CODEWITHSUNDEEP
cffmpeg
Aditya P
Aditya P

Reputation: 1942

ffmpeg.c what are pts and dts ? what does this code block do in ffmpeg.c?

What does this code bit do in ffmpeg.c , what is its purpose?

01562    ist->next_pts = ist->pts = picture.best_effort_timestamp;
01563    if (ist->st->codec->time_base.num != 0) {
01564        int ticks= ist->st->parser ? ist->st->parser->repeat_pict+1 : ist->st->codec->ticks_per_frame;
01565        ist->next_pts += ((int64_t)AV_TIME_BASE *
01566                         ist->st->codec->time_base.num * ticks) /
01567                         ist->st->codec->time_base.den;
01568    }

Upvotes: 95

Views: 72716

Answers (3)

fuzzyTew
fuzzyTew

Reputation: 3768

I found it clearer to find information in the answer page rather than a link.

The presentation time (PTS) is the correct one. The frames can be stored out-of-order in the file, and the data may need to be read or written out-of-order to be reconstructed. The DTS time stores this other codec-related ordering.

The pasted code snippet retrieves the PTS of a frame and calculates its duration so as to compute the PTS of the next one.

Upvotes: 1

ShijithR
ShijithR

Reputation: 11

B frames are predicted from I and P frames. B frames usually have more errors compared to I and P and hence are not recommended for prediction, though they might be closer in time. There are algorithms in which B is used for prediction but it is from a past B frame and not future B frames.

So in a sequence of I P B1 B2, Decode order is I P B1 B2 and Display order is I B1 B2 P. P is predicted from I, B1 from both I and P, B2 again from I and P.

Upvotes: -7

Bart
Bart

Reputation: 20028

Those are the decoding time stamp (DTS) and presentation time stamp (PTS). You can find an explanation here inside a tutorial.

So let's say we had a movie, and the frames were displayed like: I B B P. Now, we need to know the information in P before we can display either B frame. Because of this, the frames might be stored like this: I P B B. This is why we have a decoding timestamp and a presentation timestamp on each frame. The decoding timestamp tells us when we need to decode something, and the presentation time stamp tells us when we need to display something. So, in this case, our stream might look like this:

   PTS: 1 4 2 3
   DTS: 1 2 3 4
Stream: I P B B

Generally the PTS and DTS will only differ when the stream we are playing has B frames in it.

Upvotes: 171

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