bold? @var{font}
Return @code{#t} if @var{font} is a bold font.
@end deffn

@deffn {Procedure} font-italic? @var{font}
Return @code{#t} if @var{font} is an italicized font.
@end deffn

@deffn {Procedure} draw-text @var{font} @var{text} @var{position}
       [#:origin] [#:scale] [#:rotation] [#:blend-mode]
       [#:start 0] [#:end @code{(string-length text)}]

Draw the string @var{text} with the first character starting at
@var{position} using @var{font}.

@example
(draw-text font "Hello, world!" (vec2 128.0 128.0))
@end example

To render a substring of @var{text}, use the @var{start} and @var{end}
arguments.

Refer to @code{draw-sprite} (@pxref{Sprites}) for information about
the other arguments.
@end deffn

@node Blending and Depth Testing
@subsection Blending and Depth Testing

@node Vertex Arrays
@subsection Vertex Arrays

@node Shaders
@subsection Shaders

Shaders are programs for the GPU to evaluate.  They are written in the
OpenGL Shading Language, or GLSL.  Chickadee does not currently
provide a Scheme-like domain specific language for writing shaders.
Since shaders must be written in GLSL and not Scheme, they are
considered an advanced feature.

@node Framebuffers
@subsection Framebuffers

A framebuffer is a chunk of memory that the GPU can render things
onto.  By default, the framebuffer that is used for rendering is the
one belonging to the game window, but custom framebuffers can be used
as well.  A common use-case for custom framebuffers is applying
post-processing effects: The entire scene is rendered to a
framebuffer, and then the contents of that framebuffer are applied to
a post-processing shader and rendered to the game window.  The
post-processing shader could do any number of things: scaling,
antialiasing, motion blur, etc.

@deffn {Procedure} make-framebuffer @var{width} @var{height} [#:min-filter 'linear] [#:mag-filter 'linear] [#:wrap-s 'repeat] [#:wrap-t 'repeat]

Create a new framebuffer that is @var{width} pixels wide and @var{height} pixels high.

@var{min-filter} and @var{mag-filter} determine the scaling algorithm
applied to the framebuffer when rendering.  By default, linear scaling
is used in both cases. To perform no smoothing at all, use
@code{nearest} for simple nearest neighbor scaling.  This is typically
the best choice for pixel art games.
@end deffn

@deffn {Procedure} framebuffer? @var{obj}
Return @code{#t} if @var{obj} is a framebuffer.
@end deffn

@deffn {Procedure} framebuffer-texture @var{fb}
Return the texture backing the framebuffer @var{fb}.
@end deffn

@deffn {Procedure} framebuffer-viewport @var{fb}
Return the default viewport (@pxref{Viewports}) used by the
framebuffer @var{fb}.
@end deffn

@deffn {Procedure} null-framebuffer
The default framebuffer.
@end deffn

@node Viewports
@subsection Viewports

@node Scripting
@section Scripting

Game logic is a web of asynchronous events that are carefully
coordinated to bring the game world to life.  In order to make an
enemy follow and attack the player, or move an NPC back and forth in
front of the item shop, or do both at the same time, a scripting
system is a necessity.  Chickadee comes with an asynchronous
programming system in the @code{(chickadee scripting)} module.
Lightweight, cooperative threads known as ``scripts'' allow the
programmer to write asynchronous code as if it were synchronous, and
allow many such ``threads'' to run concurrently.

But before we dig deeper into scripts, let's discuss the simple act
of scheduling tasks.

@menu
* Agendas::                     Scheduling tasks.
* Scripts::                     Cooperative multitasking.
* Tweening::                    Animations.
* Channels::                    Publish data to listeners.
@end menu

@node Agendas
@subsection Agendas

To schedule a task to be performed later, an ``agenda'' is used.
There is a default, global agenda that is ready to be used, or
additional agendas may be created for different purposes.  The
following example prints the text ``hello'' when the agenda has
advanced to time unit 10.

