PIL merge of two images with alpha channels - not working as expected

Question

There\'s a bunch of questions here on SO which provide answers to the present question, however the output is not the expected.

The goal is to merge two RGBA images.

Answer 1

From your initial description, the following idea seems to be equivalent. Let X, Y be two RGBA images. Merge X and Y considering the bands RGB from X and the bands RGBA from Y, producing an image Z. Set the band A in Z to that of the band A in X. This contradicts your final statement, but it seems to give the expected output in this situation.

So, this is the code:

image = '1.png'
watermark = '2.png'

wmark = Image.open(watermark)
img = Image.open(image)

ia, wa = None, None
if len(img.getbands()) == 4:
    ir, ig, ib, ia = img.split()
    img = Image.merge('RGB', (ir, ig, ib))
if len(wmark.getbands()) == 4:
    wa = wmark.split()[-1]

img.paste(wmark, (0, 0), wmark)
if ia:
    if wa:
        # XXX This seems to solve the contradiction, discard if unwanted.
        ia = max_alpha(wa, ia)
    img.putalpha(ia)

img.save('result.png')

where the function max_alpha is:

def max_alpha(a, b):
    # Assumption: 'a' and 'b' are of same size
    im_a = a.load()
    im_b = b.load()
    width, height = a.size

    alpha = Image.new('L', (width, height))
    im = alpha.load()
    for x in xrange(width):
        for y in xrange(height):
            im[x, y] = max(im_a[x, y], im_b[x, y])
    return alpha

This new function seems to take into consideration the contradiction mentioned.