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