@example
(at 10 (display "hello\n"))
@end example

Most of the time it is more convenient to schedule tasks relative to
the current time.  This is where @code{after} comes in handy:

@example
(after 10 (display "hello\n"))
@end example

Time units in the agenda are in no way connected to real time.  It's
up to the programmer to decide what agenda time means.  A simple and
effective approach is to map each call of the update hook
(@pxref{Kernel}) to 1 unit of agenda time, like so:

@example
(add-hook! update-hook (lambda (dt) (update-agenda 1)))
@end example

It is important to call @code{update-agenda} periodically, otherwise
no tasks will ever be run!

In addition to using the global agenda, it is useful to have multiple
agendas for different purposes.  For example, the game world can use a
different agenda than the user interface, so that pausing the game is
a simple matter of not updating the world's agenda while continuing to
update the user interface's agenda.  The current agenda is dynamically
scoped and can be changed using the @code{with-agenda} special form:

@example
(define game-world-agenda (make-agenda))

(with-agenda game-world-agenda
  (at 60 (spawn-goblin))
  (at 120 (spawn-goblin))
  (at 240 (spawn-goblin-king)))
@end example

@deffn {Procedure} make-agenda
Return a new task scheduler.
@end deffn

@deffn {Procedure} agenda? @var{obj}
Return @code{#t} if @var{obj} is an agenda.
@end deffn

@deffn {Procedure} current-agenda
@deffnx {Procedure} current-agenda @var{agenda}
When called with no arguments, return the current agenda.  When called
with one argument, set the current agenda to @var{agenda}.
@end deffn

@deffn {Syntax} with-agenda @var{agenda} @var{body} @dots{}
Evaluate @var{body} with the current agenda set to @var{agenda}.
@end deffn

@deffn {Procedure} agenda-time
Return the current agenda time.
@end deffn

@deffn {Procedure} update-agenda @var{dt}
Advance the current agenda by @var{dt}.
@end deffn

@deffn {Procedure} schedule-at @var{time} @var{thunk}
Schedule @var{thunk}, a procedure of zero arguments, to be run at
@var{time}.
@end deffn

@deffn {Procedure} schedule-after @var{delay} @var{thunk}
Schedule @var{thunk}, a procedure of zero arguments, to be run after
@var{delay}.
@end deffn

@deffn {Procedure} schedule-every @var{interval} @var{thunk} [@var{n}]
Schedule @var{thunk}, a procedure of zero arguments, to be run every
@var{interval} amount of time.  Repeat this @var{n} times, or
indefinitely if not specified.
@end deffn

@deffn {Syntax} at @var{time} @var{body} @dots{}
Schedule @var{body} to be evaluated at @var{time}.
@end deffn

@deffn {Syntax} after @var{delay} @var{body} @dots{}
Schedule @var{body} to be evaluated after @var{delay}.
@end deffn

@deffn {Syntax} every @var{interval} @var{body} @dots{}
@deffnx {Syntax} every (@var{interval} @var{n}) @var{body} @dots{}
Schedule @var{body} to be evaluated every @var{interval} amount of
time.  Repeat this @var{n} times, or indefinitely if not specified.
@end deffn

@node Scripts
@subsection Scripts

Now that we can schedule tasks, let's take things to the next level.
It sure would be great if we could make procedures that described a
series of actions that happened over time, especially if we could do
so without contorting our code into a nest of callback procedures.
This is where scripts come in.  With scripts we can write code in a
linear way, in a manner that appears to be synchronous, but with the
ability to suspend periodically in order to let other scripts have a
turn and prevent blocking the game loop.  Building on top of the
scheduling that agendas provide, here is a script that models a child
trying to get their mother's attention:

@example
(script
  (while #t
    (display "mom!")
    (newline)
    (sleep 60))) ; where 60 = 1 second of real time
@end example

This code runs in an endless loop, but the @code{sleep} procedure
suspends the script and schedules it to be run later by the agenda.
So, after each iteration of the loop, control is returned back to the
game loop and the program is not stuck spinning in a loop that will
never exit.  Pretty neat, eh?

