Frame accurate seeking in HLS

[fredrikj]

My HLS streams (transcoded from MP4 files with AWS MediaConvert) always get a time offset of 0.213 seconds. This causes time update events and seeking to be off by this much time. Is it an inevitable deficiency of HLS to not be frame accurate, or does HLS.js have a way to solve this?

[robwalch]

This is usually the result of audio priming or video composition time pushing frame rendering times on the HTMLVideoElements timeline out a bit.

does HLS.js have a way to solve this?

On Hls.Events.BUFFER_APPENDED get the different between the start of the video buffer and the fragment start time of the first appended segment with video:

let tOffset = 0;
const getAppendedOffset = (eventName, { frag }) => {
  if (frag.type === 'main' && frag.sn !== 'initSegment' && frag.elementaryStreams.video) {
    const { start, startDTS, startPTS, maxStartPTS, elementaryStreams } = frag;
    tOffset = elementaryStreams.video.startPTS - start;
    hls.off(Hls.Events.BUFFER_APPENDED, getAppendedOffset);
    console.log('video timestamp offset:', tOffset, { start, startDTS, startPTS, maxStartPTS, elementaryStreams });
  }
}
hls.on(Hls.Events.BUFFER_APPENDED, getAppendedOffset);

[fredrikj]

Thank you! That solves my issue. Seems to me that this information deserves a place in the documentation, such as in https://github.com/video-dev/hls.js/tree/master?tab=readme-ov-file#using-hlsjs

[robwalch]

Thank you for the suggestion @fredrikj. Would you submit a PR adding that documentation?

[robwalch]

Marking as closed with #6658. Thank you @fredrikj 😃