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Fixed bug 3699 - Shaped windows are distorted unless width is divisib…

…le by 8

Bogomancer

On X11, windows created using the shaped window API appear distorted unless the width of the shape surface is divisible by 8.

Steps to reproduce:

    1) Use your favorite image editor to resize one of the images in test/shapes/ to a width that's not a multiple of 8.
    2) Compile and run test/testshape.c on the image you edited.
    3) The shaped window will appear twisted and distorted.

It appears the bug was not caught sooner because all the test images are either 640 or 256 pixels wide.

I tracked down the bug to SDL_CalculateShapeBitmap() in SDL_shape.c. The shape surface is reduced to a 1-bit-per-pixel mask, but the original code doesn't take into account that X11 apparently wants each scanline to begin on a new byte.
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slouken committed Jul 11, 2017
1 parent 34b29c6 commit 74ca16547cdd22e998f93a0021a44a5eddf7f93e
Showing with 5 additions and 3 deletions.
  1. +5 −3 src/video/SDL_shape.c
@@ -74,11 +74,14 @@ SDL_CalculateShapeBitmap(SDL_WindowShapeMode mode,SDL_Surface *shape,Uint8* bitm
int y = 0;
Uint8 r = 0,g = 0,b = 0,alpha = 0;
Uint8* pixel = NULL;
Uint32 bitmap_pixel,pixel_value = 0,mask_value = 0;
Uint32 pixel_value = 0,mask_value = 0;
int bytes_per_scanline = (shape->w + (ppb - 1)) / ppb;
Uint8 *bitmap_scanline;
SDL_Color key;
if(SDL_MUSTLOCK(shape))
SDL_LockSurface(shape);
for(y = 0;y<shape->h;y++) {
bitmap_scanline = bitmap + y * bytes_per_scanline;
for(x=0;x<shape->w;x++) {
alpha = 0;
pixel_value = 0;
@@ -98,7 +101,6 @@ SDL_CalculateShapeBitmap(SDL_WindowShapeMode mode,SDL_Surface *shape,Uint8* bitm
break;
}
SDL_GetRGBA(pixel_value,shape->format,&r,&g,&b,&alpha);
bitmap_pixel = y*shape->w + x;
switch(mode.mode) {
case(ShapeModeDefault):
mask_value = (alpha >= 1 ? 1 : 0);
@@ -114,7 +116,7 @@ SDL_CalculateShapeBitmap(SDL_WindowShapeMode mode,SDL_Surface *shape,Uint8* bitm
mask_value = ((key.r != r || key.g != g || key.b != b) ? 1 : 0);
break;
}
bitmap[bitmap_pixel / ppb] |= mask_value << (7 - ((ppb - 1) - (bitmap_pixel % ppb)));
bitmap_scanline[x / ppb] |= mask_value << (x % ppb);
}
}
if(SDL_MUSTLOCK(shape))

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