Scripts can suspend to any capable handler, not just the agenda.
The @code{yield} procedure will suspend the current script and pass
its ``continuation'' to a handler procedure.  This handler procedure
could do anything.  Perhaps the handler stashes the continuation
somewhere where it will be resumed when the user presses a specific
key on the keyboard, or maybe it will be resumed when the player picks
up an item off of the dungeon floor; the sky is the limit.

Sometimes it is necessary to abruptly terminate a script after it has
been started.  For example, when an enemy is defeated their AI routine
needs to be shut down.  When a script is spawned, a handle to that
script is returned that can be used to cancel it when desired.

@example
(define script (script (while #t (display "hey\n") (sleep 60))))
;; sometime later
(cancel-script script)
@end example

@deffn {Procedure} spawn-script @var{thunk}
Apply @var{thunk} as a script and return a handle to it.
@end deffn

@deffn {Syntax} script @var{body} @dots{}
Evaluate @var{body} as a script and return a handle to it.
@end deffn

@deffn {Procedure} script? @var{obj}
Return @code{#t} if @var{obj} is a script handle.
@end deffn

@deffn {Procedure} script-cancelled? @var{obj}
Return @code{#t} if @var{obj} has been cancelled.
@end deffn

@deffn {Procedure} script-running? @var{obj}
Return @code{#t} if @var{obj} has not yet terminated or been
cancelled.
@end deffn

@deffn {Procedure} script-complete? @var{obj}
Return @code{#t} if @var{obj} has terminated.
@end deffn

@deffn {Procedure} cancel-script @var{co}
Prevent further execution of the script @var{co}.
@end deffn

@deffn {Procedure} yield @var{handler}
Suspend the current script and pass its continuation to the
procedure @var{handler}.
@end deffn

@deffn {Procedure} sleep @var{duration}
Wait @var{duration} before resuming the current script.
@end deffn

@deffn {Procedure} channel-get @var{channel}
Wait for a message from @var{channel}.
@end deffn

@deffn {Syntax} forever @var{body} @dots{}
Evaluate @var{body} in an endless loop.
@end deffn

@node Tweening
@subsection Tweening

Tweening is the process of transitioning something from an initial
state to a final state over a pre-determined period of time.  In other
words, tweening is a way to create animation.  The @code{tween}
procedure can be used within any script like so:

@example
(define x 0)
(script
  ;; 0 to 100 in 60 ticks of the agenda.
  (tween 60 0 100 (lambda (y) (set! x y))))
@end example

@deffn {Procedure} tween @var{duration} @var{start} @var{end} @var{proc} [#:step 1 #:ease @code{smoothstep} #:interpolate @code{lerp}]
Transition a value from @var{start} to @var{end} over @var{duration},
sending each succesive value to @var{proc}.  @var{step} controls the
amount of time between each update of the animation.

To control how the animation goes from the initial to final state, an
``easing'' procedure may be specified.  By default, the
@code{smoothstep} easing is used, which is a more pleasing default
than a simplistic linear function.  @xref{Easings} for a complete list
of available easing procedures.

The @var{interpolate} procedure computes the values in between
@var{start} and @var{end}.  By default, linear interpolation (``lerp''
for short) is used.
@end deffn

@node Channels
@subsection Channels

Channels are a tool for communicating amongst different scripts.  One
script can write a value to the channel and another can read from it.
Reading or writing to a channel suspends that script until there is
someone on the other end of the line to complete the transaction.

Here's a simplistic example:

@example
(define c (make-channel))

(script
 (forever
  (let ((item (channel-get c)))
    (pk 'got item))))

(script
 (channel-put c 'sword)
 (channel-put c 'shield)
 (channel-put c 'potion))
@end example

@deffn {Procedure} make-channel
Return a new channel
@end deffn

@deffn {Procedure} channel? @var{obj}
Return @code{#t} if @var{obj} is a channel.
@end deffn

@deffn {Procedure} channel-get @var{channel}
Retrieve a value from @var{channel}.  The current script suspends
until a value is available.
@end deffn

@deffn {Procedure} channel-put @var{channel} @var{data}
Send @var{data} to @var{channel}.  The current script suspends until
another script is available to retrieve the value.
@end deffn