Saving to apng has wrong durations

[Yay295]

Using https://github.com/python-pillow/Pillow/blob/main/Tests/images/apng/different_durations.png

from PIL import Image, ImageSequence

ddp = Image.open('different_durations.png')
print([frame.info.get('duration') for frame in ImageSequence.Iterator(ddp)]) # [4000.0, 1000.0]
ddp.save('different_durations.gif',save_all=True)

ddg = Image.open('different_durations.gif')
print([frame.info.get('duration') for frame in ImageSequence.Iterator(ddg)]) # [4000, 1000]
ddg.save('same_durations.png',save_all=True)

sdp = Image.open('same_durations.png')
print([frame.info.get('duration') for frame in ImageSequence.Iterator(sdp)]) # [1000.0, 1000.0]

Also, ImageMagick (version 7.1.1-33) says the duration of the second frame is actually 990, not 1000.

> magick identify -format %T\n apng:different_durations.png
400
99

I'm not sure what's going on there though, because the duration parsing isn't complicated.

Pillow/src/PIL/PngImagePlugin.py

Lines 669 to 672 in 219add0

	delay_num, delay_den = i16(s, 20), i16(s, 22)
	if delay_den == 0:
	delay_den = 100
	self.im_info["duration"] = float(delay_num) / float(delay_den) * 1000

[radarhere]

Regarding saving to APNG, I've created #8104

I'm having trouble convincing ImageMagick to tell me the durations on my local machine or on GitHub Actions. I opened different_durations.png with https://davidmz.github.io/apng-js/ and it reported 4000 and 1000. If you think that ImageMagick isn't reading the duration according to the specification, then maybe that's something for you to raise with ImageMagick?

[Yay295]

I don't know why I didn't check this before, but saving from APNG to APNG has the same issue; it not only when the source is a GIF.

from PIL import Image, ImageSequence
ddp = Image.open('different_durations.png')
ddp.save('ddc.png',save_all=True)
ddc = Image.open('ddc.png')
print([frame.info.get('duration') for frame in ImageSequence.Iterator(ddc)]) # [1000.0, 1000.0]

I think it probably is an issue in ImageMagick, or perhaps in libpng. I've filed an issue with ImageMagick though.

[radarhere]

The ImageMagick issue is ImageMagick/ImageMagick#7371

[radarhere]

ImageMagick has made a change to fix their problem.