Commit 84808ea4bbd12ac212ff7a4ef82d426f6d771e99
2021-10-22T17:48:32
video: Add SDL_PremultiplySurfaceAlphaToARGB8888() A number of video backends need to get ARGB8888 formatted surfaces with premultiplied alpha, typically for mouse cursors. Add a new function to do this, based loosely on legacy_alpha_premultiply_ARGB8888() from the KMSDRM backend. The new function, SDL_PremultiplySurfaceAlphaToARGB8888() takes two arguments: - src: an SDL_Surface to be converted. - dst: a buffer which is filled with premultiplied ARGB8888 data of the same size as the surface (assuming pitch = w). This is not heavily optimised: it just repeatedly calls SDL_GetRGBA() to do the conversion, but should do for now.
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diff --git a/src/video/SDL_pixels.c b/src/video/SDL_pixels.c
index ccb024d..c7b4d32 100644
--- a/src/video/SDL_pixels.c
+++ b/src/video/SDL_pixels.c
@@ -1227,4 +1227,34 @@ SDL_CalculateGammaRamp(float gamma, Uint16 * ramp)
}
}
+/* Creates a copy of an ARGB8888-format surface's pixels with premultiplied alpha */
+void
+SDL_PremultiplySurfaceAlphaToARGB8888(SDL_Surface *src, Uint32 *dst)
+{
+ Uint8 A, R, G, B;
+ int x, y;
+
+ if (SDL_MUSTLOCK(src))
+ SDL_LockSurface(src);
+
+ for (y = 0; y < src->h; ++y) {
+ Uint8 *src_px = (Uint8*)(src->pixels) + (y * src->pitch);
+ for (x = 0; x < src->w; ++x) {
+ /* Component bytes extraction. */
+ SDL_GetRGBA(*(Uint32*)src_px, src->format, &R, &G, &B, &A);
+ src_px += src->format->BytesPerPixel;
+
+ /* Alpha pre-multiplication of each component. */
+ R = (float)A * ((float)R /255);
+ G = (float)A * ((float)G /255);
+ B = (float)A * ((float)B /255);
+
+ /* ARGB8888 pixel recomposition. */
+ (*dst++) = (((Uint32)A << 24) | ((Uint32)R << 16) | ((Uint32)G << 8)) | ((Uint32)B << 0);
+ }
+ }
+ if (SDL_MUSTLOCK(src))
+ SDL_UnlockSurface(src);
+}
+
/* vi: set ts=4 sw=4 expandtab: */
diff --git a/src/video/SDL_pixels_c.h b/src/video/SDL_pixels_c.h
index 1974797..91db5b4 100644
--- a/src/video/SDL_pixels_c.h
+++ b/src/video/SDL_pixels_c.h
@@ -43,6 +43,7 @@ extern void SDL_InvalidateAllBlitMap(SDL_Surface *surface);
extern void SDL_DitherColors(SDL_Color * colors, int bpp);
extern Uint8 SDL_FindColor(SDL_Palette * pal, Uint8 r, Uint8 g, Uint8 b, Uint8 a);
extern void SDL_DetectPalette(SDL_Palette *pal, SDL_bool *is_opaque, SDL_bool *has_alpha_channel);
+extern void SDL_PremultiplySurfaceAlphaToARGB8888(SDL_Surface *src, Uint32 *dst);
#endif /* SDL_pixels_c_h_ */