[ttml2] Add informative note to tts:luminanceGain recommending that the content author override the default value (#1118)

jmccrossan-fox has just created a new issue for https://github.com/w3c/ttml2:

== Add informative note to tts:luminanceGain recommending that the content author override the default value  ==
Currently the definition of luminanceGain states:
Note:
The [SRGB] color space specifies an absolute reference white point luminance of 80 cd∙m-2. As a result, colors specified in the [SRGB] space cannot be reliably composited with high-dynamic range (HDR) content intended for display with significantly greater peak luminance, such as content that uses the perceptual quantizer electro-optical transfer function (PQ EOTF) specified in [ITU BT.2100-1]. Specifically,
- [SRGB] colors may appear too dim if they remain unscaled and the average luminance of the scene exceeds 80 cd∙m-2; and
- [SRGB] colors may appear too bright if they are uniformly scaled to an arbitrary large luminance, such as 10,000 cd∙m-2.

Consider adding to the above informative note that the initial value defined for tts:luminanceGain ('1.0') results in an unscaled presentation of sRGB colors. A content author should consider providing a specific value for tts:luminanceGain if HDR presentation is to be supported – for example by defining an explicit tts:luminaceGain value for each region element in the document or by using the [`initial`](https://w3c.github.io/ttml2/index.html#styling-vocabulary-initial) mechanism.

Please view or discuss this issue at https://github.com/w3c/ttml2/issues/1118 using your GitHub account

Received on Thursday, 13 June 2019 19:33:16 UTC