[ttml2] Add support for compositing a TTML2 document onto HDR video

palemieux has just created a new issue for 
https://github.com/w3c/ttml2:

== Add support for compositing a TTML2 document onto HDR video ==
Colors in TTML2 documents are specified using sRGB. This yields 
ambiguities when compositing such documents onto HDR video, whose 
allows peak luminance much higher than that associated with sRGB, e.g.
 ITU Rec 2100 allows luminance values up to 10000 cd/m^2 while sRGB 
specifies a reference white of 80 cd/m^2.

As a practical example, white sRGB caption text should rarely, if 
ever, be composited onto HDR video assuming that sRGB (255,255,255) 
maps to peak HDR white: the resulting text would be unacceptably 
brighter than the rest of scene, whose average luminance is typically 
far below peak HDR white. At what luminance level should sRGB captions
 be composited onto HDR images -- without having to define new 
colorspaces and encodings for TTML2?

The following proposes to add a tts:hdrAbsoluteLuminance attribute, 
which allows the author to specify the peak white luminance, i.e. the 
absolute luminance of the sRGB triplet (255, 255,255), when 
compositing a region on an HDR image:

- tts:hdrAbsoluteLuminance applies to <region> only
- the value of tts:hdrAbsoluteLuminance is a decimal number between 0 
and 100
- the initial value of tts:hdrAbsoluteLuminance is 1
- the peak white luminance is equal to 100 * tts:hdrAbsoluteLuminance 
cd/m^2

It would also be useful to note that, in the case of image-based 
subtitles, tts:hdrAbsoluteLuminance is not necessary if the image is 
natively stored as an HDR image.

Please view or discuss this issue at 
https://github.com/w3c/ttml2/issues/233 using your GitHub account

Received on Wednesday, 11 January 2017 23:55:39 UTC