Just done some more experiment. This issue only affect __Chromium__ but not Google Chrome (Compiled from AUR, google-chrome package).

Google Chrome can play APNG normally in all three situations.

Google Chrome version: 60.0.3112.90, 64 bit, stable

hcm@, can you clarify this?

I'm not sure why you'd see different results on Chrome vs Chromium...something else coming into play at build time?  I've tested versions 59, 60, and 61 on Ubuntu and cannot repro...

My first thought was that this was the same as  issue 722072 , but I cannot repro even if I revert  issue 722072 's fix

Just tried testing this out on different Linux system inside virtual box. Can only reproduce this bug under Arch linux.

Tried under following versions without issue:
Ubuntu 16.04.3 LTS
Ubuntu 17.04
Fedora 26

Arch linux with i3 as WM have issue.


Another test I have tried is to run chromium using this script, inside my host system with arch linux. No issue on playing APNG with that build.

The script: https://github.com/scheib/chromium-latest-linux

I suspect this is an issue with Arch Linux's build. Should I file a report to them?

Yes, please file against the Arch linux distribution. It only happens there and it's not a distribution we work with directly.

Let's close this as WontFix, please go to Arch Linux bug tracker for
reporting the issue.
If the real culprit remains in Chromium side, please feel free to file a
new separate bug, or reopen this if appropriate.

Filed to Arch issue tracker: https://bugs.archlinux.org/task/55093

Just to make a reference ;)

Great, thanks!

The issue appears to be related to the fact that libpng on Arch is patched for APNG support. [1] (With non-patched system libpng, the images are animated correctly.)

The following warnings (originating from code in the libpng-apng patch [1]) are printed repeatedly when viewing the bouncing ball image:

    libpng warning: size in first frame's fcTL must match the size in IHDR
    libpng warning: Skipped (ignored) a chunk between APNG chunks

Could Chromium's PNG decoder be made to work with system libpng carrying the apng patch?

[1] https://sourceforge.net/projects/libpng-apng/

I'm not sure what Chromium's policy is for working with alternative versions of libraries that we're not testing.

That said, I have an idea of what might be going wrong here. Chromium works without the APNG patch by modifying the data it passes to libpng - e.g. it uses a modified IHDR with the size of the fcTL, so that they always match. Let's say the IDAT is part of the image, and we're decoding a frame after that, whose size does not match the IHDR chunk. We'll modify the IHDR to match the size of the fcTL in PNGImageReader::StartFrameDecoding [1]. The problem is, we then tell libpng to process the other chunks leading up to the IDAT. Since the first frame is part of the image, it looks something like:

IHDR
fcTL (matches IHDR)
IDAT
fcTL (does not match IHDR)
fdAT

So at [2], we (incidentally) tell libpng to process the first fcTL, which does not match the modified IHDR (which matches the second fcTL).

A couple of possible solutions:
- Make this method skip over APNG chunks, so it treats the image as a standard PNG
- Tell libpng to ignore APNG chunks. IIUC, png_set_keep_unknown_chunks can be used to tell libpng to let the client handle (most) chunks. I don't think this will currently work because the APNG-patched libpng checks for fcTL [3] before looking at the user-chunks [4]. Standard libpng already makes an exception - user cannot override handling of IDAT or IEND. Not sure whether it was a deliberate decision to disallow overriding APNG chunks.

[1] https://chromium.googlesource.com/chromium/src/+/03bbf72bf0b90103dd7f016f378de5e373a7892f/third_party/WebKit/Source/platform/image-decoders/png/PNGImageReader.cpp#192
[2] https://chromium.googlesource.com/chromium/src/+/03bbf72bf0b90103dd7f016f378de5e373a7892f/third_party/WebKit/Source/platform/image-decoders/png/PNGImageReader.cpp#218
[3] https://sourceforge.net/p/libpng-apng/code/ci/master/tree/pngpread.c#l238
[4] https://sourceforge.net/p/libpng-apng/code/ci/master/tree/pngpread.c#l367