Commit 0ebda874255c946dc7d39779317d1467377d083f
2023-01-06T13:27:53
Fixed bug #6990: fix computation of alpha in BlitRGBtoRGBPixelAlphaMMX
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diff --git a/src/video/SDL_blit_A.c b/src/video/SDL_blit_A.c
index acfc1a4..08a9d4e 100644
--- a/src/video/SDL_blit_A.c
+++ b/src/video/SDL_blit_A.c
@@ -332,9 +332,19 @@ static void BlitRGBtoRGBPixelAlphaMMX(SDL_BlitInfo *info)
Uint32 ashift = sf->Ashift;
Uint64 multmask, multmask2;
- __m64 src1, dst1, mm_alpha, mm_zero, mm_alpha2;
+ __m64 src1, dst1, mm_alpha, mm_zero, mm_alpha2, mm_one_alpha;
mm_zero = _mm_setzero_si64(); /* 0 -> mm_zero */
+ if (amask == 0xFF000000) { /* 1 in the alpha channel -> mm_one_alpha */
+ mm_one_alpha = _mm_set_pi16(1, 0, 0, 0);
+ } else if (amask == 0x00FF0000) {
+ mm_one_alpha = _mm_set_pi16(0, 1, 0, 0);
+ } else if (amask == 0x0000FF00) {
+ mm_one_alpha = _mm_set_pi16(0, 0, 1, 0);
+ } else {
+ mm_one_alpha = _mm_set_pi16(0, 0, 0, 1);
+ }
+
multmask = 0x00FF;
multmask <<= (ashift * 2);
multmask2 = 0x00FF00FF00FF00FFULL;
@@ -361,14 +371,33 @@ static void BlitRGBtoRGBPixelAlphaMMX(SDL_BlitInfo *info)
mm_alpha = _mm_or_si64(mm_alpha2, *(__m64 *) & multmask); /* 0F0A0A0A -> mm_alpha */
mm_alpha2 = _mm_xor_si64(mm_alpha2, *(__m64 *) & multmask2); /* 255 - mm_alpha -> mm_alpha */
- /* blend */
+ /*
+ Alpha blending is:
+ dstRGB = (srcRGB * srcA) + (dstRGB * (1-srcA))
+ dstA = srcA + (dstA * (1-srcA)) *
+
+ Here, 'src1' is:
+ srcRGB * srcA
+ srcA
+ And 'dst1' is:
+ dstRGB * (1-srcA)
+ dstA * (1-srcA)
+ so that *dstp is 'src1 + dst1'
+
+ src1 is computed using mullo_pi16: (X * mask) >> 8, but is approximate for srcA ((srcA * 255) >> 8).
+
+ need to a 1 to get an exact result: (srcA * 256) >> 8 == srcA
+ */
+ mm_alpha = _mm_add_pi16(mm_alpha, mm_one_alpha);
+
+ /* blend */
src1 = _mm_mullo_pi16(src1, mm_alpha);
src1 = _mm_srli_pi16(src1, 8);
dst1 = _mm_mullo_pi16(dst1, mm_alpha2);
dst1 = _mm_srli_pi16(dst1, 8);
dst1 = _mm_add_pi16(src1, dst1);
dst1 = _mm_packs_pu16(dst1, mm_zero);
-
+
*dstp = _mm_cvtsi64_si32(dst1); /* dst1 -> pixel */
}
++srcp;
@@ -664,14 +693,14 @@ static void BlitRGBtoRGBPixelAlphaMMX3DNOW(SDL_BlitInfo *info)
mm_alpha2 = _mm_xor_si64(mm_alpha2, *(__m64 *) & multmask2); /* 255 - mm_alpha -> mm_alpha */
- /* blend */
+ /* blend */
src1 = _mm_mullo_pi16(src1, mm_alpha);
src1 = _mm_srli_pi16(src1, 8);
dst1 = _mm_mullo_pi16(dst1, mm_alpha2);
dst1 = _mm_srli_pi16(dst1, 8);
dst1 = _mm_add_pi16(src1, dst1);
dst1 = _mm_packs_pu16(dst1, mm_zero);
-
+
*dstp = _mm_cvtsi64_si32(dst1); /* dst1 -> pixel */
}
++srcp;
@@ -1024,7 +1053,7 @@ static void Blit555to555SurfaceAlphaMMX(SDL_BlitInfo *info)
dst2 = _mm_and_si64(dst2, rmask); /* dst2 & MASKRED -> dst2 */
mm_res = dst2; /* RED -> mm_res */
-
+
/* green -- process the bits in place */
src2 = src1;
src2 = _mm_and_si64(src2, gmask); /* src & MASKGREEN -> src2 */
@@ -1166,7 +1195,7 @@ static void BlitARGBto565PixelAlpha(SDL_BlitInfo *info)
compositioning used (>>8 instead of /255) doesn't handle
it correctly. Also special-case alpha=0 for speed?
Benchmark this! */
- if (alpha) {
+ if (alpha) {
if (alpha == (SDL_ALPHA_OPAQUE >> 3)) {
*dstp = (Uint16)((s >> 8 & 0xf800) + (s >> 5 & 0x7e0) + (s >> 3 & 0x1f));
} else {
@@ -1212,7 +1241,7 @@ static void BlitARGBto555PixelAlpha(SDL_BlitInfo *info)
compositioning used (>>8 instead of /255) doesn't handle
it correctly. Also special-case alpha=0 for speed?
Benchmark this! */
- if (alpha) {
+ if (alpha) {
if (alpha == (SDL_ALPHA_OPAQUE >> 3)) {
*dstp = (Uint16)((s >> 9 & 0x7c00) + (s >> 6 & 0x3e0) + (s >> 3 & 0x1f));
} else {