@menu
* Kernel::                      The fundamental components.
* Input::                       Keyboard, mouse, and controller input.
* Math::                        Linear algebra and more.
* Graphics::                    Eye candy.
* Audio::                       Sound effects and music.
* Buffers::                     Splitting games into logical components.
* 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 creating
and managing the game window, dispatching input events, ensuring that
the game is updated at the desired interval, and rendering graphics.
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 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 draw something
to the game window, a hacker with a penchant for game development must
latch onto extension points built into the kernel, called ``hooks'',
and specify what action ought to be taken for any given event.  For
example, the @code{key-press-hook} can be used to respond to the
@code{a} key being pressed by swinging the player's mighty sword.
There are many hooks available, so read on to learn about all of them.
For information about using Guile's hook API, see @xref{Hooks,,,
guile, GNU Guile Reference Manual}.

@deffn {Procedure} run-game [#:window-title "Chickadee!"] @
       [#:window-width 640] [#:window-height 480] [#:window-fullscreen? #f] @
       [#:update-hz 60]

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

The @code{update-hook} will be run @var{update-hz} times per second.

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}.
@end deffn

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

@deffn {Procedure} time
Return the current game time in milliseconds.
@end deffn

@defvr {Variable} load-hook
A hook that is run once when the event loop boots, before any other
hook is run.  This hook is run with zero arguments.

@example
(add-hook! load-hook (lambda () (display "hello!\n")))
@end example

@end defvr

@defvr {Variable} update-hook
A hook that is run every time the game simulation should be advanced.
This hook is run with a single argument @var{dt}, the fixed timestep
that was configured when the event loop was started, in milliseconds.

@example
(add-hook! update-hook (lambda (dt) (display "tick!\n")))
@end example

@end defvr

@defvr {Variable} before-draw-hook
A hook that is run before a frame is rendered.  This hook is run with
zero arguments.

@example
(add-hook! before-draw-hook (lambda () (display "about to draw!\n")))
@end example

@end defvr

@defvr {Variable} after-draw-hook
A hook that is run after a frame is rendered.  This hook is run with
zero arguments.

@example
(add-hook! after-draw-hook (lambda () (display "done drawing!\n")))
@end example

Combined with @code{before-draw-hook}, one can perform a frames per
second calculation to monitor game performance and stability.

@end defvr

@defvr {Variable} draw-hook
A hook that is run each time a frame should be rendered.  This hook is
run with a single argument @var{alpha}, 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.

@c TODO: Add example of linear interpolation

@example
(add-hook! draw-hook (lambda (alpha) (display "<(._.<) \n")))
@end example

@end defvr

@defvr {Variable} quit-hook
A hook that is run when the user clicks the close button on the game
window.  This hook is run with zero arguments.

@example
(add-hook! quit-hook (lambda () (display "bye!\n")))
@end example

@end defvr

@defvr {Variable} key-press-hook
A hook that is run when a key is pressed on the keyboard.  This hook
is run with four arguments:

@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 binds 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

@example
(add-hook! key-press-hook
           (lambda (key scancode modifiers repeat?)
             (display "pressed key: ")
             (display key)
             (newline)))
@end example

@end defvr

@defvr {Variable} key-release-hook
A hook that is run when a key is released on the keyboard.  This hook
is run with three arguments:

@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

@end defvr

@defvr {Variable} text-input-hook
A hook that is run when printable text is typed on the keyboard.  This
hook is run with a single argument, @var{text}, a string containing
the text that was entered.
@end defvr

@defvr {Variable} mouse-press-hook
A hook that is run when a mouse button is pressed.  This hook is run
with four arguments:

@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

@end defvr

@defvr {Variable} mouse-release-hook
A hook that is run when a mouse button is released.  This hook is run
with three arguments:

@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

@end defvr

@defvr {Variable} mouse-move-hook
A hook that is run when the mouse is moved.  This hook is run with
five arguments:

@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

@end defvr

@defvr {Variable} controller-add-hook
A hook that is run when a game controller is connected.  This hook is
run with a single argument, @var{controller}, the controller that was
connected.
@end defvr

@defvr {Variable} controller-remove-hook
A hook that is run when a game controller is disconnected.  This hook
is run with a single argument, @var{controller}, the controller that
was disconnected.
@end defvr

@defvr {Variable} controller-press-hook
A hook that is run when a button on a game controller is pressed.
This hook is run with two arguments:

@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

@end defvr

@defvr {Variable} controller-release-hook
A hook that is run when a button on a game controller is released.

