[Matroska-devel] requesting "AvgTimePerFrame" or "FieldRate" header field

madshi madshi at gmail.com
Sun Dec 16 11:09:40 CET 2012


 > Wasn't your original goal to display the content at the original
> frame rate it was recorded ?

My original goal was simply to have a reliable frame rate indicator for MKV
video tracks, which is currently not available.

It is true that I want to use this to decide the display refresh rate etc.
But that is just my personal end goal. From a design point of view the new
frame rate indicator should ideally be compatible to all other containers
to make development easy. Other containers usually just pass the
h264/MPEG2/VC-1 frame rate information through, so MKV should do that, too,
to stay compatible.

> If so I fail to understand how writing 30000/1001 for 24p
> content is good.

For true 24p content (e.g. Blu-Ray) 24000/1001 should be written.

I suppose you're wondering about soft-telecined content (which is encoded
as 24 frames with telecine flags)? This content type practically tells the
decoder to decode the 24 frames and then to interlace them to 30i and
output them this way. Whether the decoder really does that, whether a
deinterlacer is sitting behind the decoder etc are things that the MKV
muxer can't possibly know. So a frame rate of 30000/1001 should be written
to the header (or 60000/1001 as the *field* rate).

Basically we have to find a crystal clear definition of what the new MKV
frame/field rate field specifies. And since the MKV muxer doesn't know
whether a deinterlacer will be in the display chain and what the
deinterlacer will do exactly, it doesn't make sense to specify the "final
display refresh rate" in the MKV because the MKV muxer just doesn't know
that. Instead it makes sense to store the frame rate that the decoder
should output, because there's no confusion about that. And that happens to
match exactly how h264/MPEG2 and VC-1 specify the frame rate in their
bitstream.

Best regards, Mathias.
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