Files not seekable

[jech]

Galène (jech/galene) uses this package for saving videos to disk. It appears that the files are not terminated correctly:

• when played back in a web browser or a media player, the files are not seekable;
• mediainfo reports that the files are truncated.

Is this an intrinsic limitation of the library, or am I doing something wrong?

[at-wat]

To make seekable webm, SeekHead element (which contains position of Info/Track/Cues elements) must be put in front of the Segment.
And Cues element must contain video keyframes information and should be put before Clusters.
(https://www.webmproject.org/docs/container/#muxer-guidelines)

It usually requires two-pass processing as the contents of SeekHead and Cues are not deterministic at the point of writing them.
There is an implementation to make webm seekable (TypeScript): https://github.com/legokichi/ts-ebml#try-to-convert-a-mediarecorder-webm-to-seekable-webm
You can implement similar process using ebml-go. (My company internally has it.)

[jech]

Thanks for the info, that's clear.

What about mediainfo indicating that the file is truncated?

[at-wat]

I don't know what mediainfo is actually saying, but I guess it says data size of Segment and Cluster elements are unknown. (https://github.com/ietf-wg-cellar/ebml-specification/blob/master/specification.markdown#unknown-data-size)
As webm.NewSimpleBlockWriter() is designed to use io.WriteCloser (without seek), size of Segment and Clusters are unknown when the size header is written to the stream.
To make webm seekable for all player, these data sizes must be known value.

[jech]

Thanks a lot for your explanations. May I suggest that all of this should be documented.