Transparency and GIF

[LingHu01]

• OS: windows 10 (should not matter)
• Python: 3.11
• Pillow: 9.3.0

i'm trying to resize a GIF, unpacking every single frame i had no issue, they all have the correct transparent background, but when i try to create/save the GIF only the first frame will have the correct transparency, the other ones will have black background.

from PIL import Image

def process_gif(in_path, out_path= 'out.gif', size= (32,32)):
    with Image.open(in_path) as im :
        images = []
        durations = []
        for i in range(im.n_frames - 1, -1, -1):
            im.seek(i)
            im.thumbnail(size, Image.Resampling.LANCZOS) # noqa
            im_temp = im.copy()
            print(im_temp.info)
            images.insert(0, im_temp)
            durations.append(im.info['duration'])
        images[0].save(
            out_path,
            format='gif',
            save_all=True,
            append_images=images[1 :],
            duration=durations,
            disposal=2,
            optimize=False
        )
        for i, image in enumerate(images):
            image.save(f'{i}.png')

i iterate backwards because thumbnail will raise an error if i iterate normaly after one cicle(i think is because im.size pick the first frame for reference)

imput image :

those are the single frame my code produced:

output:

I tried to include background= (0, 0, 0, 0) but getcolor in ImagePalette.py will raise an ValueError: cannot add non-opaque RGBA color to RGB palette

upon further inspection using print(im_temp.info) before inserting the fram i got this result in console

{'version': b'GIF89a', 'background': 0, 'loop': 0, 'duration': 70}  #last frame
{'version': b'GIF89a', 'background': 0, 'loop': 0, 'duration': 70}
{'version': b'GIF89a', 'background': 0, 'loop': 0, 'duration': 70}
{'version': b'GIF89a', 'background': 0, 'loop': 0, 'duration': 70, 'transparency': 48, 'extension': (b'NETSCAPE2.0', 219)} #first frame

it may be related to transparency attribute?

[LingHu01]

using this after reading #6832 solved my problem

from PIL import GifImagePlugin
GifImagePlugin.LOADING_STRATEGY = GifImagePlugin.LoadingStrategy.RGB_AFTER_DIFFERENT_PALETTE_ONLY

[radarhere]

As an explanation, by default, GifImagePlugin does try to detect the transparency in your images, but after it is converted to P, the first pixel is (0, 0, 0, 1) - translucent, rather than transparent, which we can't express in a GIF.

If the default behaviour was to be improved here, it would be a conversion problem.