Re: [css-gcpm][css-color] device-cmyk() interaction with RGB-space colors.

On Tue, Sep 24, 2013 at 3:25 PM, Tab Atkins Jr. <jackalmage@gmail.com>wrote:

> On Tue, Sep 24, 2013 at 3:14 PM, Rik Cabanier <cabanier@gmail.com> wrote:
> > On Tue, Sep 24, 2013 at 2:58 PM, Tab Atkins Jr. <jackalmage@gmail.com>
> > wrote:
> >> On Tue, Sep 24, 2013 at 2:31 PM, Rik Cabanier <cabanier@gmail.com>
> wrote:
> >> > The compositing/blending spec doesn't talk about converting colors [1]
> >> > All
> >> > the formulas (except for 4 special ones) work on individual color
> >> > channels,
> >> > regardless of what they represent.
> >>
> >> Those channels appear to be assumed to be RGB.
> >
> > Can you point out where that is so I can update the spec?
>
> Hmm, I guess it actually doesn't.  The few examples that give explicit
> color numbers are all written with RGB, but there's nothing outside
> the examples that actually talks about the color value.
>

Great. I made a not to add CMYK (and spot?) examples for the next version.


>
> >> If it's compatible
> >> with CMYK, cool, but that needs to be specified (and again, needs to
> >> be defined how you blend/composite between RGB and CMYK).
> >>
> >> I doubt that this "just use the channels" approach is compatible with
> >> some other color spaces, like Lab, though.
> >
> > It is compatible. You can try it in PhotoShop :-)
>
> How does that work, though?  I mean, HSL doesn't work in a "just use
> the channels" approach, which is why the HSL blend modes are
> non-separable.  Why would Lab work?
>

Yes, those HSL blend modes are out for anything but the RGB colorspace.

I might take back do-it-per-channel for Lab blending. (I thought we did it
per channel in Acrobat. PhotoShop seems doing something else and the PDF
reference manual says that you shouldn't do it).



>
> >> > I agree we need to define a way how and when individual elements are
> >> > converted to the document's color space.
> >>
> >> Sure, something more advanced than just "always convert to RGB
> >> immediately" might be worthwhile.  We talked about this at the f2f,
> >> though, and some of the ideas (like converting individual pixels to
> >> RGB as necessary, or converting entire elements to RGB as soon as you
> >> need to convert some part of it) were all horrible and not worth the
> >> benefit, so I didn't put those ideas into the spec.
> >
> > It seems we could define something simple. People have a tendency to
> > overcomplicate color management...
>
> Ideas welcome.
>
> ~TJ
>