[MMUSIC] About overlaying subtitles with the "ParallaxInfo" attribute

[Bert Greevenbosch <[e-mail]>]

Hi Jonathan,

During the Prague meeting on the draft-greevenbosch-mmusic-parallax-attribute-00 draft, you commented that there were some issues about the overlay of the subtitles and the video, and how to indicate which overlays which.

In the RFC that defines 3GPP timed text streaming over RTP (RFC 4396), an attribute "layer" is defined. It defines the render hierarchy of timed text, i.e. when two timed text streams overlap, the one with the more negative "layer" value will be rendered over the one with the less negative value. This means that the first perceived stream as being in front of the second stream. The "layer" attribute is optional, and specific to RFC 4396.

For streams with the "ParallaxInfo" attribute, I have always thought of them as overlapping the main video content. I hinted at this in the informative section, but maybe we should write this more explicitly in the draft.

There is also a relationship between parallax and layers, especially since a more negative parallax value also indicates positioning nearer to the viewer. However, the "ParallaxInfo" contains more information than the "layer" attribute, as the first describes the exact positioning in the virtual 3D space, whereas the second only defines whether the stream is closer or further away than other streams. But in general we could say that if the "ParallaxInfo" attribute of one stream is more negative than of another stream, the "layer" attribute of the first should be more negative than the "layer" attribute of the second too.

What do you think?

Best regards,
Bert