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from: http://dev.gameres.com/Program/Visual/2D/IntelAlpha.htm
Intel官方网站提供。

unsigned char *lpAlpha, // 256 级alpha通道
unsigned int iAlpPitch, // alpha通道的pitch
unsigned char *lpSrc, // 原色彩缓冲
unsigned int iSrcX, //
unsigned int iSrcY, // 原色彩位置
unsigned int iSrcPitch, // 原色彩pitch
unsigned char *lpDst, // 目标缓冲
unsigned int iDstX,
unsigned int iDstY, // 目标位置
unsigned int iDstW,
unsigned int iDstH, // 目标缓冲的尺寸
unsigned int iDstPitch // 目标缓冲的pitch
void ablend_565(unsigned char *lpAlpha, unsigned int iAlpPitch,
unsigned char *lpSrc, unsigned int iSrcX, unsigned int iSrcY,
unsigned int iSrcPitch, unsigned char *lpDst,
unsigned int iDstX, unsigned int iDstY,
unsigned int iDstW, unsigned int iDstH,
unsigned int iDstPitch) {

//Mask for isolating the red,green, and blue components
static __int64 MASKB=0x001F001F001F001F;
static __int64 MASKG=0x07E007E007E007E0;
static __int64 MASKSHIFTG=0x03F003F003F003F0;
static __int64 MASKR=0xF800F800F800F800;

//constants used by the integer alpha blending equation
static __int64 SIXTEEN=0x0010001000100010;
static __int64 FIVETWELVE=0x0200020002000200;
static __int64 SIXONES=0x003F003F003F003F;

unsigned char *lpLinearDstBp=(iDstX<<1)+(iDstY*iDstPitch)+lpDst; //base pointer for linear destination
unsigned char *lpLinearSrcBp=(iSrcX<<1)+(iSrcY*iSrcPitch)+lpSrc; //base pointer for linear source
unsigned char *lpLinearAlpBp=iSrcX+(iSrcY*iAlpPitch)+lpAlpha; //base pointer for linear alpha

_asm{
mov esi,lpLinearSrcBp; //src
mov edi,lpLinearDstBp; //dst
mov eax,lpLinearAlpBp; //alpha
mov ecx,iDstH; //ecx=number of lines to copy
mov ebx,iDstW; //ebx=span width to copy

test esi,6; //check if source address is qword aligned

//since addr coming in is always word aligned(16bit)
jnz done; //if not qword aligned we don't do anything

primeloop:
movd mm1,[eax]; //mm1=00 00 00 00 a3 a2 a1 a0
pxor mm2,mm2; //mm2=0;
movq mm4,[esi]; //g1: mm4=src3 src2 src1 src0
punpcklbw mm1,mm2; //mm1=00a3 00a2 00a1 00a0

loopqword:
mov edx,[eax];
test ebx,0xFFFFFFFC; //check if only 3 pixels left
jz checkback; //3 or less pixels left

//early out tests
cmp edx,0xffffffff; //test for alpha value of 1
je copyback; //if 1's copy the source pixels to the destination

test edx,0xffffffff; //test for alpha value of 0
jz leavefront; //if so go to the next 4 pixels

