.avi with DIV3 FourCC and no audio track gets no preview and no thumbnail

[kickmeagainlater]

3.10 on macOS 26.5.2 (25F84), Apple Silicon.

A video-only AVI tagged DIV3 gets neither a preview nor a thumbnail (QuickTime says "not compatible"). Retagging the same bitstream to MP43 fixes it:

ffmpeg -f lavfi -i "testsrc2=size=640x480:rate=25:duration=20" \
  -c:v msmpeg4 -vtag DIV3 -b:v 1204k div3.avi
ffmpeg -i div3.avi -c copy -vtag MP43 mp43.avi   # works

tag	audio	result
DIV3	none	no preview, no thumbnail
DIV3	mp3	ok
MP43 (retag, same bitstream)	none	ok
XVID	none	ok
MP42	none	ok

Same at 624x352/29.97 as at 640x480/25, so it isn't geometry.

With no audio track the formatreader is never invoked at all — zero output on subsystem == "uk.org.marginal.qlvideo" so AVFoundation is demuxing and the track gets identified by FourCC. DIV3 isn't in videodecoder/Info.plist (or anywhere in the repo); DIVX, MP41, MP42, MP43, FMP4 are. Add an audio track and the formatreader does get the file, resolves it to MP43 via the catch-all after extract_extradata bails on msmpeg4v3, and it plays.

Possibly the same underlying thing as #180, though the symptom differs: XVID files with audio like that one preview fine here, and this fails with no thumbnail rather than a spinner. I assume it's #175 territory, and I realize you've said it's usually too late by the time AVFoundation gives up. What made me file anyway: the tag alone decides it on a byte-identical bitstream. If AVFoundation were claiming the codec I'd expect MP43 to fail the same way. Does it special-case DIV3 in particular?DIV3 is the common tag in the wild and MP43 is rare, so that would explain it,otherwise this may just be a missing registration:

<key>CodecType</key>
<string>DIV3</string>

0x4449_5633: AV_CODEC_ID_MSMPEG4V3,  // 'DIV3'

Untested unfortunately, as it seems mediaextension entitlements can't be provisioned on a personal team, and an ad-hoc build registers but never launches. 🥲

riff.c also maps MPG3, DIV4, DIV5, DIV6, DVX3, AP41, COL0, COL1 to msmpeg4v3, and DIV2 to msmpeg4v2.

[Marginal]

I'm working through various DivX issues for the next release. It's still not completely clear to me what AVFoundation is doing with .avi files, and the difference in behaviour depending on whether there's an audio channel is weird. But the absence of DIV3 as a registered CodecType certainly won't help.

I can test by retagging some of my existing MP43 sample files, but can you point to any publicly-available genuine DIV3 samples?

[kickmeagainlater]

Attached a genuine DIV3 sample, 5 seconds cut with -c copy out of an old Badi VHS rip. Audio stripped since that's the case that actually breaks.

Also, I stripped the audio off the DIV5 sample from samples.ffmpeg.org and it fails identically to DIV3, so it's not just that one tag. Same file with -vtag MP43 plays fine.

On the audio thing being weird: video-only XVID .avi gets no loadTrackInfo calls from the formatreader at all, but the videodecoder still gets invoked. So AVFoundation seems to demux those itself and hand the track over anyway, it just needs the fourcc registered to bite. I could be wrong, that's just from peaking at logs.

div3-genuine-sample.zip