[cairo] Subtractive API, part 0

Arjen Nienhuis a.g.nienhuis at gmail.com
Thu Jan 28 13:53:12 PST 2010

On Thu, Jan 28, 2010 at 7:40 PM, Chris Murphy <lists at colorremedies.com> wrote:
> On Jan 28, 2010, at 9:47 AM, Arjen Nienhuis wrote:
>> Does anyone know what Photoshop does in that case: what happens if you
>> draw with a cmyk(0,0,1,1) brush with 50% opacity? Or with the borders?
> If you are using CMYK color mode, it will reduce all channels equally. They are not "gray balanced" to be well behaved. For that the color model needs to be RGB. Many of photoshop's plugins only work in RGB, and those that let you work in CMYK can produce color shifts that are not present when the same plugin is used in RGB mode.
>> The point of my message was that It's bad to do composition in CMYK
>> without a full blown CMS. If we don't want a CMS, we can use the SVG
>> model with sRGB fallbacks.
> Photoshop does compositing of CMYK, using just numerical values, not CIE values. However, it does show the effect on-screen with a CIE based preview, since it always tags CMYK with an ICC profile and there is no way not to.

I tried scribus 1.3.5:

For cmyk(0,0,100,100) with 50% opacity it shows (slightly yellow) gray
as I would expect.
For cmyk(0,0,100,100) with 50% shade it shows beige.

In the PS output cmyk(0,0,100,100) with 50% opacity shows as
cmyk(0,0,100,100) with 100% opacity.

In the PDF output cmyk(0,0,100,100) with 50% opacity appears beige on
screen in acroread, the same as cmyk(0,0,100,100) with 50% shade. (?!)

> CMYK is extremely device dependent. So I have no problem with using it for composition, but ignoring the color meaning of CMYK values is just not useful in my opinion.
> Chris Murphy
> Color Remedies (TM)
> New York, NY
> ----------------------------------------------------------------------
> Co-author "Real World Color Management, 2nd Ed"
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