Re: Pre-meeting Thoughts on HDR Canvas

It is easy to assume that blending in gamma space is always undesirable.
History tells a different story. Blending in non-linear space is common.
For example, in Photoshop you can select to blend in linear or non-linear space.
It’s a creative choice. It seems most users select the non-linear option.
AFAIK video fades are typically in code space, not linear space.
So it seems we should retain this option.

Lars

From: Justin Novosad <junov@google.com>
Date: Wednesday, January 27, 2021 at 7:59 AM
To: Chris Lilley <chris@w3.org>
Cc: "public-colorweb@w3.org" <public-colorweb@w3.org>
Subject: Re: Pre-meeting Thoughts on HDR Canvas
Resent-From: <public-colorweb@w3.org>
Resent-Date: Wednesday, January 27, 2021 at 7:59 AM

Thanks Chris for correcting me.

I realize this is an old problem, but I think that adding HDR capabilities to canvas risks exacerbating it greatly because we will end up with inconsistent blending and gradient interpolation based on the canvas's working profile.
For example, if a hypothetical app selects a canvas working profile to match the output device's capabilities, these behavior discrepancies risk being perceived as bugs and will be unpleasant to work around for web developers.

On Wed, Jan 27, 2021 at 12:46 PM Chris Lilley <chris@w3.org<mailto:chris@w3.org>> wrote:

On 2021-01-27 18:32, Justin Novosad wrote:
> The CSS and SVG specifications do not explicitly address the issue of
> gamma-correct blending, but the examples in the CSS spec suggest doing
> things the "wrong" way, which ignores gamma correctness.

The SVG specification explicitly says that filer operations are in
linear-light sRGB by default (with an option to change to sRGB, where
speed is more important than getting the right result); and that all
other operations are (sadly) in gamma-encoded sRGB by default (with an
opt-in for linear-light sRGB).

The CSS Compositing specification, sadly, requires operations in
gamma-encoded sRGB. This choice was primarily driven by backwards
compatibility with existing content; and secondarily with compatibility
of blend modes, as popularized in Adobe Photoshop, which are also
computed in gamma-encoded RGB spaces.

CSS Compositing thus needs to add an opt-in for linear-light compositing.

--
Chris Lilley
@svgeesus
Technical Director @ W3C
W3C Strategy Team, Core Web Design
W3C Architecture & Technology Team, Core Web & Media