Re: Pre-meeting Thoughts on HDR Canvas

FWIW, opt-in blending and interpolation behaviors can always be added
later, once there is proven demand. The only question that really needs to
be resolved now, is: what should be the default behavior for HDR?  The
default is harder to change later.

On Thu, Jan 28, 2021 at 8:55 AM Lea Verou <lea@verou.me> wrote:

> I sent my response too quickly.
> Yes, blending in perceptually uniform spaces definitely has its uses, as
> you point out.
> What I *meant* to have written is that blending in gamma-encoded sRGB
> space is never optimal, since it's neither perceptually uniform, nor linear
> light. There are cases where it's *acceptable*, but I have not yet come
> across cases where it's *better* (for any definition of "better") over both
> perceptually uniform spaces like Lab *and* linear light spaces like XYZ.
>
> Cheers,
> Lea
>
> Lea Verou ✿ http://lea.verou.me ✿ Twitter & elsewhere: @leaverou
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> On 28Jan, 2021, at 01:55, Andrew Somers <andy@generaltitles.com> wrote:
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> Hi Lea,
>
> I've written some articles on this with examples.
>
>
>    - There are use cases where linear is often best: emulating real
>    light, image compositing
>
>
>
>    - There are use cases where a perceptual space is more useful:
>    creating gradients, simple color mixing, calibrating user controls.
>
>
> And…. there are exceptions to those cases as well. And desirable or better
> is… dependent on what one is trying to achieve.
>
> An example from a gist I wrote on a related subject:
> <https://gist.github.com/Myndex/10caff6a68e844591e83eadeebfb4347>
>
> ----------------
>
> These four gradients were created using (from left to right) sRGB, xyY
> (linear), L*a*b*, and LCh (from Lab).
>
> <100867985-d07f5a80-344f-11eb-85ee-868d635356cd.png>
> <https://user-images.githubusercontent.com/42009457/100867985-d07f5a80-344f-11eb-85ee-868d635356cd.png>
>
> Four different spaces/methods, and four very different results. On first
> glance it might seem that the LCh is the most dynamic, good if that is your
> design goal... but look at this next slide, from saturated color to
> achromatic...
>
> <100868304-4b487580-3450-11eb-8cf6-a47afd2a6a02.png>
> <https://user-images.githubusercontent.com/42009457/100868304-4b487580-3450-11eb-8cf6-a47afd2a6a02.png>
>
> YIKES!! LCh mangled that all to hell! When going from a saturated to a
> less saturated or monochromatic color, you probably don't want nearly as
> much hue movement. But also notice that the sRGB and the LAB look almost
> the same. This is to be expected. The gamma for sRGB is roughly 2.2 and the
> effective gamma for LAB is about 2.38, in other words, when it comes to the
> lightness curve, there is very little difference between sRGB and L*, if
> the gradient is not shifting hue, then you are not likely to notice much of
> a difference.
>
> ---------
>
> *More Colorful Examples:*
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> *Gradients In Space* <https://www.myndex.com/WEB/Gradients>
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> *Lstar Wars Episode IV - A New Color*
> <https://www.myndex.com/WEB/GradientsPartTwo>
>
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> Cheers,
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> Andy
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> On Jan 27, 2021, at 2:49 PM, Lea Verou <lea@verou.me> wrote:
>
> Common and desirable are not necessarily correlated. If non linear
> blending is the application default, most users are “choosing” it by never
> making a conscious decision. I’m having trouble finding cases where non
> linear blending is desirable or produces a better result, for any
> definition of “desirable” or “better”…
>
> --
> Lea Verou ✿ http://lea.verou.me ✿ @leaverou
>
> On Jan 28, 2021, at 00:38, Leonard Rosenthol <lrosenth@adobe.com> wrote:
>
> 
> I would say that pretty much every content authoring tool – be it graphics
> (Photoshop, Inkscape, Scribus) or documents (MSOffice, GDocs, PDF, etc.)
> uses gamma/non-linear blending – though also in SDR.  To change that in the
> web space (at least for SDR content) would also require a change of all of
> those authoring tools or users won’t be able to author content that matches.
>
> I do agree, however, that in the context on HDR content it may well make
> sense to consider alternatives since the authoring isn’t there yet….
> Provided that we fully describe a mixed SDR/HDR stack and how compositing
> would work in such a scenario – especially when multiple colorspaces are
> also involved…
>
> Leonard
>
> *From: *Lars Borg <borg@adobe.com>
> *Date: *Wednesday, January 27, 2021 at 2:23 PM
> *To: *Justin Novosad <junov@google.com>, "chris@w3.org" <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 2:23 PM
>
> 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> 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
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