Commit 7bf4319eb271363ee711d209d1db76e55d54fedd
2023-01-06T21:24:24
fix computation of alpha in BlitRGBtoRGBPixelAlphaMMX3DNOW (see #6990) ( same as BlitRGBtoRGBPixelAlphaMMX )
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diff --git a/src/video/SDL_blit_A.c b/src/video/SDL_blit_A.c
index 08a9d4e..04dfb7c 100644
--- a/src/video/SDL_blit_A.c
+++ b/src/video/SDL_blit_A.c
@@ -658,9 +658,19 @@ static void BlitRGBtoRGBPixelAlphaMMX3DNOW(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;
@@ -692,6 +702,24 @@ static void BlitRGBtoRGBPixelAlphaMMX3DNOW(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 */
+ /*
+ 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);