Calculate PTS before frame encoding in FFmpeg

Question

How to calculate correct PTS value for frame before encoding in FFmpeg C API?

For encoding I\'m using function avcodec_encode_video2 and then writing i

Answer 1

Time is measured not in seconds or milliseconds or any standard unit. Instead, it is measured by the avCodecContext's timebase.

So if you set the codecContext->time_base to 1/1, it means using second for measurement.

cctx->time_base = (AVRational){1, 1};

Assuming you want to encode at a steady fps of 30. Then, the time when a frame is encoded is framenumber * (1.0/fps)

But once again, the PTS is also not measured in seconds or any standard unit. It's measured by avStream's time_base.

In the question, the author mentioned 90k as the standard resolution for pts. But you will see that this is not always true. The exact resolution is saved in avstream. you can read it back by:

    if ((err = avformat_write_header(ofctx, NULL)) < 0) {
        std::cout << "Failed to write header" << err << std::endl;
        return -1;
    }

    av_dump_format(ofctx, 0, "test.webm", 1);
    std::cout << stream->time_base.den  << " " << stream->time_base.num << std::endl;

The value of stream->time_stamp is only populated after calling avformat_write_header

Therefore, the right formula for calculating PTS is:

//The following assumes that codecContext->time_base = (AVRational){1, 1};
videoFrame->pts = frameduration * (frameCounter++) * stream->time_base.den / (stream->time_base.num * fps);

So really there are 3 components in the formula,

1. fps
2. codecContext->time_base
3. stream->time_base

so pts = fps*codecContext->time_base/stream->time_base

I have detailed my discovery here