Re: HTML Canvas - Transforms for HDR and WCG

It is quite possible, even likely, for the ICC profile’s perceptual intent to include the tone mapping from HLG to SDR. 

In such case, the output should not be tone mapped further, and clamping shouldn’t be needed.

 

Lars

 

From: Christopher Cameron <ccameron@google.com>
Date: Saturday, May 1, 2021 at 2:19 PM
To: Chris Lilley <chris@w3.org>
Cc: "public-colorweb@w3.org" <public-colorweb@w3.org>
Subject: Re: HTML Canvas - Transforms for HDR and WCG
Resent-From: <public-colorweb@w3.org>
Resent-Date: Saturday, May 1, 2021 at 2:19 PM

 

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".