Support chapter (m4a)

I'm not sure if other formats also contain chapters, but I have a 2-hour-long M4A file with chapters. I want to display a list where I can navigate to the chapters position and see the current chapter title instead the file title. Therefore, it would be helpful if I could access the chapter metadata. It's not required to load this on folder scan. I would call another method to get advanced metadata. Btw. I would also load lyrics laters in separate method. But I don't know. Just some thoughts for optimizations. As I read, to read the chapters is a bit more complex. So I really would extract these with a separate method the dev can call when required. Same for lyrics, because it may contain a lot of text.

[ClementBeal]

Hi,

I just publishes 1.5.0 that should support the M4A chapters!

About the lyrics and the chapters having their own function, it's not planned yet. I think the library is already fast enough to load on the fly the full metadata of one file.

[domske]

Great, but I have two files.

Works

00 00 00 1C 66 74 79 70 4D 34 41 20 00 00 02 00 
4D 34 41 20 69 73 6F 6D 69 73 6F 32 00 00 00 08 
66 72 65 65 05 14 07 87 6D 64 61 74 00 1F First chapter title

Doesn't work

00 00 00 1C 66 74 79 70 4D 34 41 20 00 00 02 00 
4D 34 41 20 69 73 6F 6D 69 73 6F 32 00 00 00 08 
66 72 65 65 06 DD 8B FF 6D 64 61 74 00 0E First chapter title

both created with ffmpeg. weird.

[ClementBeal]

Great, but I have two files.

Works

00 00 00 1C 66 74 79 70 4D 34 41 20 00 00 02 00 
4D 34 41 20 69 73 6F 6D 69 73 6F 32 00 00 00 08 
66 72 65 65 05 14 07 87 6D 64 61 74 00 1F

Doesn't work

00 00 00 1C 66 74 79 70 4D 34 41 20 00 00 02 00 
4D 34 41 20 69 73 6F 6D 69 73 6F 32 00 00 00 08 
66 72 65 65 06 DD 8B FF 6D 64 61 74 00 0E

both created with ffmpeg. weird.

Could you send me the second file or the ffmpeg command please?

[domske]

I can't. Every attempt to censor the file resolves the issue and makes debugging impossible. I can't upload copyrighted content. It might be a problem with skipping right atom boxes when the order isn't as expected. For example skipping the mdat block. I fixed the problem for this file using faststart (moves moov atom to the beginning of the file).

ffmpeg -i input.m4a -c copy -movflags +faststart output.m4a

Btw, M4A files exported from GarageBand (iPad) have no metadata detected by audio_metadata_reader. I think that's because of the free block. So there may indeed be an issue with skipping, but I haven't reviewed your code yet. Maybe you try to read just some bytes at the beginning of the file or don't skip the right blocks? I will try to check that and create demo files as soon as I have enough time. If you want, you could try exporting an m4a file from GarageBand on the iPad, where +faststart also fixed the problem.

What I mean is that the problem is not just the chapters themselves, but a general problem that metadata is not always read, possibly due to the position of the metadata in the file. In case of the m4a created with GarageBand no metadata is read, not even the title. Some files also works without faststart, maybe because of different length of binary blocks, etc. In my optinion faststart should be the default, no other options. Weird decisions.

Update: The m4a before faststart didn't included "chpl" (Nero), but "chap" (QuickTime). Often hidden for some players becuase of nesting, which cleans up with faststart. But e.g. IINA (macOS) can read all the chapters. I also wrote a M4A Chapter Parser for this case, which reads that without issues.