question about RGB / CMYK colors in color picker

F
Posted By
freedda
Sep 2, 2008
Views
792
Replies
15
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Closed
In the color picker, if I enter R255-G80-B0, the CMYK color reads M83-Y100. However, if I instead enter the M83-Y100 in CMYK first, then the RBG numbers are R240-G83-B35, and if I enter these numbers first, then the CMYK numbers change again.

Why is this happening? Shouldn’t the RBG translate to to just one CMYK color and vis versa?

David.

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CB
charles badland
Sep 2, 2008
No. Your first color (R255-G80-B0) is out of gamut for CMYK. It can’t be created using CMYK colors.
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freedda
Sep 2, 2008
Now to be clear, I’m not converting from RGB to CMYK color space or image file type, I’m simply reading the color numbers in the color picker. The CMYK numbers are filling in when I enter the RGB numbers, and the LAB and HSB numbers are filling in as well.

If I am picking an out of gamut color for CMYK, is there a way I can tell this beforehand?

Best, David.
CB
charles badland
Sep 2, 2008
Now to be clear, I’m not converting from RGB to CMYK color space or image file type, I’m simply reading the color numbers in the color picker.

I understand. But you can’t describe an out of gamut color inputing into the CMYK field. If a color is out of gamut from the Color Picker, you’ll see a little color box next to the preview swatch with a small triangle warning-sign/exclamation -mark above it.
JM
J_Maloney
Sep 2, 2008
I’m not converting from RGB to CMYK color space

Yes, you are. Change your color settings and watch the numbers change too.

J
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freedda
Sep 2, 2008
Yes, you are. Change your color settings and watch the numbers change too.

J, I don’t understand what you are saying. I’m working with an RGB image, and just picking colors.
F
freedda
Sep 2, 2008
charles badland – 12:42pm Sep 2, 08 PST (#1 of 5)

No. Your first color (R255-G80-B0) is out of gamut for CMYK. It can’t be created using CMYK colors.

So Charles, if I have in a photoshop image that are out of gamut, how does my printer handle/render them? Maybe too big a question for this discussion but I’m curious, since I have a specific reason for selecting these colors.

Best, David
JM
J_Maloney
Sep 2, 2008
I’m saying your working RGB color space (in your case, sRGB) and your working CMYK color space (in your case, SWOP Coated) will have bearing on what RGB values translate to what CMYK values. You are, in effect, converting an RGB image to CMYK and back when you play in the picker.

Your out-of-gamut question depends on your process and intent. Are you trying to match corporate spot colors? An existing real-world object? Your screen? Are you going to print on a digital press or offset?

If you’re trying to match a specific color, your best bet is probably to let the printer know. You might be better off giving them a sample of the match and having them "correct" your file, rather than fighting through rounds of proofs. You can almost guarantee that you will not get a good result if the press operator & pre-press team have no idea what you’re trying to match.

J
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freedda
Sep 2, 2008
Okay, thanks J. I don’t fully grok it, but that’s okay, especially since I’m only talking about inkjet printing.

Best, David.
JM
J_Maloney
Sep 2, 2008
Stick with RGB then. Your inkjet has a considerably larger gamut than SWOP and might do OK without too much fiddling.

J
J
Jim
Sep 2, 2008
wrote in message
charles badland – 12:42pm Sep 2, 08 PST (#1 of 5)

No. Your first color (R255-G80-B0) is out of gamut for CMYK. It can’t be created using CMYK colors.

So Charles, if I have in a photoshop image that are out of gamut, how does my printer handle/render them? Maybe too big a question for this discussion but I’m curious, since I have a specific reason for selecting these colors.

Best, David
The printer or photoshop converts the out of gamut colors to the nearest approximation of the original in the target gamut.
Jim
CB
charles badland
Sep 3, 2008
…how does my printer handle/render them?

Your printer? Do as "J" says: keep your image in RGB. 99.9% of all desktop inkjet printers are RGB devices: they expect RGB files.
DF
David_Freed
Sep 3, 2008
Charles and J, I should be more clear about my intent.

I’m making transparencies for use in making photo-etching plates. I’m doing this on my Epson R2400 printer, using the Quadtone RIP to control ink colors and resolution. I’ve found that certain colors are better at filter UV light, (necessary for my process, as I’m exposing the plates with the transparencies on top in a UV exposure unit)

Based on colors which I know the RGB values for (e.g. B255, G80) and which I believe are better at filtering UV light, I’m trying to find the corresponding CMYK values so I can use the Quadtone RIP to select which CMY inks to print with. So, getting back to my original post, I’d try to a ratio of 83 magenta and 100 yellow to print with. I’m not really messing at all with the file color space.

I don’t know if that changes any of the info. you provided, but I thought I’d clarify.

Best, David
CB
charles badland
Sep 3, 2008
David,
I’m not quite sure I understand what you are trying to do. I know a number of people who use an R2400 to output; none use a RIP.
A RIP can certainly allow control on ink levels on output… Does a RIP for the 2400 control of the individual non-process inks? Such as Magenta and Light Magenta?
Sorry, but a little out of my experience.
CB
F
freedda
Sep 3, 2008
Charles, I’m doing an "alternative photo process" to be sure, which is why its a bit confusing. I’m using a light-sensitive film to create an etching plate (based on a digital image) that I then ink up and run through an etching press. It’s sort of a modern version of photogravure, but not nearly as detailed.

So, what I’m trying to do is create a transparency made up of ‘dots’ of color, with ‘spaces’ of clear film around them (similar to a bitmap image), which gives me the "etch" on my plate–I then use this transparency to expose my plate in a UV exposure unit.

The reason I’m using the Quadton RIP is because I have to print at a lower resolution (using 1440 dpi) than I can do with the Epson native program to create the "etch" in my print. The Quadtone RIP also always me to select any of the ink cartridges (from all of them, including non-process colors) and to determine how much of each ink will be laid down. The native Epson driver for my printer does not offer this level of control.

The Quadtone RIP is an inexpensive program that is mainly using by people doing fine art black and white images. It gives you control over the inks and how they are laid down. Some people also use 3rd party ink sets or bulk ink systems along with this. See: <http://www.quadtonerip.com/html/QTRoverview.html> if you want to know more.

Having said all this, I may have found a solution to my problem with a Photoshop plug-in that creates stochastic screened (also called an aquatint screen) images from my photos. See Toby Thain, "bitmap and mimicing stochastic/aquatint screen" #1, 2 Sep 2008 8:00 pm </webx?13/0>

Sorry if I’m rambling, but seemed like it needed more explaination than I gave before.

Regards, David.

I know a number of people who use an R2400 to output; none use a RIP.

Does a RIP for the 2400 control of the individual non-process inks such as Magenta and Light Magenta?

What if you just used some default standard color management and sent an RGB file to the printer without a RIP?
CB
charles badland
Sep 3, 2008
Interesting. Might try out that RIP.
Good luck.
CB

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