Support paletted PNG with alpha

[pekkavaa]

Right now trying to save an image with an RGBA palette as PNG raises an exception

from PIL import Image
import numpy as np
palette = np.random.random_integers(0, 255, size=(16,4)).astype(np.uint8)
indices = np.random.random_integers(0, 16, size=(20, 10)).astype(np.uint16)
img = Image.fromarray(indices, mode='PA')
img.putpalette(palette)
img.save("test.png")

OSError: cannot write mode PA as PNG

But at least Wikipedia states this should be possible:

With indexed color images, the palette always stores trichromatic colors at a depth of 8 bits per channel (24 bits per palette entry). Additionally, an optional list of 8-bit alpha values for the palette entries may be included;

[radarhere]

I think this comes back to the same distinction as #9242 - PA images store a palette index and an independent alpha value for each pixel.

Your highlighted quote says, emphasis mine,

an optional list of 8-bit alpha values for the palette entries

What I think your highlighted quote is describing is P mode image with an RGBA palette, where the alpha values are for the palette entries, not for the pixel values. So if I had a palette with only two entries, (255, 0, 0, 127) and (0, 255, 0, 255), I must choose between translucent red and opaque green. With just those entries, I can't have opaque red.

That type of image can be saved as a PNG image with Pillow.

from PIL import Image
import numpy as np
palette = np.random.random_integers(0, 255, size=(16,4)).astype(np.uint8)
indices = np.random.random_integers(0, 16, size=(20, 10)).astype(np.uint16)
img = Image.fromarray(indices, mode='P')
img.putpalette(palette, 'RGBA')
img.save("test.png")

If you're interested in evidence from the specification,

[pekkavaa]

Thank you for the explanation. Indeed the difference between RGBA 'P' and 'PA' images was unclear to me.