//the alpha blend starts
//green
//i=a*sg+(63-a)*dg;
//i=(i+32)+((i+32)>>6)>>6;
//red
//i=a*sr+(31-a)*dr;
//i=(i+16)+((i+16)>>5)>>5;
movq mm5,[edi]; //g2: mm5=dst3 dst2 dst1 dst0
psrlw mm1,2; //mm1=a?>>2 nuke out lower 2 bits
movq mm7,MASKSHIFTG; //g3: mm7=1 bit shifted green mask
psrlw mm4,1; //g3a: move src green down by 1 so that we won't overflow
movq mm0,mm1; //mm0=00a3 00a2 00a1 00a0
psrlw mm5,1; //g3b: move dst green down by 1 so that we won't overflow
psrlw mm1,1; //mm1=a?>>1 nuke out lower 1 bits
pand mm4,mm7; //g5: mm4=sg3 sg2 sg1 sg0
movq mm2,SIXONES;//g4: mm2=63
pand mm5,mm7; //g7: mm5=dg3 dg2 dg1 dg0
movq mm3,[esi]; //b1: mm3=src3 src2 src1 src0
psubsb mm2,mm0; //g6: mm2=63-a3 63-a2 63-a1 63-a0
movq mm7,MASKB; //b2: mm7=BLUE MASK
pmullw mm4,mm0; //g8: mm4=sg?*a?
movq mm0,[edi]; //b3: mm0=dst3 dst2 dst1 dst0
pmullw mm5,mm2; //g9: mm5=dg?*(1-a?)
movq mm2,mm7; //b4: mm2=fiveones
pand mm3,mm7; //b4: mm3=sb3 sb2 sb1 sb0
pmullw mm3,mm1; //b6: mm3=sb?*a?
pand mm0,mm7; //b5: mm0=db3 db2 db1 db0
movq mm7,[esi]; //r1: mm7=src3 src2 src1 src0
paddw mm4,mm5; //g10: mm4=sg?*a?+dg?*(1-a?)
pand mm7,MASKR; //r2: mm7=sr3 sr2 sr1 sr0　
psubsb mm2,mm1; //b5a: mm2=31-a3 31-a2 31-a1 31-a0
paddw mm4,FIVETWELVE; //g11: mm4=(mm4+512) green
pmullw mm0,mm2; //b7: mm0=db?*(1-a?)
movq mm5,mm4; //g12: mm5=mm4 green
psrlw mm7,11; //r4: shift src red down to position 0
psrlw mm4,6; //g13: mm4=mm4>>6
paddw mm4,mm5; //g14: mm4=mm4+mm5 green
paddw mm0,mm3; //b8: mm0=sb?*a?+db?*(1-a?)
movq mm5,[edi]; //r3: mm5=dst3 dst2 dst1 dst0
paddw mm0,SIXTEEN; //b9: mm0=(mm0+16) blue
pand mm5,MASKR; //r5: mm5=dr3 dr2 dr1 dr0
psrlw mm4,5; //g15: mm4=0?g0 0?g0 0?g0 0?g0 green
movq mm3,mm0; //b10: mm3=mm0 blue
psrlw mm0,5; //b11: mm0=mm0>>5 blue
psrlw mm5,11; //r6: shift dst red down to position 0
paddw mm0,mm3; //b12: mm0=mm3+mm0 blue
psrlw mm0,5; //b13: mm0=000b 000b 000b 000b blue
pmullw mm7,mm1; //mm7=sr?*a?
pand mm4,MASKG; //g16: mm4=00g0 00g0 00g0 00g0 green
pmullw mm5,mm2; //r7: mm5=dr?*(31-a?)
por mm0,mm4; //mm0=00gb 00gb 00gb 00gb

add eax,4; //move to next 4 alphas
add esi,8; //move to next 4 pixels in src
add edi,8; //move to next 4 pixels in dst

movd mm1,[eax]; //mm1=00 00 00 00 a3 a2 a1 a0
paddw mm5,mm7; //r8: mm5=sr?*a?+dr?*(31-a?)
paddw mm5,SIXTEEN; //r9: mm5=(mm5+16) red
pxor mm2,mm2; //mm2=0;
movq mm7,mm5; //r10: mm7=mm5 red
psrlw mm5,5; //r11: mm5=mm5>>5 red
movq mm4,[esi]; //g1: mm4=src3 src2 src1 src0
paddw mm5,mm7; //r12: mm5=mm7+mm5 red
punpcklbw mm1,mm2; //mm1=00a3 00a2 00a1 00a0
psrlw mm5,5; //r13: mm5=mm5>>5 red
psllw mm5,11; //r14: mm5=mm5<<10 red
por mm0,mm5; //mm0=0rgb 0rgb 0rgb 0rgb
sub ebx,4; //polished off 4 pixels
movq [edi-8],mm0; //dst=0rgb 0rgb 0rgb 0rgb

jmp loopqword; //go back to start

copyback:
movq [edi],mm4; //copy source to destination

leavefront:
add edi,8; //advance destination by 4 pixels
add eax,4; //advance alpha by 4
add esi,8; //advance source by 4 pixels
sub ebx,4; //decrease pixel count by 4
jmp primeloop;

checkback:
test ebx,0xFF; //check if 0 pixels left
jz nextline; //done with this span