This hook is run with two arguments:

@enumerate

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

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

@end enumerate

@end defvr

@defvr {Variable} controller-move-hook
A hook that is run when an analog stick or trigger on a game
controller is moved.  This hook is run with three arguments

@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

@end defvr

@node Input
@section Input

Chickadee can handle input events from the keyboard, mouse, and game
controllers.

@menu
* Keyboard::                    Keyboard input.
@end menu

@node Keyboard
@subsection Keyboard

@deffn {Procedure} key-pressed? @var{key}
Return @code{#t} if @var{key} is currently being pressed.
@end deffn

@deffn {Procedure} key-released? @var{key}
Return @code{#t} if @var{key} is not currently being pressed.
@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.  @xref{Viewports} for more
details about using viewports.
@end deffn

@deffn {Procedure} current-framebuffer
Return the currently bound framebuffer.  @xref{Framebuffers} for more
details about using framebuffers.
@end deffn

@deffn {Procedure} current-blend-mode
Return the currently bound blend mode.  @xref{Blending and Depth
Testing} for more details about using blend modes.
@end deffn

@deffn {Procedure} current-depth-test
Return @code{#t} if depth testing is currently enabled.
@xref{Blending and Depth Testing} for more details about using the
depth test.
@end deffn

@deffn {Procedure} current-texture
Return the currently bound texture.  @xref{Textures} for more details
about using textures.
@end deffn

@deffn {Procedure} current-projection
Return the currently bound projection matrix.  @xref{Matrices} for
more details about matrices.
@end deffn

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

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

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

@deffn {Syntax} with-projection @var{projection} @var{body} ...
Evaluate @var{body} with the current projection matrix bound to
@var{projection}.
@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

@node Viewports
@subsection Viewports

@node Audio
@section Audio

Chickadee has two data types for audio: samples and music.  Samples
are for short sound effects like explosions.  Music is for, well,
uh@dots{}, music.

Supported file formats include WAV and OGG.

@deffn {Procedure} load-sample @var{file}
Load audio sample from @var{file}.
@end deffn

@deffn {Procedure} set-sample-volume! @var{volume}
Set the volume that all samples are played at to @var{volume}, an
integer value between 0 and 128.
@end deffn

@deffn {Procedure} play-sample @var{sample}
Play @var{sample}.  Pretty straightforward!
@end deffn

@deffn {Procedure} load-music @var{file}
Load music from @var{file}.
@end deffn

@deffn {Procedure} music-volume
Return the volume level for music, an integer value between 0 and 128.
@end deffn

@deffn {Procedure} set-music-volume! @var{volume}
Set the volume that music is played at to @var{volume}, an integer
value between 0 and 128.
@end deffn

@deffn {Procedure} play-music @var{music} [@var{loop?}]
Play @var{music}.  If @var{loop?}, play it over and over and over and
over and@dots{}
@end deffn

@deffn {Procedure} pause-music
Pause the current music track.
@end deffn

@deffn {Procedure} resume-music
Resume the current music track.
@end deffn

@deffn {Procedure} rewind-music
estart the current music track from the beginning.
@end deffn

@deffn {Procedure} stop-music
Stop playing the current music track.
@end deffn

@deffn {Procedure} music-playing?
Return @code{#t} if music is currently playing.
@end deffn

@deffn {Procedure} music-paused?
Return @code{#t} if music is currently paused.
@end deffn

@node Buffers
@section Buffers

Games are big state machines and they can get complex quickly, thus we
need tools to manage that complexity.  One useful technique is to
separate the many ``screens'' of a game (such as the main menu, player
select screen, high score table, game over screen, etc.) from each
other, so that it is possible to program the logic for one section of
the game independent of another.  In Chickadee, these ``screens'' are
called ``buffers''.  In other game engines this same construct may be
called a ``scene'' or ``room''.

A buffer knows how to update its state, render, handle input events,
etc.  Only one buffer is ever active at a time, and the current buffer
can be changed with procedures like @code{push-buffer},
@code{replace-buffer}, and @code{pop-buffer}.

Buffers are implemented using Guile's object oriented programming
system (@pxref{GOOPS,,, guile, GNU Guile Reference Manual}).  Each
type of buffer is represented as a subclass of the @code{<buffer>}
class.  Once a new buffer class has been created, there are many
methods that can be specialized for that class.

To install buffer support and set the initial buffer, call the
@code{use-buffers!} procedure.

