Which bytes of video data refer to the current playback position

Question

I'm working with media source extensions to play a video in the browser.

My JavaScript program divides a video file in fragments of 1200 bytes. The video is encoded in the following codec: video/mp4; codecs="avc1.42E01E, mp4a.40.2".
Giving a time X in seconds I want to find the corresponding first fragment that is needed to play the video at position X (Or a fragment that is near to but not after this position). More concrete, I want to find out which fragment belongs to the time in the currentTime property of HTMLMediaElement. The video file can be analysed beforehand.

Answer 1

There is a multitude of ways for the duration->fragment mapping to be specified in the mp4 file.

• The subsegment index (sidx for short) box defined in ISO/IEC 14496-12 associates bytes ranges from the input file with the corresponding durations.

Here is a snippet of a sidx box content:

subsegment 0 size 726463 dur 450450
subsegment 1 size 723260 dur 450450
subsegment 2 size 713278 dur 450450
subsegment 3 size 441644 dur 450450
... and so on ...

Finding the fragment for given time X is trivial.

• most mp4 files don't have a sidx box but rather a stsz and stts boxes that map a series of samples (video or audio frames) to a duration + size. Here is and example:

stsz box:

sample 0 size 105222
sample 1 size 1554
sample 2 size 2153

stts box:

3 sample each with dur 512

The stts box give the duration for the samples defined in stsz

• there might be other ways. This list is not exhaustive