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Fwd: [css-color] wider/deeper colors

From: Rik Cabanier <cabanier@gmail.com>
Date: Wed, 23 Mar 2016 15:15:09 -0700
Message-ID: <CAGN7qDCKO9MRw78+mX3dGaXSL7ud3w9Yw9m=Z1-9KjRp_XY5=w@mail.gmail.com>
To: www-style list <www-style@w3.org>
[sorry. My last email only went out to Florian]

On Tue, Mar 22, 2016 at 10:41 PM, Rik Cabanier <cabanier@gmail.com> wrote:

> On Tue, Mar 22, 2016 at 9:13 PM, Florian Rivoal <florian@rivoal.net>
> wrote:
>> On Mar 23, 2016, at 11:10, Rik Cabanier <cabanier@gmail.com> wrote:
>> On Tue, Mar 22, 2016 at 5:34 PM, Florian Rivoal <florian@rivoal.net>
>> wrote:
>>> > On Mar 23, 2016, at 01:56, aaron.www-style-@infinite-source.de wrote:
>>> >
>>> > On 22.03.2016 08:45, Florian Rivoal wrote:
>>> >>
>>> >>> On Mar 22, 2016, at 11:34, Rik Cabanier <cabanier@gmail.com> wrote:
>>> >>>
>>> >>> It could be a new CSS property, for instance: compositing-space:
>>> "p3"/"sRGB"/"uncalibrated"/url(..)
>>> >>
>>> >> Shouldn't that be "compositing space: sRGB | output | CIELab" ?
>>> >>
>>> >> * sRGB: legacy "make everything dull" mode
>>> >>
>>> >> * CIELab: because if you're compositing things coming form various
>>> color spaces or wide gamut color spaces, this is how you get the least
>>> color distortion.
>>> >>
>>> >> Say your sources are in a gamut wider than the output gamut, and you
>>> have semi-transparent very saturated red on top of very saturated blue. If
>>> you composite in CIELab, then convert to the output gamut, you'll get a not
>>> very saturated color that may not need clipping to fit the output gamut.
>>> >>
>>> >> If you fit the colors to output gamut first, then do composition, the
>>> result will be different (and worse).
>>> >>
>>> >> * output: because if all the things you are compositing do fit in the
>>> output device's gamut, doing composition directly in that space will be
>>> close enough to what you'd get using CIELab, but is computationally more
>>> efficient.
>>> >
>>> > Compositing in the output gamut is not really close to compositing in
>>> > Lab. Try generating a saturated red-green gradient or rainbow in sRGB
>>> > and in Lab.
>>> Right. I should have qualified that some more. Compositing in Lab gives
>>> better results, and compositing in the output gamut is sorta of close
>>> in some, but certainly not all scenarios. Depending on what you're
>>> doing, it may give be good enough, or maybe not.
>>> If we had no compat constraint nor performance constrain, I'd say we
>>> should
>>> always do composition/color math in CIELab, but we may not have that
>>> liberty. We should certainly explore whether we have it though, as it would
>>> be better.
>> No, compositing in CIELab will give you unexpected results.
>> If you have access to Photoshop try the following:
>> - create an rgb document (16bit is best)
>> - create a red rectangle
>> - put a green rectangle inside, give it 50% opacity
>> - use the eye dropper on the green rectangle. It measures 50% green and
>> 50% red, no blue (as expected)
>> - now switch to the document colorspace to Lab. Do not flatten
>> - notice how the transparent green rectangle changes color. It now is 78%
>> red, 67% green
>> I'd argue that's expected.
>> By definition, compositing in the output space gives you unsurprising
>> **numbers**, with half red and half green giving 50% of each.
>> By definition as well, compositing in another space will do different
>> math, leading to different numbers. Because of the way CIELab is
>> constructed, it gives you unsurprising **colors**.
>> If you look at the colors, not with the eye dropper, but with your actual
>> eyes, the result when using compositing in Lab is much nicer. RGB
>> composition gives us a dark and muddy color, while Lab composition gives us
>> a brighter color as the result of combining these two bright colors.
> True.
> Let me do some internal research on why we offer Lab as an option in
> Photoshop and who is using it.

I got some more information from one of our main Photoshop engineers:


why is Photoshop supporting the Lab colorspace in addition to the normal
RGB/CMYK color spaces.


There are a lot of reasons: LAB is more perceptually uniform, separates the
luminance from chrominance almost completely, allows other tricks that you
can’t do in RGB or CMYK. LAB support has been in Photoshop since version 3.


Is it a popular color mode for Photoshop? Is it used a lot?


Yes, LAB is used by retouchers and researchers — there are even a couple of
books on using LAB in Photoshop. [1] (maybe 10% of users know how to work
with it).


The CSS working group is considering allowing Lab as a compositing space in
HTML. Is that a good idea?


They probably don’t want to do that.  Compositing in LAB is not as
straightforward as it looks (because only the L channel matches what you

Received on Wednesday, 23 March 2016 22:15:39 UTC

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