The first thing we need to do, is open a window, and set up some restarts so that errors don't kill the connection if we don't want it to. I also add some sbcl specific code to let slime keep responding even while the main loop is running.
(defmacro restartable (&body body) "helper macro since we use continue restarts a lot (remember to hit C in slime or pick the restart so errors don't kill the app)" `(restart-case (progn ,@body) (continue () :report "Continue" ))) (defun draw () "draw a frame" (gl:clear :color-buffer-bit) ;; draw a triangle (gl:with-primitive :triangles (gl:color 1 0 0) (gl:vertex 0 0 0) (gl:color 0 1 0) (gl:vertex 0.5 1 0) (gl:color 0 0 1) (gl:vertex 1 0 0)) ;; finish the frame (gl:flush) (sdl:update-display)) (defun main-loop () (sdl:with-init () (sdl:window 320 240 :flags sdl:sdl-opengl) ;; cl-opengl needs platform specific support to be able to load GL ;; extensions, so we need to tell it how to do so in lispbuilder-sdl (setf cl-opengl-bindings:*gl-get-proc-address* #'sdl-cffi::sdl-gl-get-proc-address) (sdl:with-events () (:quit-event () t) (:idle () ;; this lets slime keep working while the main loop is running ;; in sbcl using the :fd-handler swank:*communication-style* ;; (something similar might help in some other lisps, not sure which though) #+(and sbcl (not sb-thread)) (restartable (sb-sys:serve-all-events 0)) (restartable (draw)))))) (main-loop)
OK, now the window is open, but the triangle is a bit off-center. Luckily we are using lisp, so we can just fix and recompile the function while it runs:
(defun draw () "draw a frame" (gl:clear :color-buffer-bit) ;; draw a triangle (gl:with-primitive :triangles (gl:color 1 0 0) (gl:vertex -1 -1 0) ;; hmm, i need to add an option to the syntax (gl:color 0 1 0) (gl:vertex 0 1 0) ;; highlighting to show changed lines... (gl:color 0 0 1) (gl:vertex 1 -1 0)) ;; (this one changed too) ;; finish the frame (gl:flush) (sdl:update-display))
Note that the way this code draws triangles (using gl:with-primitive, gl:color, gl:vertex, etc., known as "immediate mode") is one of the least efficient ways to draw things on modern hardware, so it should be avoided for anything more complex than a few triangles.