* 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 (2d game)
                  (2d sprite)
                  (2d vector2)
                  (2d window))

     (define sprite
       (load-sprite "images/p1_front.png"
                    #:position (vector2 320 240)))

     (add-hook! draw-hook (lambda (dt alpha) (draw-sprite sprite)))

     (with-window (make-window #:title "Simple Sprite Demo")
       (run-game-loop))
   #+END_SRC

** Features

*** The Game Loop
    Guile-2d's game loop doesn't tie drawing and updating
    together. Instead, updates happen on a fixed timestep (60 ticks
    per second by default) while drawing happens as many times as
    possible. A framerate indepedent loop mitigates slow down that the
    user might experience when updating the game takes longer than
    drawing a frame at the desired rate. Instead of slowing to a
    crawl, some frames are dropped and the loop tries to catch up on
    updates. Additionally, a fixed timestep allows for a more
    deterministic simulation than a variable timestep.

    To start up the game loop, simply call =(run-game-loop)=. It's a
    good idea to set up the game window prior to starting the loop via
    the =with-window= form.

    #+BEGIN_SRC scheme
      (with-window (make-window #:title "Best Game Ever"
                                #:resolution (vector2 640 480))
        (run-game-loop))
    #+END_SRC

*** 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
    supported formats.

    #+BEGIN_SRC scheme
      (use-modules (2d sprite))

      (define sprite
        (load-sprite "cirno.png"
                     #:position (vector2 320 240)
                     #:scale (1 1)
                     #:rotation 45
                     #:color white
                     #: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 (vector2 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 draw-sprite sprites))
    #+END_SRC

*** Keyboard and Mouse Input
    There are hooks within the =(2d keyboard)= and =(2d mouse)=
    modules that can be used to respond to user input.

    #+BEGIN_SRC scheme
      (use-modules (2d keyboard)
                   (2d mouse))

      ;; Quit when ESC is pressed.
      (add-hook! key-press-hook
                 (lambda (key unicode)
                   (when (eq? key 'escape)
                     (quit-game))))

      ;; Print coordinates when the mouse is moved.
      (add-hook! mouse-move-hook
                 (lambda (x y)
                   (format #t "pos: (~d, ~d)\n" x y)))
    #+END_SRC

    In the future, there will be more convenient ways to respond to
    user input similar to how keymaps work in Emacs.

*** 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
      (use-modules (2d agenda)
                   (2d coroutine))

      (schedule-next
       (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. =schedule-next= accepts a
    thunk (a procedure that takes 0 arguments) and schedules it to be
    executed the next time the agenda is ticked. There are other ways
    to schedule procedures such as =schedule=, =schedule-interval=,
    and =schedule-every=.

    Since guile-2d enforces a fixed timestep and updates 60 times per
    second by default, 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.

   Guile-2d uses a modified REPL server that is integrated with the
   game loop. A REPL server is started when the game loop starts. To
   connect to it, use the [[http://www.nongnu.org/geiser/][Geiser]] extension for GNU Emacs or telnet.

   *Geiser*

   #+BEGIN_SRC fundamental
    M-x connect-to-guile
   #+END_SRC

   Use the default host and port settings.

   *Telnet*

   #+BEGIN_SRC sh
     telnet localhost 37146
   #+END_SRC

** 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

   See =INSTALL.org= for more detailed installation instructions.

** 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 modules in the source directory (useful
   when developing):

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

   To quit an example:
   - Close the window
   - Press the =ESCAPE= key

** Platforms

   Guile-2d supports GNU/Linux currently. OS X support is in the
   works, but there are problems with guile-sdl. See
   https://github.com/davexunit/guile-2d/issues/2 for more details.

** Dependencies

   - GNU Guile >= 2.0.9
   - [[https://gitorious.org/guile-figl/guile-figl][guile-figl]] (git master branch)
   - [[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+