Re: BUG IN SDL - Blitting to a packed indexed surface

SDL doesn't currently have any bitmap blitters. This should have returned
an error, and that's fixed.
https://hg.libsdl.org/SDL/rev/754cac536474

Thanks!

On Fri, Dec 23, 2016 at 6:02 PM, geoffp <[email protected]> wrote:

> I'm pretty sure the following is a bug in SDL.
>
> When SDL_BlitSurface has a target with a two-colour (i.e. 1-bit depth)
> packed (8 pixels per byte) format, it will in some situations try to write
> 1 byte per pixel. Since the target doesn't have enough memory space to
> accommodate this, pixel data may get written to regions in RAM where it has
> no business being, potentially causing a crash.
>
> A minimal program to demonstrate the effect is as follows. I ran this with
> SDL-2.0.5 on a 32-bit Windows platform.
>
>
>
>
> Code:
>
>
>  // Make test surfaces in two formats, 32-bit ARGB and 1-bit two-colour:
>  SDL_Surface *s32 = SDL_CreateRGBSurface(0, 30, 10, 32, 0xff0000,
> 0x00ff00, 0x0000ff, 0xff000000);
>  SDL_Surface *s01 = SDL_ConvertSurfaceFormat(s32,
> SDL_PIXELFORMAT_INDEX1LSB, 0);
>
>  // Set a palette for the indexed surface.
>  // (It already has a default palette (white, black), but for some reason
> this bug
>  // appears only if the palette is first changed to (black, white). I've
> no idea why.)
>  SDL_Color c[2] = { {0,0,0,0xff}, {0xff,0xff,0xff,0xff} };
>  SDL_SetPaletteColors(s01->format->palette, c, 0, 2);
>
>  // Confirm that these formats are as expected:
>  printf("s32 pitch = %d bytes per row\n", s32->pitch);   // 120 (4 bytes
> per pixel)
>  printf("s01 pitch = %d bytes per row\n", s01->pitch);   //  4  (8 pixels
> per byte)
>
>  // Place a white rectangle on the (default-initialized) transparent
> background of s32:
>  SDL_Rect rect = { 1, 1, 5, 3 };
>  SDL_FillRect(s32, &rect, 0xffffffff);
>
>  // Blit the s32 image to s01:
>  int result = SDL_BlitSurface(s32, NULL, s01, NULL);
>  printf("SDL_BlitSurface result = %d\n", result);
>
>  // Show the s01 pixel data:
>  printf("s01: %d x %d surface:\n", s01->w, s01->h);
>  for(int r = 0; r != s01->h; r++)
>  {
>    for(int b = 0; b != s01->pitch; b++)
>      printf("%x ", *((Uint8*)(s01->pixels) + r * s01->pitch + b) );
>    printf("\n");
>  }
>
>
>
> The output I get is:
>
>
>
>
> Code:
>
>
>   s32 pitch = 120 bytes per row
>   s01 pitch = 4 bytes per row
>   SDL_BlitSurface result = 0
>   s01: 30 x 10 surface:
>   0 0 0 0
>   0 1 1 1
>   1 1 1 1
>   1 1 1 1
>   1 1 0 0
>   0 0 0 0
>   0 0 0 0
>   0 0 0 0
>   0 0 0 0
>   0 0 0 0
>
>
>
>
> It seems that SDL_BlitSurface has forgotten that its target surface has a
> packed format, and is trying to write 1 byte per pixel.
>
> Note that if properly packed pixel data is put on this kind of surface, it
> blits just fine in the other direction, when it is the source instead of
> the destination.
>
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> [email protected]
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>
>

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