path: root/README.org

* 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 its 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)

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

     ;; Load a sprite and center it on the screen.
     (define sprite
       (load-sprite "sprite.png"
                    #:position (vector (/ window-width 2)
                                       (/ window-height 2))))

     (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 hooks. Lambdas are used as "trampolines" so that render
     ;; and key-down can be redefined later and the hooks will call the
     ;; updated procedures.
     (add-hook! on-render-hook (lambda () (render)))
     (add-hook! on-key-down-hook (lambda (key) (key-down key)))

     ;; 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.

    Guile-2d uses the FreeImage library and can load many different
    image formats. See the FreeImage [[http://freeimage.sourceforge.net/features.html][features page]] for a full list of
    support formats.

    #+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.

*** Actions
    Actions encapsulate a procedure that operates over a certain
    period of time. Action objects have two properties: an arbitrary
    procedure and a duration in game ticks. Action procedures accept
    one argument: a time delta in the range [0, 1]. Use actions in
    combination with coroutines for things that are a function of
    time, such as moving a sprite across the screen.

    #+BEGIN_SRC scheme
      (schedule-action
       ;; Move horizontally across the screen, starting at x=0 and moving to
       ;; x=800, in 60 ticks.
        (lerp (lambda (x)
                (set-sprite-position! sprite (vector x (/ window-height 2))))
              0 800 60))
    #+END_SRC

    =schedule-action= is used to schedule a coroutine that will
    perform the given action in the current agenda. =lerp= is a type
    of action, short for linear interpolation. =lerp= takes an
    arbitrary procedure to apply at each tick, a start value, an end
    value, and like all other actions, a duration. The code above
    interpolates from 0 to 800 over 60 ticks. The result of this
    action is a sprite moving across the screen from left to right.

    Actions can be combined to run in a sequence or in parallel.

    #+BEGIN_SRC scheme
      (schedule-action
       (action-parallel
        (lerp (lambda (x)
                (set-sprite-position! sprite (vector x (/ window-height 2))))
              0 800 60)
        ;; Rotate sprite 1080 degrees in 120 ticks.
        (lerp (lambda (angle)
                (set-sprite-rotation! sprite angle))
              0 1080 120)))
    #+END_SRC

    =action-parallel= will combine many actions into one action that
    does everything at the same time. In the example above, the sprite
    will still move across the screen from left to right, but while
    it's doing so (and for 60 ticks after), it will be rotating from 0
    to 1080 degrees.

** 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. **

** Building
   guile-2d uses the typical GNU build system. First run `autogen.sh`
   and then do the usual incantations.

   #+BEGIN_SRC sh
     ./autogen.sh
     ./configure
     make
     sudo make install
   #+END_SRC

** Running Examples
   To run an example when guile-2d has been installed:

   #+BEGIN_SRC sh
     cd examples
     guile simple.scm
   #+END_SRC

   To run an example using the not-yet-installed files (useful when
   developing):

   #+BEGIN_SRC sh
     cd examples
     guile -L .. simple.scm
   #+END_SRC

** Dependencies

   - GNU Guile >= 2.0 (>= 2.0.9 recommended)
   - [[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 19152 of
     =doc/low-level-gl.texi=. To fix, join lines 19152 and 19153
     together.

   - [[https://www.gnu.org/software/guile-sdl/index.html][guile-sdl]] >= 0.5.0
   - SDL 1.2
   - FreeImage >= 3.0
   - FTGL >= 2.1

** License

   GNU LGPL v3