transparency pixel shader

transparency:
    ld  a, (de)
    ex  af, af'
    add hl, sp
    ld  a, h
    exx
    adc hl, de
    ld  h, a
    ex  af, af'
    ld  c, a
    ld  b, (hl)
    ld  a, (bc)
    res 0, h
    add a, (hl)
    exx
    ld  (de), a
    inc de