Re: HTML Canvas - Transforms for HDR and WCG

On Thu, Apr 29, 2021 at 10:34 AM Chris Lilley <chris@w3.org> wrote:

>
> On 2021-04-28 15:23, Christopher Cameron wrote:
> > In all browsers today, for "Colorbars in HLG 203 display.png", any
> > signal value above 0.75 is clamped to SDR-white.
>
> The choice of 0.75 follows from the HLG specification - a code value of
> 0.75 is media white, while a code value of 1.0 is peak white and is 12x
> higher luminance.
>
> The choice of clamping all HDR highlights, with no tone mapping is an
> easy but bad one. However, dislaying HDR content on SDR is in general
> hard; HLG is easier here than PQ (by design).


Suppose we have a video that identifies itself as HLG (not anonymously),
and is displayed using a <video> tag on an SDR device.  Should it clamp
values above 0.75? My answer is "no".

We have established that it is inevitable that pixel values over 0.75 in an
image with the ICC profile "Colorbars in HLG 203 display" will be clamped.

To the original question of "what to do about ICC profiles specifying HLG
anonymously?". My answer would be "treat them as extended SDR, not as HLG,
and there is no way for them to truly be the same as HLG".