Let's take a look at a simple example to see how it all comes
together:

@example
(use-modules (chickadee)
             (chickadee buffer)
             (chickadee math vector)
             (chickadee render sprite)
             (chickadee render texture)
             (oop goops))

;; Define a new buffer type.
(define-class <splash-screen> (<buffer>)
  (chickadee #:accessor chickadee #:init-value #f))

;; Define the logic for when the buffer is first activated.
(define-method (start (splash <splash-screen>))
  (set! (chickadee splash) (load-image "images/chickadee.png")))

;; Define how the buffer should be rendered.
(define-method (draw (splash <splash-screen>) alpha)
  (draw-sprite (chickadee splash) (vec2 256.0 176.0)))

;; Hook into the game engine and make the splash screen the initial buffer.
(use-buffers! (make <splash-screen>))

;; Start the game!
(run-game)
@end example

@deffn {Class} <buffer>
The parent class for all buffers.
@end deffn

@deffn {Method} started? @var{buffer}
Return @code{#t} if @var{buffer} has been activated by the game engine.
@end deffn

@deffn {Method} start @var{buffer}
Called when @var{buffer} becomes the current buffer for the first time.
@end deffn

@deffn {Method} stop @var{buffer}
Called when @var{buffer} is no longer in use.
@end deffn

@deffn {Method} pause @var{buffer}
Called when @var{buffer} is no longer the current buffer but still in
use.
@end deffn

@deffn {Method} resume @var{buffer}
Called when @var{buffer} becomes the current buffer after having been
suspended.
@end deffn

The following methods are simply wrappers around the hooks described
in @xref{Kernel}, so see that section for more detail about the
arguments to these methods.  The engine calls these methods with the
current buffer as the first argument.

@deffn {Method} update @var{buffer} @var{dt}
Advance the simulation running in @var{buffer} by @var{dt} units of
time.
@end deffn

@deffn {Method} abort @var{buffer}
Called when the user tries to close the game window.
@end deffn

@deffn {Method} before-draw @var{buffer}
Called before @var{buffer} is rendered.
@end deffn

@deffn {Method} after-draw @var{buffer}
Called after @var{buffer} is rendered.
@end deffn

@deffn {Method} draw @var{buffer} @var{alpha}
Render @var{buffer}.
@end deffn

@deffn {Method} key-press @var{buffer} @var{key} @var{scancode} @var{modifiers} @var{repeat?}
Handle key press event.
@end deffn

@deffn {Method} key-release @var{buffer} @var{key} @var{scancode} @var{modifiers}
Handle key release event.
@end deffn

@deffn {Method} text-input @var{buffer} @var{text}
Handle text input event.
@end deffn

@deffn {Method} mouse-press @var{buffer} @var{button} @var{clicks} @var{x} @var{y}
Handle mouse press event.
@end deffn

@deffn {Method} mouse-release @var{buffer} @var{button} @var{x} @var{y}
Handle mouse release event.
@end deffn

@deffn {Method} mouse-move @var{buffer} @var{x} @var{y} @var{x-rel} @var{y-rel} @var{buttons}
Handle mouse move event.
@end deffn

@deffn {Method} controller-add @var{buffer} @var{controller}
Handle controller add event.
@end deffn

@deffn {Method} controller-remove @var{buffer} @var{controller}
Handle controller remove event.
@end deffn

@deffn {Method} controller-press @var{buffer} @var{controller} @var{button}
Handle controller press event.
@end deffn

@deffn {Method} controller-release @var{buffer} @var{controller} @var{button}
Handle controller release event.
@end deffn

@deffn {Method} controller-move @var{buffer} @var{controller} @var{axis} @var{value}
Handle controller move event.
@end deffn

The following procedures are used to manage the buffer stack:

@deffn {Procedure} use-buffers! @var{initial-buffer}
Install buffers into the game engine and set the current buffer to
@var{initial-buffer}.
@end deffn

@deffn {Procedure} push-buffer! @var{buffer}
Pause the current buffer and switch to @var{buffer}.
@end deffn

@deffn {Procedure} pop-buffer!
Stop the current buffer and switch back to the previously active
buffer, or terminate the game loop if the buffer stack is empty.
@end deffn

@deffn {Procedure} replace-buffer! @var{buffer}
Stop the current buffer and switch to @var{buffer}
@end deffn

@deffn {Procedure} current-buffer
Return the current buffer.
@end deffn

@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