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Added SDL_PremultiplyAlpha() to premultiply alpha on a block of SDL_PIXELFORMAT_ARGB8888 pixels

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This commit is contained in:
Sam Lantinga
2021-11-21 12:18:10 -08:00
parent 49369142bd
commit c97b721868
9 changed files with 109 additions and 34 deletions
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69
src/video/SDL_surface.c
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@@ -1381,7 +1381,12 @@ int SDL_ConvertPixels(int width, int height,
void *nonconst_src = (void *) src;
int ret;
/* Check to make sure we are blitting somewhere, so we don't crash */
if (!src) {
return SDL_InvalidParamError("src");
}
if (!src_pitch) {
return SDL_InvalidParamError("src_pitch");
}
if (!dst) {
return SDL_InvalidParamError("dst");
}
@@ -1440,6 +1445,68 @@ int SDL_ConvertPixels(int width, int height,
return ret;
}
/*
* Premultiply the alpha on a block of pixels
*
* This is currently only implemented for SDL_PIXELFORMAT_ARGB8888
*
* Here are some ideas for optimization:
* https://github.com/Wizermil/premultiply_alpha/tree/master/premultiply_alpha
* https://developer.arm.com/documentation/101964/0201/Pre-multiplied-alpha-channel-data
*/
int SDL_PremultiplyAlpha(int width, int height,
Uint32 src_format, const void * src, int src_pitch,
Uint32 dst_format, void * dst, int dst_pitch)
{
int c;
Uint32 srcpixel;
Uint32 srcR, srcG, srcB, srcA;
Uint32 dstpixel;
Uint32 dstR, dstG, dstB, dstA;
if (!src) {
return SDL_InvalidParamError("src");
}
if (!src_pitch) {
return SDL_InvalidParamError("src_pitch");
}
if (!dst) {
return SDL_InvalidParamError("dst");
}
if (!dst_pitch) {
return SDL_InvalidParamError("dst_pitch");
}
if (src_format != SDL_PIXELFORMAT_ARGB8888) {
return SDL_InvalidParamError("src_format");
}
if (dst_format != SDL_PIXELFORMAT_ARGB8888) {
return SDL_InvalidParamError("dst_format");
}
while (height--) {
const Uint32 *src_px = (const Uint32 *)src;
Uint32 *dst_px = (Uint32 *)dst;
for (c = width; c; --c) {
/* Component bytes extraction. */
srcpixel = *src_px++;
RGBA_FROM_ARGB8888(srcpixel, srcR, srcG, srcB, srcA);
/* Alpha pre-multiplication of each component. */
dstA = srcA;
dstR = (srcA * srcR) / 255;
dstG = (srcA * srcG) / 255;
dstB = (srcA * srcB) / 255;
/* ARGB8888 pixel recomposition. */
ARGB8888_FROM_RGBA(dstpixel, dstR, dstG, dstB, dstA);
*dst_px++ = dstpixel;
}
src = (const Uint8 *)src + src_pitch;
dst = (Uint8 *)dst + dst_pitch;
}
return 0;
}
/*
* Free a surface created by the above function.
*/