[FFmpeg-devel] [PATCH] lavf/mov.c: Guess video codec delay based on PTS while parsing MOV header.

Derek Buitenhuis derek.buitenhuis at gmail.com
Wed Nov 15 01:23:08 EET 2017

On 11/14/2017 10:11 PM, Sasi Inguva wrote:
> I don't know if the patch can be made more generic to work for all
> demuxers, because this patch requires that PTS of all packets be available
> in the header.  The other route is to make it very specific to codecs with
> B-frames.

All PTS may not be available in MP4 either, in the live profile fragmented
MP4 case, no? I am not familiar enough with the fragmented MP4 demuxing code
say whether we seek to each fragment and make it available or not. I assume
you've tested such a case, or can (or know the fragment code)?

It's feasible to restrict it to codecs, I suppose.

>> You do not appear to be restricting this to one specific codec, or even
>> codecs
>> with B-frames.
>> I can make it specific to only H264 / H265.

The GOP structure should be applicable to most MPEG codecs, I think.

> It is true that the patch will fail to compute the correct delay in lots of
> cases. However the consequence of wrongly computing the has_b_frames is
> pretty benign. It only increases the buffer size held for PTS  reordering.
> I can put a max-value check on the computed video_delay here. So in my
> opinion, we won't see fatal regressions where a file is not able to be
> decoded / played.

It's true it's likely benign, but it's not exactly a "fix" either...

>From a user perspective, it's kinda possible to work around it "properly"
(I use that term loosely here) by decoding enough frames to fill the max
DPB for a given codec (e.g. 16 for H.264) in avformat_find_stream_info
(or in a user's code if they don't use that function), isn't it? Though,
I feel that particular fix won't be welcomed with open arms, due to a
~16x 'slow down' in probing.

Does anyone else have an opinion™ on this? I guess it's "OK enough" if
restricted by codec...? :V

- Derek

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