Fix SDL color encoding.

Little-endian + RGBA means that red is located at the lowest-order bits,
not the highest.

The hello_world_sdl test is now portable, because we no longer write to
individual bytes.

We also add a test for a completely blank screen, which should be black, not
red.

Closes #1287, #761, #765.

4 files changed, 23 insertions, 5 deletions

diff --git a/tests/hello_world_sdl.cpp b/tests/hello_world_sdl.cpp
index b6401995..eeaad0cd 100644
--- a/tests/hello_world_sdl.cpp
+++ b/tests/hello_world_sdl.cpp
@@ -2,7 +2,7 @@
 #include <SDL/SDL.h>
 
 
-int main() {
+extern "C" int main(int argc, char** argv) {
   printf("hello, world!\n");
 
   SDL_Init(SDL_INIT_VIDEO);
@@ -11,10 +11,8 @@ int main() {
   if (SDL_MUSTLOCK(screen)) SDL_LockSurface(screen);
   for (int i = 0; i < 256; i++) {
     for (int j = 0; j < 256; j++) {
-      *((char*)screen->pixels + i*256*4 + j*4 + 0) = i;
-      *((char*)screen->pixels + i*256*4 + j*4 + 1) = j;
-      *((char*)screen->pixels + i*256*4 + j*4 + 2) = 255-i;
-      *((char*)screen->pixels + i*256*4 + j*4 + 3) = (i+j)%255; // actually ignored, since this is to the screen
+      // alpha component is actually ignored, since this is to the screen
+      *((Uint32*)screen->pixels + i * 256 + j) = SDL_MapRGBA(screen->format, i, j, 255-i, (i+j) % 255);
     }
   }
   if (SDL_MUSTLOCK(screen)) SDL_UnlockSurface(screen);
diff --git a/tests/runner.py b/tests/runner.py
index e7434479..4b8a3010 100755
--- a/tests/runner.py
+++ b/tests/runner.py
@@ -12735,6 +12735,9 @@ elif 'browser' in str(sys.argv):
     def test_cube_explosion(self):
       self.btest('cube_explosion.c', expected=['667220544', '-1543354600', '-1485258415'])
 
+    def test_sdl_canvas_blank(self):
+      self.btest('sdl_canvas_blank.c', reference='sdl_canvas_blank.png')
+
     def test_sdl_canvas_palette(self):
       self.btest('sdl_canvas_palette.c', reference='sdl_canvas_palette.png')
 
diff --git a/tests/sdl_canvas_blank.c b/tests/sdl_canvas_blank.c
new file mode 100644
index 00000000..0e7607a6
--- /dev/null
+++ b/tests/sdl_canvas_blank.c
@@ -0,0 +1,17 @@
+#include <stdio.h>
+#include <SDL/SDL.h>
+
+
+int main() {
+  SDL_Init(SDL_INIT_VIDEO);
+  SDL_Surface *screen = SDL_SetVideoMode(256, 256, 32, SDL_SWSURFACE);
+
+  if (SDL_MUSTLOCK(screen)) SDL_LockSurface(screen);
+  if (SDL_MUSTLOCK(screen)) SDL_UnlockSurface(screen);
+  SDL_Flip(screen); 
+
+  SDL_Quit();
+
+  return 0;
+}
+
diff --git a/tests/sdl_canvas_blank.png b/tests/sdl_canvas_blank.png
new file mode 100644
index 00000000..dc5a4a26
--- /dev/null
+++ b/tests/sdl_canvas_blank.png