Write SDL2 rendering functions

drawing.lisp

@@ -86,3 +86,39 @@
     (sdl2:destroy-renderer renderer)
     (sdl2:destroy-window window)
     (sdl2:quit)))
+
+(define-transform project-to-screen (screen-height)
+                  :x-transform (round x)
+                  :y-transform (round (- screen-height y)))
+
+(defmethod display-frame ((ctx graphics-context) drawing)
+  (with-slots (renderer width height) ctx
+    (sdl2:set-render-draw-color renderer 255 255 255 255)
+    (sdl2:render-clear renderer)
+    (sdl2:set-render-draw-color renderer 0 0 0 255)
+    (draw renderer
+		  (project-to-screen height
+							 (translate (/ width 2) (/ height 2) drawing)))
+    (sdl2:render-present renderer)))
+
+(defun draw (renderer drawing)
+  (defun draw-line (line)
+	(let ((points (line-points line)))
+      (sdl2:render-draw-line renderer
+							 (aref points 0) (aref points 1)
+							 (aref points 2) (aref points 3))))
+  (defun draw-polygon (polygon)
+	(let* ((points (polygon-points polygon))
+           (point-count (/ (length points) 2)))
+      (dotimes (i point-count)
+		(let* ((start (* i 2))
+               (end (if (= i (1- point-count)) 0 (+ start 2))))
+          (sdl2:render-draw-line renderer
+								 (aref points start)
+								 (aref points (1+ start))
+								 (aref points end)
+								 (aref points (1+ end)))))))
+  (dolist (primitive drawing)
+    (typecase primitive
+      (line (draw-line primitive))
+      (polygon (draw-polygon primitive)))))