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Add test & fix for unusual pitch in surface.premul_alpha() #2882

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Add test & fix for unusual pitch in surface.premul_alpha() #2882

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b742938
Add test, fix for unusual pitch in premul_alpha()
MyreMylar May 28, 2024
8b28d80
Try to please gcc
MyreMylar May 29, 2024
34f230b
Merge branch 'main' into fix-premul-alpha-bytewidth
MyreMylar Jun 9, 2024
ad833b8
Merge branch 'main' into fix-premul-alpha-bytewidth
MyreMylar Jul 25, 2024
c739cf2
formatting
MyreMylar Jul 25, 2024
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7 changes: 6 additions & 1 deletion src_c/alphablit.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2841,7 +2841,8 @@ premul_surf_color_by_alpha(SDL_Surface *src, SDL_Surface *dst)
// since we know dst is a copy of src we can simplify the normal checks
#if !defined(__EMSCRIPTEN__)
#if SDL_BYTEORDER == SDL_LIL_ENDIAN
if ((PG_SURF_BytesPerPixel(src) == 4) && pg_has_avx2()) {
if ((PG_SURF_BytesPerPixel(src) == 4) &&
(src->pitch % PG_SURF_BytesPerPixel(src) == 0) && pg_has_avx2()) {
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@ankith26 ankith26 May 18, 2025 •
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I think this test should be more robust. It should check for src->pitch == (src->w * PG_SURF_BytesPerPixel(src)) because I can imagine pitch may still be a multiple of bpp and not be equal to the minimal possible pitch.

premul_surf_color_by_alpha_avx2(src, dst);
return 0;
}
Expand Down Expand Up @@ -2873,6 +2874,8 @@ premul_surf_color_by_alpha_non_simd(SDL_Surface *src, SDL_Surface *dst)
int srcbpp = PG_FORMAT_BytesPerPixel(srcfmt);
int dstbpp = PG_FORMAT_BytesPerPixel(dstfmt);
Uint8 *src_pixels = (Uint8 *)src->pixels;
int srcskip = src->pitch - (width * srcbpp);
int dstskip = dst->pitch - (width * dstbpp);
Uint8 *dst_pixels = (Uint8 *)dst->pixels;

int srcpxskip = PG_SURF_BytesPerPixel(src);
Expand All @@ -2898,6 +2901,8 @@ premul_surf_color_by_alpha_non_simd(SDL_Surface *src, SDL_Surface *dst)
dst_pixels += dstpxskip;
},
n, width);
src_pixels += srcskip;
dst_pixels += dstskip;
}
}

Expand Down
4 changes: 4 additions & 0 deletions src_c/simd_blitters_sse2.c
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Original file line number Diff line number Diff line change
Expand Up @@ -793,7 +793,9 @@ premul_surf_color_by_alpha_sse2(SDL_Surface *src, SDL_Surface *dst)
int width = src->w;
int height = src->h;
Uint32 *srcp = (Uint32 *)src->pixels;
int srcskip = src->pitch - width * PG_SURF_BytesPerPixel(src);
Uint32 *dstp = (Uint32 *)dst->pixels;
int dstskip = dst->pitch - width * PG_SURF_BytesPerPixel(dst);

SDL_PixelFormat *srcfmt = src->format;
Uint32 amask = srcfmt->Amask;
Expand Down Expand Up @@ -846,6 +848,8 @@ premul_surf_color_by_alpha_sse2(SDL_Surface *src, SDL_Surface *dst)
++dstp;
},
n, width);
srcp = (Uint32 *)((Uint8 *)srcp + srcskip);
dstp = (Uint32 *)((Uint8 *)dstp + dstskip);
}
}

Expand Down
20 changes: 20 additions & 0 deletions test/surface_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4038,6 +4038,26 @@ def test_surface_premul_alpha(self):
),
)

def create_surface_from_byte_width(byte_width):
surf_height = 5
byte_data = bytes(byte_width * surf_height) # 50 bytes
surf_width = byte_width // 4 # width = 2

dest = pygame.image.frombuffer(
byte_data, (surf_width, surf_height), "RGBA", pitch=byte_width
)
dest.fill((120, 50, 70, 200))
return dest

test_surf = create_surface_from_byte_width(10)
test_surf = test_surf.premul_alpha()

for y in range(0, test_surf.get_height()):
for x in range(0, test_surf.get_width()):
self.assertEqual(
test_surf.get_at((x, y)), pygame.Color(94, 39, 55, 200)
)

def test_surface_premul_alpha_ip(self):
"""Ensure that .premul_alpha_ip() works correctly"""

Expand Down
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