* 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 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 *** 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 an 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) (2d game)) (coroutine (while #t (walk 'up) (wait game-agenda 60) (walk 'down) (wait game-agenda 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 an agenda. =game-agenda= is the main agenda that is ticked at each iteration of the game update loop. 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 game-agenda 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 game-agenda 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 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 ** Using the Sandbox If you want to quickly create a guile-2d environment and start experimenting, run the =sandbox= script. 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.