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Input/Output/Workflow Scanning, printing, color management, and discussing best practices for control and repeatability

Convering from RGB to CMYK

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  #11  
Old 12-02-2007, 12:11 PM
pixelzombie pixelzombie is offline
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Re: Convering from RGB to CMYK

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Originally Posted by Leo View Post
Sory, I´m new to this things. Thank you for your patience. I don´t want to send RGB because than I can´t conrol the rusults of convertin. Am I right? Well the first colorspace was Euroscale coated v2. The results were bad. Than I have changed the profile to Toyo Inks uncoated, and this works very goog and only this profile. The problem is that I don´t know which profile will be used in the printing offece. I will investigate it today. And the main problem that my picture has 90% of reds.
check the Gamut Warning in photoshop of your rgb file, chances are they may be beyond the gamut of a typical inkjet printer and having the right profile would get somewhat pleasing results..i would suggest that you desaturate the reds by blending some of the information from another channel so the reds don't move very far once you make the conversion...
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  #12  
Old 12-02-2007, 12:16 PM
pixelzombie pixelzombie is offline
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Re: Convering from RGB to CMYK

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Originally Posted by theboyk View Post
True, they use a CMYK inkset (typically CMYK, then depending on the printer/manufacturer, light cyan, light magenta, photo or matte black, light black, light light black, etc.) but they use these inks (typically with a minimum of 5 inks for lower end inkjets, and up to 12+ for pro machines, plus sometimes a gloss ink coating, such as in the Epson R1800) to output the RGB gamut on the paper. But, when you send the job to print, unless you run it through a RIP (or, direct from Photoshop w/ proof options) and have it set to simulate a CMYK output device (which, you would never want to do, because of the loss of color gamut, unless you were outputting proofs for going to a true CMYK press) then you're printing from RGB as your source file. There is no good reason (expect for proofing) that I know of where you'd want to print CMYK when you have the option to print RGB. Why would you not want to output the full gamut of colors of RGB?
you could send the printer a RGB file, but somewhere along the line it will get converted to cmyk and there is no guarantee that they will use or even have the right profile for your needs...
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  #13  
Old 12-02-2007, 12:30 PM
theboyk theboyk is offline
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Re: Convering from RGB to CMYK

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Originally Posted by pixelzombie View Post
you could send the printer a RGB file, but somewhere along the line it will get converted to cmyk and there is no guarantee that they will use or even have the right profile for your needs...
Well, I'd find a printer who knows what they're doing then.
Any decent printer outputting to canvas (we're talking about canvas, right?) would be outputting to some sort of Epson or other inkjet that handles canvas. They would much rather get the files as RGB and they would NOT print/convert to CMYK (if they wanted to simulate a CMYK output device, even then they'd just set the output simulation profile to the CMYK device you're proofing for and the original image you submit would never be converted from RGB to CMYK — but, that would be for proofing a print going to a CMYK device and that's not what we're talking about here, we're talking about outputting an image to canvas, most likely a very limited run). They'd use the RGB file you sent them (if anything, I'd get the RGB profile of the printer's RGB working space), the RIP would use the working space RGB profile (thus why I'd get their RGB working space profile so at least you can supply your RGB image in their working space) and the output profile would be set to the profile of the paper (in this case, canvas) you're printing on. This will give you your best possible print.
If you want to limit your colors (which is why you're saturated reds are loosing their brilliance — true CMYK can't replicate these brilliant saturations) then go ahead and convert to CMYK. But, if you want a nice, brilliant print on canvas using the full colors possible, then leave your image as RGB.
k.
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  #14  
Old 12-06-2007, 10:44 AM
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Leo Leo is offline
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Re: Convering from RGB to CMYK

Thank you everyone for help!
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  #15  
Old 12-06-2007, 03:42 PM
pixelzombie pixelzombie is offline
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Re: Convering from RGB to CMYK

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Originally Posted by theboyk View Post
Well, I'd find a printer who knows what they're doing then.

