Animated PNG becomes static after conversion

[romanlv]

Possible bug

Is this a possible bug in a feature of sharp, unrelated to installation?

• Running npm install sharp completes without error.
• Running node -e "require('sharp')" completes without error.

Are you using the latest version of sharp?

• I am using the latest version of sharp as reported by npm view sharp dist-tags.latest.

What is the output of running npx envinfo --binaries --system --npmPackages=sharp --npmGlobalPackages=sharp?

System:
OS: macOS 12.6
CPU: (10) arm64 Apple M1 Pro
Memory: 92.67 MB / 16.00 GB
Shell: 5.8.1 - /bin/zsh
Binaries:
Node: 16.13.2 - /usr/local/bin/node
Yarn: 1.22.19 - ~/.npm-packages/bin/yarn
npm: 8.19.2 - ~/.npm-packages/bin/npm
npmPackages:
sharp: ^0.31.3 => 0.31.3

What are the steps to reproduce?

$ curl -Os https://ezgif.com/images/format-demo/butterfly.png


$ identify apng:butterfly.png
apng:butterfly.png=>butterfly.png[0] APNG 250x250 250x250+0+0 8-bit sRGB 534048B 0.040u 0:00.040
apng:butterfly.png=>butterfly.png[1] APNG 181x178 250x250+24+20 8-bit sRGB 0.040u 0:00.040
apng:butterfly.png=>butterfly.png[2] APNG 181x178 250x250+24+20 8-bit sRGB 0.040u 0:00.039
apng:butterfly.png=>butterfly.png[3] APNG 181x178 250x250+24+20 8-bit sRGB 0.040u 0:00.039
apng:butterfly.png=>butterfly.png[4] APNG 181x178 250x250+24+20 8-bit sRGB 0.040u 0:00.038
apng:butterfly.png=>butterfly.png[5] APNG 181x178 250x250+24+20 8-bit sRGB 0.030u 0:00.037
apng:butterfly.png=>butterfly.png[6] APNG 181x178 250x250+24+20 8-bit sRGB 0.030u 0:00.037
....


$ node -e "require('sharp')('butterfly.png', { animated: true }).resize({ width: 150 }).toFile('butterfly-out.png')"


$ identify apng:butterfly-out.png
apng:butterfly-out.png=>butterfly-out.png APNG 150x150 150x150+0+0 8-bit sRGB 6056B 0.010u 0:00.001

What is the expected behaviour?

Converted image to continue to have animation

[romanlv]

it works fine for webp file formats (when input and output are webp e.g. file)

[lovell]

Please see #2375