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

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Mon, 30 Sep 2013 12:33:54 -0700
Message-ID: <CAAWBYDDUgc47sWsgYcqFdsPDbFfvuKRAVvS9ehEET3nnHBrRjQ@mail.gmail.com>
To: Chris Lilley <chris@w3.org>
Cc: Alan Gresley <alan@css-class.com>, Rik Cabanier <cabanier@gmail.com>, Simon Sapin <simon.sapin@exyr.org>, www-style list <www-style@w3.org>
On Mon, Sep 30, 2013 at 4:14 AM, Chris Lilley <chris@w3.org> wrote:
> Tab asked why some colourspaces could perform independent channel by
> chanel operations, suspected that it applied to RGB but not Lab; and I
> answered why (you can do it with ones that are additive). Lab is an
> example of an additive colourspace and can be, and is, used for
> compositing.

Okay, so, synthesizing everything I understand of the thread so far, I
think I need to make the following changes:

1. Allow computed values to be CMYKA quints, rather than RGBA quads,
when UAs wish to do so.  Normal browsers wouldn't do that, but
print-based UAs might.  (And make sure I do this in a sufficiently
generic way that UAs can hold computed values in other spaces, too,
like Lab.)

2. Define which types of operations work equally over RGBA and CMYKA
as simpler per-channel operations (and so can be done in either space,
as long as both inputs are the same type of color), and which don't.
For those that don't, require converting CMYKA to RGBA for those
operations.  When the inputs don't have matching spaces, convert them
both to RGBA and do the operation there.  (If we add more possible
spaces, we'll need rules about what to convert between - if we had
Lab, for example, I suspect we'd want to preserve it and convert the
other to Lab as well.)

3. Serialization is always in RGB.

4. Maybe have some way to define a color profile for accurate mapping
between spaces.  (Or maybe save that for Colors 5.)

Is this right?  If so, can I get a quick listing of which operations
fall in which bucket wrt #2?

~TJ
Received on Monday, 30 September 2013 19:34:42 UTC

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