Pavel A. da Mek wrote:

...

I can not enter colours like 5 % C, 50 % M, 75 % and 5 % K, the number always change upon entering the fourth value such that one value is 0 %.

This is the purpose for what the fourth (black) coordinate (ink) is used: to use an additional black filter and only two of the three color filters.

Maybe theoretically, but not in practice. In the prepress world is common to use the four values of CMYK, for example in 'rich black' (http://marvin.mrtoads.com/richblack_vs_plainblack.html) or in PANTONE to CMYK transformations. The algorithm you use is ok to convert RGB->CMYK or CMYK->RGB (without color management), and show the result in a screen, but looks different if you print it. I think inkscape only translates CMYK values to RGB values (and later from RGB to CMYK in the color palette, lossing the original CMYK colors) but really don't support the CMYK color mode.

Pavel A. da Mek wrote:

...

Maybe theoretically, but not in practice. In the prepress world is common to use the four values of CMYK, for example in 'rich black' (http://marvin.mrtoads.com/richblack_vs_plainblack.html) or in PANTONE to CMYK transformations.

Thanks for the link, it is interesting. But is all this fuss with different black colors really necessary? I would suppose that the designer should not be bothered with such matters and that the printing device should solve the corrections resulting from the fact that 100% of ink does not filter 100% of the light but, say, only 80%, and so the ideal CMYK 0,0,0,90% must be replaced by 50%,50%,50%,80%, which means 62.5%,62.5%,62.5%,100% of the maximal amount of the ink. After all, the color space perceived by humans is 3-dimensional and so there is no need to use four coordinates.

P.A.

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Every black has his uses: We can use 0%,0%,0%,100% for small text (because we don't want a blurring effect if more than 1 ink is used), and a rich black of 60%,40%,40%,100% for the backgrounds. For example a gradient from 5%,100%,100%,0% to 5%,100%,100%,100%, looks better than a gradient to 0%,0%,0%,100% or 50%,50%,50%,80%, ...or other black because only a ink is modified. And not only works for the black ink, it works for all inks.

In short, CMYK colors shouldn't be converted to RGB ones, only for screen preview.

Quoting "Pavel A. da Mek" <a.da_mek@...998...>:

Thanks for the link, it is interesting. But is all this fuss with different black colors really necessary?

After all, the color space perceived by humans is 3-dimensional and so there is no need to use four coordinates.

*puts on graphic designer hat*

It's not that simple, because the behavior of inks is funky and non-linear. There's no trivial mapping. You have to play games to get particular qualities of color sometimes.

In *theory* 100% C + 100% Y + 100% M should = 100% K when printed; in practice that is NEVER true.

*puts on Inkscape developer hat*

Of course, for right now all this is moot. SVG only natively supports the sRGB color space, so that's all we can store in the actual file. The color selector is really only there for picking RGB colors (in terms of RGB, HSV, CYMK, etc...).

We may eventually be able to take advantage of ICC colors in SVG (it makes provision for that at least), but that will require a lot of work before it's ready to use.

-mental

On Thu, Aug 25, 2005 at 09:06:34AM +0200, Tobias Hilbricht wrote:

Am Mittwoch, 24. August 2005 17:43 schrieb mental@...32...:

...

Of course, for right now all this is moot. SVG only natively supports the sRGB color space, so that's all we can store in the actual file. The color selector is really only there for picking RGB colors (in terms of RGB, HSV, CYMK, etc...).

Thank you for this information. So inkscape, due to its SVG based design, aims at creating graphics for web and onscreen presentations rather than for prepress and professional printing. The CMYK tab confused me.

Tobias, you may find it worthwhile to look at Scribus, a sister project of Inkscape's with goals of being suitable for prepress and professional printing.

Bryce

Pavel A. da Mek wrote:

...

Maybe theoretically, but not in practice. In the prepress world is common to use the four values of CMYK, for example in 'rich black' (http://marvin.mrtoads.com/richblack_vs_plainblack.html) or in PANTONE to CMYK transformations.

Thanks for the link, it is interesting. But is all this fuss with different black colors really necessary? I would suppose that the designer should not be bothered with such matters and that the printing device should solve the corrections resulting from the fact that 100% of ink does not filter 100% of the light but, say, only 80%, and so the ideal CMYK 0,0,0,90% must be replaced by 50%,50%,50%,80%, which means 62.5%,62.5%,62.5%,100% of the maximal amount of the ink. After all, the color space perceived by humans is 3-dimensional and so there is no need to use four coordinates.

P.A.

---

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Here is an example I have done in photoshop (because it has color management) to show why CMYK colors shouldn't be modified: Imagine we want to print this image: http://img398.imageshack.us/img398/8898/degradadosok1ak.png In inkscape the colors are modified: 5,100,100,0 -> 0,100,100,5 5,100,100,100 -> 0,0,0,100 50,50,50,80 -> 0,0,0,90 0,0,0,100 -> 0,0,0,100 60,40,40,100 -> 0,0,0,100 50,25,50,25 -> 33,0,33,44 And the printed work is: http://img399.imageshack.us/img399/1849/degradadosmal8ma.png

Thats why inkscape or the printing device do not have to do the corrections, because the designer wants the exact output (first image).

Am Mittwoch, 24. August 2005 13:27 schrieb Pavel A. da Mek:

...

I can not enter colours like 5 % C, 50 % M, 75 % and 5 % K, the number always change upon entering the fourth value such that one value is 0 %.

This is the purpose for what the fourth (black) coordinate (ink) is used: to use an additional black filter and only two of the three color filters.

...

Of course I do not get the colour I want.

If you get 0 % C, 48 % M, 74 % Y and 9 % K, it is (except the small rounding difference) the same color

Dear Pavel,

thank you for your response and the clarifications. It appears that the two colours mentioned are the same on the screen.

However, I would very much prefer to see the values I enter and not some recalculated values, because I would like to take over colours from other applications or colour books and swatches where the CMYK values appear in the standard way. If you have more than one colour it gets difficult to remember which colour in Inkscape corresponds to which colour in other applications.

Additionally, when I have a CMYK colour, enter a new value for Cyan and then klick into Magenta, Cyan is zeroed in the same moment - no chance to finish the new colour. It works for me only if I start from Black K and enter Yellow, Magenta and Cyan subsequently.

Perhaps other users made similar observations?

Yours sincerely

Tobias Hilbricht