[FFmpeg-user] I found the bugs

[Mark Filipak]

On 17/06/2024 13.50, Paul B Mahol wrote:
> On Mon, Jun 17, 2024 at 5:35 PM Mark Filipak <markfilipak.imdb at gmail.com>
> wrote:
> 
>> On 17/06/2024 03.16, Paul B Mahol wrote:
>>> On Mon, Jun 17, 2024 at 3:16 AM Mark Filipak <markfilipak.imdb at gmail.com
>>>
>>> wrote:
>>>
>>>> To expand on my idea:
>>>>
>>>> The video is this:
>>>> I P B B P B B P B B I P B B..
>>>> I want to change it to this:
>>>> I B B P B B P B B P I B B P..
>>>>
>>>
>>> If this is what I think it is, your are wasting your time.
>>
>> It is possible. But can FFmpeg do it? You see, Paul, I read in several
>> places from seemingly
>> authoritative people that professional videos are PTS-ordered. I've since
>> discovered that
>> professional videos are almost always DTS-ordered -- I now have a packet
>> browser and have seen it to
>> be true. However, I have seen professional videos that are PTS-ordered but
>> I didn't think too much
>> about it at the time.
> 
> PTS order is order once frames are decoded and going to be presented to
> user, thats why it is called presentation timestamps.

Yes of course.

>> My idea is to take the 4-second video -- DTS-ordered -- that I sent to
>> trac, change it to
>> PTS-ordered, and see if '-vf showinfo' and '-f show_frames' respond
>> correctly. It's a long shot, but
>> it might show something useful -- what if PTS-order made showinfo
>> show_frames and framecrc all
>> agree? Wouldn't that be something interesting?
> 
> If decoder follows specifications for decoding bitstreams correctly than
> changing order of raw encoded frames as stored in bitstream will almost
> every-time cause broken decoding.
>
>>> Its nonsense to change order of encoded frames, no manipulation of clock
>>> can fix that.
>>
>> No, no. Manipulation of PCR. Fix up the PCRs so that the result is truly
>> PTS-ordered.
> 
> If this is at format container layer, I dunno how would that work even if
> its is possible to be useful.

The container would contain a PES packet stream with PCRs like any other PES packet stream. It would 
work like any PTS-ordered stream does. You know, there _are_ PTS-ordered streams. And you know, MPEG 
is silent on the matter of what physical order frames are in the stream, only that they must be 
given to the decoder in DTS-order.

Look, I agree that it would be nonsense. I want to turn the DTS-ordered stream into a PTS-ordered 
stream merely as a test: '-f show_frames' appears to be getting DTSes and PTSes mixed up, then 
applying a fix that results in completely new DTSes. If it treats the PTS-ordered stream 
differently, that will say a lot. What is says depends on what it does. Capiche?

I thought I could do it by demuxing, then moving them like you can move frames in a filter_complex, 
and then remuxing, but I don't know how to do that to PES packets (instead of decoded pictures) and 
I don't know how to rewrite PCRs.