Add informative note to tts:luminanceGain recommending that the content author override the initial value.

[jmccrossan-fox]

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 mechanism.

[skynavga]

This sounds like a candidate for a style guide, and not a technical specification. We don't make style guide recommendations in TTML[12].

[palemieux]

The issue could be moved to IMSC