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fix computation of alpha in BlitRGBtoRGBPixelAlphaMMX3DNOW (see #6990)

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( same as BlitRGBtoRGBPixelAlphaMMX )
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Sylvain 2023-01-06 21:24:24 +01:00
parent 0ebda87425
commit 7bf4319eb2
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GPG key ID: 5F87E02E5BC0939E
1 changed files with 29 additions and 1 deletions
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30
src/video/SDL_blit_A.c
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@ -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);