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Re: [csswg-drafts] [css-color] Separation / DeviceN color support (#2023)

From: Mike Bremford via GitHub <sysbot+gh@w3.org>
Date: Thu, 14 Nov 2019 09:02:02 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-553791757-1573722121-sysbot+gh@w3.org>
For clarification here's a worked evaluation of the "duotone" example above. I have two spot colors and I'm trying to perform a blend between them. I'm a browser, so I work in RGB. What do I do?

Lets evaluate the color a quarter of the way from blue to red. At this point I have 75% of "PANTONE Reflex Blue C" - defined as `device-cmyk(1 0.723 0 0.02)`,  and 25% of "PANTONE Warm Red C" - defined as `device-cmyk(0 0.75 0.9 0)`.

As I have neither of these colors on my output device, I must convert them to their fallback ColorSpace, device-cmyk.

```javascript
var t0 = 0.75; // amount of color 0
var t1 = 0.25; // amount of color 1

var c = 1 - ( (1-(t0 * 1    )) * (1-(t1 * 0   )) );
var m = 1 - ( (1-(t0 * 0.723)) * (1-(t1 * 0.75)) );
var y = 1 - ( (1-(t0 * 0.0  )) * (1-(t1 * 0.9 )) );
var k = 1 - ( (1-(t0 * 0.02 )) * (1-(t1 * 0   )) );

// c=0.75 m=0.628 y=0.225 k=0.015
```
Conversion is now from CMYK to sRGB - a much simpler problem (let me refer you to #2022 for my thoughts on that)


Finally, there's one other justification for all this which I missed in my post above, and it's that spot colors are sometimes used in PDF output for things that aren't colors at all. We have a customer that does greeting cards - they use a separation in the output to mark score lines on the cards for folding or cutting. I believe they can also used for things like where to put glue or glitter, if you have such a machine.

These options would never be possible if spot colors were limited to those defined in ICC profiles, which would be a shame - who doesn't want more glitter on their printouts?

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Received on Thursday, 14 November 2019 09:02:07 UTC

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