* guile-2d

  Guile-2d is a 2D game programming library for GNU Guile. It is a
  layer above SDL and OpenGL that provides abstractions for common 2D
  game programming requirements such as:

  - Sprites
  - Animation
  - Tilesets
  - Tile maps
  - Scene graph
  - Input handling
  - Scripting

** Inspiration
   Every programming language should have a fun, easy to use 2D game
   library. Guile-2d draws it's inspiration from great
   libraries/frameworks such as [[http://love2d.org][LÖVE]], [[http://pygame.org][Pygame]], and [[http://pyglet.org][Pyglet]].

** Example
   Here is the simplest Guile-2d application (so far).

   #+BEGIN_SRC scheme
     (use-modules (figl gl)
                  (2d sprite)
                  (2d game-loop)
                  (2d window))

     (define window-width 800)
     (define window-height 600)
     (define sprite #f)

     (define (key-down key)
       (display key) (newline)
       (case key
         ;; Quit program when ESCAPE or Q is pressed.
         ((any-equal? key 'escape 'q)
          (close-window)
          (quit))))

     ;; Draw our sprite
     (define (render)
       (draw-sprite sprite))

     ;; Register callbacks.
     (set-render-callback (lambda () (render)))
     (set-key-down-callback (lambda (key) (key-down key)))

     ;; Open the window.
     (open-window window-width window-height)

     ;; Load a sprite and center it on the screen.
     ;; Must be done AFTER opening the window.
     (set! sprite (load-sprite "sprite.png" #:position (vector (/ window-width 2)
                                                               (/ window-height 2))))

     ;; Start the game loop.
     ;; The render callback will be called through this procedure.
     (run-game-loop)
   #+END_SRC

** Features

*** Sprites
    Sprites encapsulate the presentation of an image or a region of an
    image.

    The simplest way to get started with sprites is to use the
    =load-sprite= procedure. All arguments except the filename are
    optional keyword arguments.

    #+BEGIN_SRC scheme
      (define sprite (load-sprite "cirno.png" #:position #(0 0) #:scale (1 1)
                                  #:rotation (0) #:color #xffffff #:anchor 'center))
    #+END_SRC

    Alternatively, you can make a sprite from an existing texture. The
    same keyword arguments in =load-sprite= are also available here.

    #+BEGIN_SRC scheme
      (define sprite (make-sprite (load-texture "cirno.png")))
    #+END_SRC

    Position, scale, rotation, color, and anchor are mutable.

    #+BEGIN_SRC scheme
      (set-sprite-position! sprite #(100 100))
    #+END_SRC

    Drawing a sprite is simple.

    #+BEGIN_SRC scheme
      (draw-sprite sprite)
    #+END_SRC

*** Sprite Batches
    When drawing many sprites, it is inefficient to send them to the
    GPU individually. Sprite batches resolve this issue by sending
    sprites to the GPU in large chunks.

    To take advantage of this, create a sprite batch and use
    =with-sprite-batch=. All calls to =draw-sprite= will use the
    sprite batch within this form.

    #+BEGIN_SRC scheme
      (define sprites (make-a-ton-of-sprites))

      (define batch (make-sprite-batch))

      (with-sprite-batch batch
        (for-each
         (lambda (sprite) (draw-sprite sprite))
         sprites))
    #+END_SRC

*** Coroutines and Agendas
    The ability to write scripts is very important for most games. A
    script for an RPG NPC could look like this:

    #+BEGIN_SRC scheme
      ;; Walk up one tile and then down one tile, forever.
      (while #t
        (walk 'up)
        (walk 'down))
    #+END_SRC

    Unfortunately, running this script as it is means completely
    locking up the program in an unbounded loop. However, coroutines
    (and a scheduler known as the "agenda") are here to save the day!
    Coroutines are procedures that can be exited at any point and
    resumed later.

    It would be nice if after every call to =walk=, the NPC would wait
    for one second before taking its next step. This is where the
    agenda comes in. The agenda is used to schedule procedures to be
    run after an arbitrary number of game updates (1 by
    default). Since coroutines and the agenda go hand in hand, there
    exists a =wait= procedure to pause a coroutine and schedule it to
    be resumed later.

    Using a coroutine and the agenda, the NPC script can be rewritten
    such that it does not halt further program execution.

    #+BEGIN_SRC scheme
      (agenda-schedule
       (colambda ()
         (while #t
           (walk 'up)
           (wait 60)
           (walk 'down)
           (wait 60))))

    #+END_SRC

    =colambda= is a useful macro that is syntactic sugar for a lambda
    expression executed as a coroutine. =agenda-schedule= accepts a
    thunk (a procedure that takes 0 arguments) and schedules it to be
    executed later. In this example we do not provide a second
    argument to =agenda-schedule=, which means that the thunk will be
    executed upon the next game update.

    Since guile-2d enforces a fixed timestep and updates 60 times per
    second, waiting for 60 updates means that the NPC will wait one
    second in between each step.

** REPL Driven Development
   The read-eval-print-loop present in Guile allows you to develop
   your game while it is running! This allows you to see in real time
   what your changes do to the game without having to restart the
   program every time.

   ** This section needs to be completed. **

** Dependencies

   - [[https://gitorious.org/guile-figl/guile-figl][guile-figl]]

     No official release. guile-2d runs off of the latest master
     version.

     As of this writing, =make= will fail due to errors with building
     texinfo files. Currently there is an error on line 19188 of
     =doc/low-level-gl.texi=. To fix, join lines 19188 and 19189
     together.

   - [[https://www.gnu.org/software/guile-sdl/index.html][guile-sdl]] >= 0.5.0
   - SDL 1.2

** License

   GNU LGPL v3