//backalign: //work out back end pixels
movq mm5,[edi]; //g2: mm5=dst3 dst2 dst1 dst0
psrlw mm1,2; //mm1=a?>>2 nuke out lower 2 bits
movq mm7,MASKSHIFTG; //g3: mm7=shift 1 bit green mask
psrlw mm4,1; //g3a: move src green down by 1 so that we won't overflow
movq mm0,mm1; //mm0=00a3 00a2 00a1 00a0
psrlw mm5,1; //g3b: move dst green down by 1 so that we won't overflow
psrlw mm1,1; //mm1=a?>>1 nuke out lower 1 bits
pand mm4,mm7; //g5: mm4=sg3 sg2 sg1 sg0
movq mm2,SIXONES;//g4: mm2=63
pand mm5,mm7; //g7: mm5=dg3 dg2 dg1 dg0
movq mm3,[esi]; //b1: mm3=src3 src2 src1 src0
psubsb mm2,mm0; //g6: mm2=63-a3 63-a2 63-a1 63-a0
movq mm7,MASKB; //b2: mm7=BLUE MASK
pmullw mm4,mm0; //g8: mm4=sg?*a?
movq mm0,[edi]; //b3: mm0=dst3 dst2 dst1 dst0
pmullw mm5,mm2; //g9: mm5=dg?*(1-a?)
movq mm2,mm7; //b4: mm2=fiveones
pand mm3,mm7; //b4: mm3=sr3 sr2 sr1 sr0
pmullw mm3,mm1; //b6: mm3=sb?*a?
pand mm0,mm7; //b5: mm0=db3 db2 db1 db0
movq mm7,[esi]; //r1: mm7=src3 src2 src1 src0
paddw mm4,mm5; //g10: mm4=sg?*a?+dg?*(1-a?)
pand mm7,MASKR; //r2: mm7=sr3 sr2 sr1 sr0
psubsb mm2,mm1; //b5a: mm2=31-a3 31-a2 31-a1 31-a0
paddw mm4,FIVETWELVE; //g11: mm4=(i+512) green
pmullw mm0,mm2; //b7: mm0=db?*(1-a?)
movq mm5,mm4; //g12: mm5=(i+512) green
psrlw mm7,11; //r4: shift src red down to position 0
psrlw mm4,6; //g13: mm4=(i+512)>>6
paddw mm4,mm5; //g14: mm4=(i+512)+((i+512)>>6) green
paddw mm0,mm3; //b8: mm0=sb?*a?+db?*(1-a?)
movq mm5,[edi]; //r3: mm5=dst3 dst2 dst1 dst0
paddw mm0,SIXTEEN; //b9: mm0=(i+16) blue
pand mm5,MASKR; //r5: mm5=dr3 dr2 dr1 dr0
psrlw mm4,5; //g15: mm4=0?g0 0?g0 0?g0 0?g0 green
movq mm3,mm0; //b10: mm3=(i+16) blue
psrlw mm0,5; //b11: mm0=(i+16)>>5 blue
psrlw mm5,11; //r6: shift dst red down to position 0
paddw mm0,mm3; //b12: mm0=(i+16)+(i+16)>>5 blue
psrlw mm0,5; //b13: mm0=000r 000r 000r 000r blue
pmullw mm7,mm1; //mm7=sr?*a?
pand mm4,MASKG; //g16: mm4=00g0 00g0 00g0 00g0 green
pmullw mm5,mm2; //r7: mm5=dr?*(31-a?)
por mm0,mm4; //mm0=00gb 00gb 00gb 00gb

add eax,4; //move to next 4 alphas

//stall
paddw mm5,mm7; //r8: mm5=sr?*a?+dr?*(31-a?)
paddw mm5,SIXTEEN; //r9: mm5=(i+16) red
movq mm7,mm5; //r10: mm7=(i+16) red
psrlw mm5,5; //r11: mm5=(i+16)>>5 red
paddw mm5,mm7; //r12: mm5=(i+16)+((i+16)>>5) red
psrlw mm5,5; //r13: mm5=(i+16)+((i+16)>>5)>>5 red
psllw mm5,11; //r14: mm5=mm5<<10 red
por mm0,mm5; //mm0=0rgb 0rgb 0rgb 0rgb

test ebx,2; //check if there are 2 pixels
jz oneendpixel; //goto one pixel if that's it

movd [edi],mm0; //dst=0000 0000 0rgb 0rgb
psrlq mm0,32; //mm0>>32
add edi,4; //edi=edi+4
sub ebx,2; //saved 2 pixels
jz nextline; //all done goto next line

oneendpixel: //work on last pixel
movd edx,mm0; //edx=0rgb
mov [edi],dx; //dst=0rgb

nextline: //goto next line
dec ecx; //nuke one line
jz done; //all done

mov eax,lpLinearAlpBp; //alpha
mov esi,lpLinearSrcBp; //src
mov edi,lpLinearDstBp; //dst
add eax,iAlpPitch; //inc alpha ptr by 1 line
add esi,iSrcPitch; //inc src ptr by 1 line
add edi,iDstPitch; //inc dst ptr by 1 line
mov lpLinearAlpBp,eax; //save new alpha base ptr
mov ebx,iDstW; //ebx=span width to copy
mov lpLinearSrcBp,esi; //save new src base ptr
mov lpLinearDstBp,edi; //save new dst base ptr
jmp primeloop; //start the next span

done:
emms
}
}