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

* Add test, fix for unusual pitch in premul_alpha()

fixes #2750

* Try to please gcc

* formatting

* added @ankith suggestion

* Formatting.

Author: ankith26

src_c/alphablit.c

@@ -1714,7 +1714,9 @@ 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 == (src->w * PG_SURF_BytesPerPixel(src))) &&
+        pg_has_avx2()) {
         premul_surf_color_by_alpha_avx2(src, src_format, dst);
         return 0;
     }
@@ -1749,6 +1751,8 @@ premul_surf_color_by_alpha_non_simd(SDL_Surface *src,
     int srcbpp = PG_FORMAT_BytesPerPixel(src_format);
     int dstbpp = PG_FORMAT_BytesPerPixel(dst_format);
     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 = srcbpp;
@@ -1775,6 +1779,8 @@ premul_surf_color_by_alpha_non_simd(SDL_Surface *src,
                 dst_pixels += dstpxskip;
             },
             n, width);
+        src_pixels += srcskip;
+        dst_pixels += dstskip;
     }
 }
 

src_c/simd_blitters_sse2.c

@@ -794,7 +794,9 @@ premul_surf_color_by_alpha_sse2(SDL_Surface *src, PG_PixelFormat *srcfmt,
     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);
     Uint32 amask = srcfmt->Amask;
     Uint64 ones;
 
@@ -845,6 +847,8 @@ premul_surf_color_by_alpha_sse2(SDL_Surface *src, PG_PixelFormat *srcfmt,
                 ++dstp;
             },
             n, width);
+        srcp = (Uint32 *)((Uint8 *)srcp + srcskip);
+        dstp = (Uint32 *)((Uint8 *)dstp + dstskip);
     }
 }
 

test/surface_test.py

@@ -4004,6 +4004,26 @@ 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"""