Question

I noticed that with some video files the PTS timestamps returned inside the AVPacket structure do not start at 0, but some time later. E.g. at 3.128 or something. 99% of the video files I tested has PTS timestamps starting at 0 but very few files have some strange timestamps that start at 3.128 or 1.2 or something. How am I supposed to handle these cases? Should I just record the PTS timestamp of the very first packet and then subtract this PTS from all following timestamp values to get a 0-based PTS value? Or what should I do with these non-0-based timestamps? Thanks for your help!

Answer 1

Libavcodec/avformat is just giving you the data that’s in the file. Unfortunately (or perhaps fortunately, depending on your perspective), many file formats do not require timestamps to start at 0. In fact it can be important to have them start at other values if several files each make up part of a longer stream and you want to be able to non-destructively put them back together.

If you want 0-based timestamps, then like you said, you’ll need to save the lowest/first timestamp and subtract that value from all timestamps. Note however that for some really ugly formats (like DVD video) it’s common for timestamps to reset in the middle of the content, and this could even result in getting negative timestamps with your approach. If you expect you might be dealing with such content, you’ll need to detect discontinuities and patch them up. Last I worked with avcodec/avformat, they didn’t have a feature to do this for you automatically, but they might now. I’d look into it if you think you might need that.