You are right, AVI container has no aspect ratio flagging.
But MKV, MOV/QT, MP4 and WebM has it.
So...?
Aspect ratio when cropping:
If my orig frame has a SAR of 1 (=quadratic) and I chop off the upper half,
the SAR (= frame dimension ratio) will be halved.
If I keep the DAR constant then, the frame will be
stretched vertically by 2 when displaying.
As I said: Confused.
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