What does WebM DocType Version Signify?

Question

If a WebM's DocTypeVersion is 4, what does that signify and correlate to? Is that version 4 of the Matroska specification draft? Or does WebM have its own versioning independent of Matroska? I couldn't find any information on webmproject.org

After analyzing a WebM file created by ffmpeg, I see the "Document Type Version" is 4:

$ ffmpeg -i audio.opus -a:c copy audio.webm
$ mkvinfo audio.webm

+ EBML head
|+ EBML version: 1
|+ EBML read version: 1
|+ Maximum EBML ID length: 4
|+ Maximum EBML size length: 8
|+ Document type: webm
|+ Document type version: 4             <=  This is what I'm referencing
|+ Document type read version: 2

Answer 1

WebMs is a type of EBML document.

From https://github.com/cellar-wg/ebml-specification/blob/master/specification.markdown

The version of the EBML Body is found in EBMLDocTypeVersion. A parser for the particular DocType format can read the EBML Document if it can read either the EBMLDocTypeVersion version of that format or a version equal or higher than the one found in EBMLDocTypeReadVersion.

FFmpeg will set version to 4 for WebM if there's an audio stream with codec Opus or if video stream has an alpha channel stored in an additional block or is a stereo video.

Answer 2

The WebM Container Guidelines strongly imply that these versions are the same as the Matroska verion numbers in their note on DocReadTypeVersion:

• DocReadTypeVersion SHOULD follow the Matroska specification.
  • Example: Files with v2 elements should have a DocReadTypeVersion of 2.

As far as the meaning of and differences between DocTypeVersion and related DocTypeReadVersion, section 21 of version 4 of the draft Matroska Specifications has a good explanation.

DocTypeVersion MUST be equal to or greater than the highest Matroska version number of any Element present in the Matroska file. For example, a file using the SimpleBlock Element MUST have a DocTypeVersion equal to or greater than 2. A file containing CueRelativePosition Elements MUST have a DocTypeVersion equal to or greater than 4.

The DocTypeReadVersion MUST contain the minimum version number that a reading application can minimally support in order to play the file back -- optionally with a reduced feature set. For example, if a file contains only Elements of version 2 or lower except for CueRelativePosition (which is a version 4 Matroska Element), then DocTypeReadVersion SHOULD still be set to 2 and not 4 because evaluating CueRelativePosition is not necessary for standard playback -- it makes seeking more precise if used.

To add to that, SimpleBlock Elements are version 2 Elements that contain actual video or audio data, which is why the DocTypeReadVersion should be 2 if they are used - they have the actual file content! So a player that doesn't support version 2 wouldn't be able to play back the content in those Elements.