Bug 20566 - CEA-708 supports multiple grids
CEA-708 supports multiple grids
Status: RESOLVED FIXED
Product: TextTracks CG
Classification: Unclassified
Component: Conversion of 608/708 captions to WebVTT
unspecified
PC All
: P2 normal
: ---
Assigned To: Silvia Pfeiffer
This bug has no owner yet - up for the taking
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Blocks:
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Reported: 2013-01-04 21:14 UTC by Silvia Pfeiffer
Modified: 2013-02-02 22:23 UTC (History)
2 users (show)

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Description Silvia Pfeiffer 2013-01-04 21:14:27 UTC
[Feedback provided by David Ronca, Netflix:]

CEA-608 mandates a 32x15 grid in a 4:3 display area, CEA-708 allows either 32x15 (4:3) or 42:15 (16:9). If the input was CEA-708, how would a client know whether the display region should be 4:3 or 16:9.
Comment 1 Silvia Pfeiffer 2013-01-04 21:15:33 UTC
Suggest adding a metadata header field AspectRatio=4:3 or AspectRatio=16:9 to capture this and then make the position calculation for 708 dependent on this.
Comment 2 Silvia Pfeiffer 2013-01-22 07:49:17 UTC
Added some further description of how to convert the two different grids in CEA-708 to WebVTT and introduced a AspectRatio metadata header.

https://dvcs.w3.org/hg/text-tracks/rev/f11322440708
Comment 3 David Singer 2013-01-29 08:18:33 UTC
Surely we can say that VTT measurements are on uniformly-sampled axes, and have the author correct the non-uniform numbers to uniform ones?  Do we need this complication?
Comment 4 Silvia Pfeiffer 2013-02-02 22:23:38 UTC
David: This does not change WebVTT. It only explains better how to transcode a CEA-708 file to WebVTT. The AspectRatio metadata field is also just an hint for the video player in case it needs it. The conversion spec uses only percentage values for positioning, so it ends up being uniformly sampled.