Converting animated gif to webp produces faulty images

[madeddy]

What did you do?

Converted some animated gifs to animated webp. Used different qualtity settings.

What did you expect to happen?

Getting a identical converted image. Beside the(logical) format and possible size change.

What actually happened?

Webp outputs are defect. Three different problems appear.
The output images:

1. play too slow
2. do not play in a image viewer (gwenview for me OR in Pillow's Image.show() method)
3. some images have after convert also visual defects like wrong pixels (flickering and line style artifacts)

ps.: Converting the same images with the same settings with webp CLI tools works without issues.

What are your OS, Python and Pillow versions?

• OS: Kubuntu 18.04.3
• Python: 3.6.9
• Pillow: 6.2.1

Code example

Python:

from pathlib import Path as pt
from PIL import Image

src_f = pt("animated.gif")
dst_f = pt(src_f).with_suffix('.webp')

# other used quality settings for fast copy past:
# {'lossless': True} {'allow_mixed': True}
quali = {'quality': 90}

Image.open(src_f).save(dst_f, 'webp', **quali, save_all=True)

Shell variant:

gif2webp -v -lossy -q 90 paris_metro.gif -o paris_metro.webp

Example images

Content of the archive:

• Two input gifs
• both output files (converted with "quality 90") with problems 1. and 2.
• paris_metro.webp has also problem 3. present. (pixel errors are to see while playing)
  anim.tar.gz

[radarhere]

I'm unable to replicate 2. The following code works for me with your attached images.

from PIL import Image
Image.open("animated.webp").show()
Image.open("paris_metro.webp").show()

For the record, 3 isn't a problem with WEBP, it's a problem with Pillow loading the GIF.

[radarhere]

To fix 1, you just need to pass in the duration -

im = Image.open(src_f)
im.save(dst_f, 'webp', **quali, duration=im.info["duration"], save_all=True)

[madeddy]

OK. Lets see.

1. With duration passed in it works now correct. (What the ...! Not true. I come back to this.)
2. Was a two pronged problem:

• Not playing with .show() - i did'n notice it uses a viewer app from underlying OS. So its not pillow related - totally my fault (For me it uses imagemagick and still doesn't play.)
• Did'n work in Kubuntu's gwenview, but now it does play. Was imo caused by 1.

3. Um... Does this mean the convert is ok, however the input gets corrupted because of a faulty read in?

Problem 1. is solvable with the mentioned workaround but i consider this now even more a bug as before. Pillow has the input file and can get all needed infos by himself. Or whats next?

Do you find it a acceptable approach if i would make a enhancement issue out of it? Chances? So basically pillow takes the duration in the above fashion, if not passed in by user.

And thanks for taking the time.
Greets

[radarhere]

1 - Pillow values backwards compatibility. So I don't think that changing defaults is likely to be accepted.
3 - Saving is ok, but reading is not. This may or may not be helpful to you, but it's more an explanation here.

[madeddy]

For 3) Ah no, thats good. The explanation is appreciated. Even if i cannot do anything about it, but i hope you folks find the problem.
For 1) I feared something like this. https://pillow.readthedocs.io/en/latest/reference/Image.html#PIL.Image.info A pity.
To get this under control, Pillow would imho need a extended analysis module for the input image attributes with tons of checks and rechecks and compares and tests etc.

So to clears this up: Basically are the problems 1 and 2 resolved by a workaround. The docs should mention this in a more visible way.

[radarhere]

Testing, I find 3 is resolved by #5333

[radarhere]

I've created #5338 to change the default behaviour for 1, since I realised that GIF already behaves in this way.

[madeddy]

Really great! 🥳
Thanks for your work and getting back to this. (This can then be closed with the successful pull merge.)

Greets