save the canvas in preloadedImages, and do canvas blits in IMG_Load etc.

emcc

@@ -899,7 +899,7 @@ try:
             canvas.height = img.height;
             var ctx = canvas.getContext('2d');
             ctx.drawImage(img, 0, 0);
-            preloadedImages['%(filename)s'] = ctx.getImageData(0, 0, canvas.width, canvas.height);
+            preloadedImages['%(filename)s'] = canvas;
             window.URL.revokeObjectURL(url);
             removeRunDependency();
           };

src/library_sdl.js

@@ -442,8 +442,14 @@ mergeInto(LibraryManager.library, {
     assert(!srcrect && !dstrect); // TODO
     var srcData = SDL.surfaces[src];
     var dstData = SDL.surfaces[dst];
-    assert(srcData.width === dstData.width && srcData.height === dstData.height);
-    {{{ makeCopyValues('dstData.buffer', 'srcData.buffer', 'srcData.width*srcData.height*4', 'i8', null, 1) }}}
+    assert(!!srcData.locked == !!dstData.locked); // we support the case of both locked or both not locked
+    if (srcData.locked) {
+      // Just support blitting everything
+      assert(srcData.width === dstData.width && srcData.height === dstData.height);
+      {{{ makeCopyValues('dstData.buffer', 'srcData.buffer', 'srcData.width*srcData.height*4', 'i8', null, 1) }}}
+    } else {
+      dstData.ctx.drawImage(srcData.canvas, 0, 0, srcData.width, srcData.height, 0, 0, dstData.width, dstData.height);
+    }
     return 0;
   },
 
@@ -522,10 +528,8 @@ mergeInto(LibraryManager.library, {
     var raw = preloadedImages[filename];
     assert(raw, 'Cannot find preloaded image ' + filename);
     var surf = SDL.makeSurface(raw.width, raw.height, 0);
-    // XXX Extremely inefficient!
-    for (var i = 0; i < raw.width*raw.height*4; i++) {
-      {{{ makeSetValue('SDL.surfaces[surf].buffer', 'i', 'raw.data[i]', 'i8') }}}
-    }
+    var surfData = SDL.surfaces[surf];
+    surfData.ctx.drawImage(raw, 0, 0, raw.width, raw.height, 0, 0, raw.width, raw.height);
     return surf;
   },
 

tests/sdl_image.c

@@ -7,8 +7,6 @@ int main() {
   SDL_Init(SDL_INIT_VIDEO);
   SDL_Surface *screen = SDL_SetVideoMode(600, 450, 32, SDL_SWSURFACE);
 
-  if (SDL_MUSTLOCK(screen)) SDL_LockSurface(screen);
-
   SDL_Surface *image = IMG_Load("screenshot.jpg");
   if (!image)
   {
@@ -18,7 +16,6 @@ int main() {
   SDL_BlitSurface (image, NULL, screen, NULL);
   SDL_FreeSurface (image);
 
-  if (SDL_MUSTLOCK(screen)) SDL_UnlockSurface(screen);
   SDL_Flip(screen); 
 
   printf("you should see an image.\n");