Black color_RGB to CMYK

Hi,

I have a quick question concerning color space conversion:

I noticed that a color in the RGB color space has "multiple mapping" in CMYK space. Sorry if I am using the wrong term. What I mean is that, say, a black color in RGB is 0,0,0 but in CMYK, you can have the correponding color either as C:75, M:68, Y:67 K:90 or C:50; M:50; Y:50, K:100, etc.. I also noticed that in Photoshop, it always converts a black color into C:75, M:68, Y:67 K:90 regardless whatever I set it in color picker.

I am only concerned with the black color in a RGB mode. So my question is: is C:75, M:68, Y:67 K:90 the "darkest"/"most black" in RGB mode? SOrry for my silly question.

Regards,

tweaker
idOleAsked:
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idOleAuthor Commented:
Sorry, I forgot:

The second concern would be:
How do I arbitrarily set the "darkest" color to, say: C:100%, M:100%, Y:100%, K:100% for R:0 G:0 B:0, as shown in "info" in Photoshop, instead of C:75, M:68, Y:67 K:90 as it is now?

Thanks!
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dearsinaCommented:
1. Yes, C:75, M:68, Y:67 K:90 is indeed the darkest colour when Photoshop automatically converts RGB to CMYK. Try setting it to for instance K:100, it turns out to be this kind of brownish colour. I am sure someone else can explain better why this is.

2. The best way is to do it manually. You have more control over how your image turns out.

sina
london

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weedCommented:
There are a few caveats to converting RGB to CMYK.

1) Process black is often not pure black because its using 3 or 4 colors to create a muddy sort of soup which ends up being a close approximation of black. You're mixing C, M, and Y together to get "black" which just doesnt work.

2) By laying down all those colors to reproduce a black you end up with so much ink on the paper (CMYK is intended for print remember) that it would turn the paper into a soggy mess.

3) K=%100 would result in the same as problem 2. Alot of solid ink on the paper and a soggy mess. This is where black point compensation comes in. Even if you assign 100% black youll end up with 95% to avoid the oversaturation of the paper. Not a pure black but that's better than a soggy mess.

K:100 IS pure black if youre printing it with black ink and not a mixture of CMY.
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idOleAuthor Commented:
Hi thanks all! I was aware that this might be something relate to the color management setting in Photoshop; which controls on what media the final product is designed for. So might be the "darkest color" might be changed accordingly if I changed the profile in color management seeting?

Sina, how to "do it manually", which I suppose can override the C:75, M:68, Y:67 K:90 default setting? Both weed and sina were approaching this from a "print design"'s point of view. My concern is, if I used C:75, M:68, Y:67 K:90 in an alpha channel for texturing a model in a 3D modeler or masking a layer in a video composition, will it be 100% transparent? I used it in a shockwave 3D file and it seemed not to be 100% transparent but I am not sure if this is the reason. That's why I am asking this silly question (to try to change the Black to k100% to see if the problem remains). Thanks folks again.

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weedCommented:
If you turn off black point compensation in your color management you should be able to get %100k.

If youre not using this for print there is no reason to be using CMKY colors. Use RGB.
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dearsinaCommented:
I must agree with 'weed' CMYK is a necessary print evil, I can't think why on earth you would want to use it on screen. That said, what I meant by doing it manually was to convert the image, then 'colour correct it' by either simply paint bucketing or using the 'Replace Colour' tool, something that I just played around with (concerning CMYK converts) and found out wasn't as easy as I thought, but hey, you're the one wrestling the (un)necessary evil.

sina
london

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fbordsCommented:
umm...why not just use pantone black. it should be the same for screen and print.
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weedCommented:
Because when you go to print youll get C,M,Y,K and a spot color plate. That's a pain.
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fbordsCommented:
true....then configure the colors manually in the pallette
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idOleAuthor Commented:
Thank all guys and I am sorry  I can only give credits to one answer but who cares:)
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