Manually alpha blending an RGBA pixel with an RGB pixel

Question

I am trying to do an alpha blend operation of an RGBA image (foreground image), on to a RGB image (background image). However, while doing so I think I may be doing the wron

Answer 1

alpha_blend_sse:)

;example usage
;mov eax, 0xabcdef
;mov edx, 0xAAAAAADD
;call alph_blend_see


; In\   EAX = background color (ZRBG) 32bit (Z mean zero, always is zero)
; In\   EDX = foreground color (RBGA) 32bit
; Out\  EAX = new color
alph_blend_sse:
    xor r14, r14
    xor r12, r12
    xor r13, r13

    movzx r15, dl               ; av: alpha number (0x00--->0xFF)
    movzx ecx, dl
    not ecx                     ; faster than 255 - dl
    mov r14b, cl                ; rem

    shr edx, 8
    and edx, 0x00FFFFFF
    mov r12d, edx
    mov r13d, eax               ; RBGA ---> ZRGB

    mov rax, 0x0000FF
    movq xmm3, rax

    ; s: eax
    ; d: edx

    ;=============================red = ((s >> 16) * rem + (d >> 16) * av) >> 8;
    movq xmm0, r12
    psrld xmm0, 0x10
    movq xmm1, r14
    pmuludq xmm1, xmm0
    movq xmm0, r13
    psrld xmm0, 0x10
    movq xmm2, r15
    pmuludq xmm2, xmm0
    addps xmm2, xmm1
    psrld xmm2, 0x8
    movq rax, xmm2
    mov r9b, al
    shl r9d, 8

    ;=============================green = (((s >> 8) & 0x0000ff) * rem + ((d >> 8) & 0x0000ff) * av) >> 8;
    movq xmm0, r12
    psrld xmm0, 0x8
    andps xmm0, xmm3
    movq xmm1, r14
    pmuludq xmm1, xmm0
    movq xmm0, r13
    psrld xmm0, 0x8
    andps xmm0, xmm3
    movq xmm2, r15
    pmuludq xmm2, xmm0
    addps xmm2, xmm1
    psrld xmm2, 0x8
    movq rax, xmm2
    mov r9b, al
    shl r9d, 8

    ;=============================blue = ((s & 0x0000ff) * rem + (d & 0x0000ff) * av) >> 8;
    movq xmm0, r12
    andps xmm0, xmm3
    movq xmm1, r14
    andps xmm1, xmm3
    pmuludq xmm1, xmm0
    movq xmm0, r13
    andps xmm0, xmm3
    movq xmm2, r15
    andps xmm2, xmm3
    pmuludq xmm2, xmm0
    addps xmm2, xmm1
    psrld xmm2, 0x8
    movq rax, xmm2
    mov r9b, al

    mov eax, r9d
    ret

Answer 2

A typical "Over" blend is done in the following way:

outputRed = (foregroundRed * foregroundAlpha) + (backgroundRed * (1.0 - foregroundAlpha));

And then repeating for the blue and green channels. Do this for every pixel.

Answer 3

It seems that you have missed out (1-Alpha) multiplier for background (currentPixelColor)