Superimpose function does not superimpose

[Fresheyeball]

The superimpose function in Graphics.Image.Processing does not superimpose. It replaces the pixels in the target image with the overlay image. While useful, this is not the expected behavior. Especially, with superimposed images with alpha channels.

[Fresheyeball]

superimpose'
  :: Array arr RGBA e
  => AlphaSpace RGBA e
  => Fractional e => Ord e
  => (Int, Int)     -- ^ @(i, j)@ starting index from within a source image.
  -> Image arr RGBA e -- ^ Image to be positioned above the source image.
  -> Image arr RGBA e -- ^ Source image.
  -> Image arr RGBA e
superimpose' (!i0, !j0) !imgA !imgB = traverse2 imgB imgA const newPx where
  !(m, n) = dims imgA
  newPx getPxB getPxA (i, j) =
    let !(i', j') = (i - i0, j - j0)
        new = getPxA (i', j')
        old = getPxB (i, j)
    in if i' >= 0 && j' >= 0 && i' < m && j' < n then overlayAlpha old new else old


overlayAlpha :: Elevator e => Ord e => Fractional e => Pixel RGBA e -> Pixel RGBA e -> Pixel RGBA e
overlayAlpha base overlay = setPxC (liftPx2 f base overlay) AlphaRGBA 1
  where f b o = b + (getAlpha overlay * (o - b))

This works, but makes me wish that AlphaSpace came with a setAlpha so this could be more abstract.

[lehins]

Thanks for submitting this. I'll make sure to get it fixed in a new version of hip, once I do get to the rewrite I have planned.
For now you could probably simulate setAlpha with setAlpha a = addAlpha a . dropAlpha

[Javran]

Thanks @Fresheyeball for this workaround - it worked fine but it generates a greenish color when there are section where pixels of both image are transparent. So I checked Wikipedia, following https://en.wikipedia.org/wiki/Alpha_compositing I have an improved version of overlayAlpha that works as expected:

overlayAlpha
  :: (Elevator e, Ord e, Fractional e)
  => Pixel RGBA e
  -> Pixel RGBA e
  -> Pixel RGBA e
overlayAlpha bPxa oPxa =
  addAlpha rAlp (liftPx2 f (dropAlpha bPxa) (dropAlpha oPxa))
  where
    bAlp = getAlpha bPxa
    oAlp = getAlpha oPxa
    rAlp = oAlp + bAlp * (1 - oAlp)
    f b o =
      if rAlp == 0
        then 0
        else (o * oAlp + b * bAlp * (1 - oAlp)) / rAlp