path: root/README.org

* guile-2d

  Guile-2d is a free software 2D game engine written in GNU Guile
  Scheme.  It provides an abstraction layer above SDL and OpenGL 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/engines 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 deterministic
    simulation, unlike a variable timestep.

    To start up the game loop, simply call =(start-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))
        (start-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

*** 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))

      (coroutine
       (while #t
         (walk 'up)
         (wait 60)
         (walk 'down)
         (wait 60)))

    #+END_SRC

    =coroutine= is a useful macro that evaluates a block of code as a
    coroutine.  =wait= aborts the procedure and schedules the
    continuation inside of the agenda.  In this example, the script is
    paused for 1 second after each step.  Since guile-2d enforces a
    fixed timestep and updates 60 times per second by default, 60
    ticks is equivalent to 1 second.

    You can also use the agenda to schedule the evaluation of any
    thunk even if it isn't a coroutine.

    #+BEGIN_SRC scheme
      (define (hello)
        (display "Hello, world!  Sorry I'm late!\n"))

      (schedule hello 600)
    #+END_SRC

    =schedule= accepts a thunk (a procedure that takes no arguments)
    and schedules it to be applied after a certain number of ticks, or
    after 1 tick by default.  In this example, the text "Hello, world!
    Sorry I'm late!" is displayed after 10 seconds.  There are other
    ways to schedule procedures, too.  =schedule-interval= applies a
    thunk periodically, and =schedule-each= applies a thunk upon every
    tick.

*** Functional Reactive Programming
    Games are composed of values that evolve as time passes.  The
    player's score, the current stage, an enemy's hit points, etc. all
    change in response to events that happen at discrete points in
    time.  Typically, this means that a number of callback procedures
    are registered to react to events which mutate data structures
    and/or assign to variables.  However, this approach, while simple
    and effective, comes at the price of readability and
    comprehension.  Instead of explicitly mutating data and entering
    "callback hell", guile-2d abstracts and formalizes the process
    using a functional reactive programming style.

    Time-varying values are called "signals", and they are created in
    a declarative and functional manner.  Rather than describing the
    process of mutation procedurally, one describes the relationship
    between signals instead.  Signal relationships are described in a
    functional style using =signal-map=, =signal-fold=,
    =signal-filter=, and others.

    Example:
    #+BEGIN_SRC scheme
      (define-signal position
        (signal-fold v+ (vector2 320 240)
                     (signal-map (lambda (v)
                                   (vscale v 4))
                                 (signal-sample 1 key-arrows))))
    #+END_SRC

    This signal describes a relationship between the arrow keys on the
    keyboard and the position of the player.  =signal-sample= is used
    to trigger a signal update upon every game tick that provides the
    current state of the arrow keys.  =key-arrows= is a vector2 that
    maps to the current state of the arrow keys, allowing for 8
    direction movement.  This vector2 is then scaled 4x to make the
    player move faster.  Finally, the scaled vector is added to the
    previous player position via =signal-fold=.  The player's position
    is at (320, 240) initially.  As you can see, there are no
    callbacks and explicit mutation needed.  Those details have been
    abstracted away, freeing the programmer to focus on more important
    things.

    As an added bonus, signals adapt to changes in their environment
    when defined using the =define-signal= form.  This means that a
    signal can be re-defined at the REPL and other dependent signals
    will take notice and re-evaluate themselves automagically.

*** 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 integrates Guile's cooperative REPL server with the game
   loop.  To activate this feature, import the =(2d repl)= module.  To
   connect to the REPL server, 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 without installing guile-2d (useful when
   developing):

   #+BEGIN_SRC sh
     cd examples
     ../pre-inst-env guile simple.scm
   #+END_SRC

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

** Using the Sandbox
   If you want to quickly create a guile-2d environment and start
   experimenting, run =./pre-inst-env sandbox=.  It will import many
   useful modules, start a REPL server, open a window, and start the
   game loop.  Simply connect to the REPL server and start hacking!

** 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
   - [[http://www.gnu.org/software/guile-opengl/][guile-opengl]] >= 0.1.0
   - [[https://www.gnu.org/software/guile-sdl/index.html][guile-sdl]] >= 0.5.0
   - SDL 1.2
   - FreeImage >= 3.0

** License

   GNU GPL v3+

   See =COPYING= for the full license text.