Transparent png converted to green

[German-Corpaz]

Hi, I'm using libimagequant library to optimize a png, and the result is perfect, the only problem is that pngs with transparent background are converted to a green background, i don't know why this is happening, the palette generated is correct is there a solution for this, apart from replacing colors?

Code:

img = img.convert('RGBA')
attr = liq.Attr()
input_image = attr.create_rgba(img.tobytes(), img.width, img.height, 0)
result = input_image.quantize(attr)
out_pixels = result.remap_image(input_image)
out_palette = result.get_palette()
out_img = Image.frombytes('P', (img.width, img.height), out_pixels)
palette_data = []
for color in out_palette:
    palette_data.append(color.r)
    palette_data.append(color.g)
    palette_data.append(color.b)
out_img.putpalette(palette_data)
out_img.save('output.png')

[hugovk]

Please can you share an example image that causes this?

Your code isn't self-contained. Please share runnable example code. For example, we don't know what's in img, or what liq is.

[German-Corpaz]

Sorry about that

from PIL import Image
import libimagequant as liq

img = Image.open("test.png")
img = img.convert('RGBA')
attr = liq.Attr()
input_image = attr.create_rgba(img.tobytes(), img.width, img.height, 0)
result = input_image.quantize(attr)
out_pixels = result.remap_image(input_image)
out_palette = result.get_palette()
out_img = Image.frombytes('P', (img.width, img.height), out_pixels)
palette_data = []
for color in out_palette:
    palette_data.append(color.r)
    palette_data.append(color.g)
    palette_data.append(color.b)
out_img.putpalette(palette_data)
out_img.save('output.png')

[wiredfool]

Have you tried something like this:

img = Image.open("test.png")
quantized = img.quantize(method=Image.LIBIMAGEQUANT)
quantized.save('out.png')

https://github.com/python-pillow/Pillow/blob/master/src/PIL/Image.py#L1039

[German-Corpaz]

I didnt build pillow to use libimagequant, thats why im doing it the other way.

But i think the error of the bg from transparent to green is coming in the putpalette method, i dont know why but its not using transparency

[wiredfool]

There are a few ways to do transparency, either using 'PA' mode (paletted w/ alpha) or setting the im.info['transparency'] value.

But the basic palletted image doesn't have any transparency, as you can see by passing in the rgb values, but not alpha.

[ghost]

Issue fixed by RoadrunnerWMC. Please see RoadrunnerWMC/libimagequant-python#1.

[gofr]

putpalette doesn't look like it supports alpha. But there's an apparently undocumented function that does. Add the following line before the save() at the end and I think it works:

out_img.im.putpalettealphas(bytes(c.a for c in out_palette))

[radarhere]

Using the suggestion from @gofr as a starting point, I've put together PR #5089. If that is merged, then the last few lines of your code could be changed to

palette_data = []
for color in out_palette:
    palette_data.append(color.r)
    palette_data.append(color.g)
    palette_data.append(color.b)
    palette_data.append(color.a)
out_img.putpalette(palette_data, "RGBA")
out_img.save('output.png')

and the correct result would be generated.