A script to do alpha-Blending of 2 RGBA values

rgba_blend.rb

def comp(_A, _R, _G, _B)
    _A<<24 | _R<<16 | _G<<8 | _B;
end

def decomp( num )
    _A = (num >> 24) & 0xFF;
    _R = (num >> 16) & 0xFF;
    _G = (num >> 8) & 0xFF;
    _B = (num >> 0) & 0xFF;
    [_A,_R,_G,_B]
end

def blend(srcARGB, backgroundARGB)
    src = srcARGB.hex
    dst = backgroundARGB.hex
    #assume premultiplied;
    srcA, srcR, srcG, srcB = decomp( src )
    dstA, dstR, dstG, dstB = decomp( dst )
    resultA = srcA + (255-srcA)*dstA/255;
    resultR = srcR + (255-srcA)*dstR/255;
    resultG = srcG + (255-srcA)*dstG/255;
    resultB = srcB + (255-srcA)*dstB/255;
    #
    result = comp(resultA, resultR, resultG, resultB)
    result.to_s(16)
end

result = blend(ARGV[0], ARGV[1])
puts "#{result}"