If you want to limit your colors (which is why you're saturated reds are loosing their brilliance — true CMYK can't replicate these brilliant saturations) then go ahead and convert to CMYK. But, if you want a nice, brilliant print on canvas using the full colors possible, then leave your image as RGB.
k.
so let me get this straight, you'd rather send a RGB file to the rip instead of a cmyk file?
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  #16  
Old 12-06-2007, 04:04 PM
theboyk theboyk is offline
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Re: Convering from RGB to CMYK

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Originally Posted by pixelzombie View Post
so let me get this straight, you'd rather send a RGB file to the rip instead of a cmyk file?
If printing to any sort of inkjet based printer (which is pretty much what you'd be using if printing to a canvas) and you want to print the full color gamut of your image without restricting it to the limited gamut of CMYK, then yes, without a doubt you would want to send to the RIP as RGB and it would print using the entire RGB gamut (based on that printer type profile). Even if you're trying to proof the image as if it were going to be printed on a traditional CMYK press (offset, web, etc.), you'd send it as RGB and let the RIP simulate the CMYK printing (based on the CMYK press profile supplied by your printer). As soon as you convert your image to CMYK you've lost a massive amount of color data because of the limited CMYK gamut. Keep it in RGB.

And even if you were sending your files to a traditional printer (CMYK offset press, web press, etc.) if you're lucky and you have a good printer with a color managed workflow, you'll get the best results by leaving your images in RGB (again, based on their RGB working space/profile) and again let their RIP manage the conversion on the fly from RGB to CMYK (thus, never having to convert your image out of the RGB color space). If your printer doesn't have an RGB color managed workflow and you *have* to convert to CMYK, then you should, at the very least, get their CMYK press profile for converting your images. Otherwise, under most cases, your randomly converted CMYK profile will be ignored by their RIP and their working CMYK profile will be assigned (and this could result in an unwanted color shift in your image because color numbers are preserved rather than converted).

So, in short, whenever possible, stay in RGB. I know this sounds crazy because these days everyone thinks RGB is for the web and CMYK is for print, but that's just not true. RGB is the way to go whenever possible (and when you do have to convert to CMYK, ALWAYS keep a copy of your RGB original).

k.
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  #17  
Old 12-06-2007, 06:45 PM
pixelzombie pixelzombie is offline
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Re: Convering from RGB to CMYK

i've never heard anyone recommend sending an rgb file to a RIP unless you were sending it to an RGB output device...
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  #18  
Old 12-06-2007, 07:00 PM
theboyk theboyk is offline
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Re: Convering from RGB to CMYK

Wow, that's too bad. You should look into it.
k.
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  #19  
Old 12-06-2007, 07:22 PM
pixelzombie pixelzombie is offline
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Re: Convering from RGB to CMYK

i used to work for Schawk and i'm sure they know the proper way to send a file, but i will run a test when i get a chance...
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  #20  
Old 12-06-2007, 10:18 PM
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Re: Convering from RGB to CMYK

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Originally Posted by pixelzombie View Post
so let me get this straight, you'd rather send a RGB file to the rip instead of a cmyk file?
Some boxes rip both RGB and CMYK. The calibration of the RIP will affect both in a different way.

The box changes all inputs to RGB, then does the calibration offsets and prints in CMYK.

CMYK is the standard for basic-grade colour printing (printing as in printing, rather than printing as in photography).

Send the files as RGB unless otherwise requested by the printer - it is a fallacy that all printers print in CMYK.

EDIT - Your photographic images are held in additive (RGB) form. The simple rule is that for all normal photographic printing purposes you should hand them to the printer driver or RIP as RGB and let it sort out how to use the inks at its disposal. Your control over that is through the colour management facilities in the software you are using. Do not try to do the CMYK conversion yourself. In any event, if you are using a modern printer it is not CMYK that you are converting to anyhow, but (in the Canon case) C.pC.M.pM.Y.R.G.K.
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