fix bug with locking an sdl surface not reading the canvas pixel data properly; fixes #664

src/library_sdl.js

@@ -722,16 +722,15 @@ var LibrarySDL = {
     surfData.locked++;
     if (surfData.locked > 1) return 0;
 
-    if (!surfData.image) {
-      surfData.image = surfData.ctx.getImageData(0, 0, surfData.width, surfData.height);
-      if (surf == SDL.screen) {
-        var data = surfData.image.data;
-        var num = data.length;
-        for (var i = 0; i < num/4; i++) {
-          data[i*4+3] = 255; // opacity, as canvases blend alpha
-        }
+    surfData.image = surfData.ctx.getImageData(0, 0, surfData.width, surfData.height);
+    if (surf == SDL.screen) {
+      var data = surfData.image.data;
+      var num = data.length;
+      for (var i = 0; i < num/4; i++) {
+        data[i*4+3] = 255; // opacity, as canvases blend alpha
       }
     }
+
     if (SDL.defaults.copyOnLock) {
       // Copy pixel data to somewhere accessible to 'C/C++'
       if (surfData.isFlagSet(0x00200000 /* SDL_HWPALETTE */)) {

tests/runner.py

@@ -9124,6 +9124,9 @@ elif 'browser' in str(sys.argv):
     def test_sdl_canvas_palette(self):
       self.btest('sdl_canvas_palette.c', reference='sdl_canvas_palette.png')
 
+    def test_sdl_canvas_twice(self):
+      self.btest('sdl_canvas_twice.c', reference='sdl_canvas_twice.png')
+
     def test_sdl_maprgba(self):
       self.btest('sdl_maprgba.c', reference='sdl_maprgba.png', reference_slack=3)
 

tests/sdl_canvas_twice.c

@@ -0,0 +1,26 @@
+#include <SDL/SDL.h>
+
+#if EMSCRIPTEN
+#include <emscripten.h>
+#endif
+
+int main(int argc, char **argv) {
+  SDL_Init(SDL_INIT_VIDEO);
+  SDL_Surface *screen = SDL_SetVideoMode(40, 40, 32, SDL_SWSURFACE);
+
+  SDL_FillRect(screen, NULL, 0xff0000ff);
+  SDL_LockSurface(screen);
+  *((int*)screen->pixels + 95) = 0;
+  SDL_UnlockSurface(screen);
+
+  SDL_FillRect(screen, NULL, 0x00ff00ff); // wipe out previous pixel and fill
+  SDL_LockSurface(screen);
+  *((int*)screen->pixels + 205) = 0;
+  SDL_UnlockSurface(screen);
+
+  SDL_Flip(screen);
+
+  while(1) { SDL_WaitEvent(NULL); }
+
+  return 0;
+}