Testing subtitle sync

[nicholas-fr]

I'm preparing a script to add IMSC1 ('im1t') subtitles to any mezzanine stream. It would add 2 things:

• Subtitles displaying a rectangle in the top right corner, flashing in sync with the flash in the video, for example:

• Subtitles with text identical to the text in the video, for example:

The ISMC1 content is generated by the script based on the selected mezzanine stream. The main purpose is to test subtitle timing and not correct rendering of subtitle content. For testing subtitle rendering, I believe we will need dedicated subtitle mezzanine content for each WAVE subtitle profile.

This is work in progress, but after the initial discussion in today's call, there are already a couple of points I will follow up on:

• As this initial script generates frame-synchronous subtitles, they may update too frequently for some clients or beyond what is allowed/recommended by the specification.
• Need to confirm that the TTML generated by the script is compliant with IMSC1.

Feedback and suggestions are welcome.

[haudiobe]

Does the rendering model provide any constraints.

Yes.

See clause 10 at IMSC (https://www.w3.org/TR/ttml-imsc1.1/#hypothetical-render-model)

Best,

-- Pierre

[haudiobe]

Hi Thomas,

Yes, we developed this back in DECE to bound the glyph rendering rate. The formula was refined when it moved to W3C.

A subtitle/frame is not necessarily a violation as it is glyph dependent.

Unfortunately the public validators do not check this today, and there are other complexities that are not bounded.

I am aware of one notable encoder manufacturer that creates samples (and thus redraws the screen) as often as 10-20 times/sec, which is a violation. They've been informed and they are fixing it.

Mike

[gitwjr]

Deferred for future work. @nicholas-fr will provide an update.