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

Converting CMYK to RGB and back to CMYK.

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  #11  
Old 11-01-2010, 08:53 PM
bakerser bakerser is offline
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Re: Converting CMYK to RGB and back to CMYK.

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Originally Posted by mistermonday View Post
Converting CMYK to RGB will incurr slight loss because both color spaces are not absolute and there will be some round-off error in the conversion process. You also need to make sure that whatever processing you do in RGB does not move some colors out of the CMYK gamut otherwise you will have significant loss when you convert back to CMYK. I assume your final conversion back to CMYK is necessary because printing will be done on a commercial press.
+1

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Originally Posted by xxxmen View Post
It sounds from the responses so far that there is a small risk in converting to RGB. Do most pro retouchers utilize a strict CMYK workflow? How do they workaround a situation in where they require using a filter or effect that is only allowed to be used in RGB mode?
IMO, using an RGB flavor which most closely matches your final CMYK gamut is the way to go for accessibility, safety, and quality. If you're soft-proofing and/or using gamut warning, CMYK conversion should be the last step in the game (if at all).

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Originally Posted by TommyO View Post
RGB cannot represent all the colors of CMYK. So, in your thinking, yes there will be out of gamut problems.
OOC, what CMYK color can't be represented by an RGB triplet in any color space?
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Originally Posted by TommyO View Post
Most high end inkjet printers that work with CMYK inks do not require you to convert the file to CMYK for printing; even the high end printers at the local printshop, i.e Fuji or Noritsu. They are fully aware most output is RGB and internally compensate for that fact, doing all the final conversions necessary on the fly. It can actually be worse to convert to CMYK (if you do, be sure to soft proof using an actual profile from the printshop).
+1. Unless you absolutely have to (or really really know what you're doing), let the RIP handle the conversion.
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  #12  
Old 11-01-2010, 11:56 PM
pixelzombie pixelzombie is offline
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Re: Converting CMYK to RGB and back to CMYK.

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Originally Posted by bakerser View Post


OOC, what CMYK color can't be represented by an RGB triplet in any color space?
the really clean cyans, i'll take a snapshot of the 2 spaces laid over one another at work tomorrow...
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  #13  
Old 11-02-2010, 12:06 AM
bakerser bakerser is offline
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Re: Converting CMYK to RGB and back to CMYK.

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Originally Posted by pixelzombie View Post
the really clean cyans, i'll take a snapshot of the 2 spaces laid over one another at work tomorrow...
I look forward to it.
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  #14  
Old 11-02-2010, 04:16 PM
pixelzombie pixelzombie is offline
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Re: Converting CMYK to RGB and back to CMYK.

here is a 2D look comparing sheetfed to sRGB, the white box represents cmyk and the larger gray box is RGB, i have also posted a comparison in a 3D form...you can easily see the area outside of the gray box that is only represented by CMYK..the differences would be even greater if you were comparing an actual output device such as an inkjet versus a Lambda...
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File Type: jpg 3d.jpg (22.7 KB, 6 views)
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  #15  
Old 11-02-2010, 04:18 PM
bakerser bakerser is offline
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Re: Converting CMYK to RGB and back to CMYK.

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Originally Posted by pixelzombie View Post
here is a 2D look comparing sheetfed to sRGB, the white box represents cmyk and the larger gray box is RGB, i have also posted a comparison in a 3D form...
Nice graphics.

But I'm confused then. Why don't you think that a larger-gamut color space can represent that range in RGB?
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  #16  
Old 11-02-2010, 04:20 PM
pixelzombie pixelzombie is offline
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Re: Converting CMYK to RGB and back to CMYK.

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Originally Posted by bakerser View Post
Nice graphics.

But I'm confused then. Why don't you think that a larger-gamut color space can represent that range in RGB?
not sure what you mean
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  #17  
Old 11-02-2010, 04:30 PM
bakerser bakerser is offline
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Re: Converting CMYK to RGB and back to CMYK.

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Originally Posted by pixelzombie View Post
not sure what you mean
You've compared the colorspace of a flavor of CMYK to that of sRGB, which is one of the smaller RGB colorspaces. But does AdobeRGB encompass all those values? Does ProPhotoRGB?

It seems (as someone who is still getting used to being here again) that this conversation has become a bit confused (not just you and I, but generally) as to the difference between a color mode (RGB, CMYK, LAB, etc.) and a color space - something which is absolute, and exists independent of the mode by which the color itself is being represented or manipulated. So, to say that a color cannot be recreated in RGB (a color mode) wouldn't be correct, while saying that it can't be recreated in sRGB (a color space) could be. I felt for a long time that that was an arbitrary and somewhat anal distinction, but when we start talking about different CMYK profiles, different conversion methods, etc., it starts to become very, very important to understanding how color can and should be represented.

...

Hopefully that makes more sense.
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  #18  
Old 11-02-2010, 04:41 PM
pixelzombie pixelzombie is offline
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Re: Converting CMYK to RGB and back to CMYK.

i only used sRGB as it is one of the defaults in photosho and we still get files in that colorspace...yes, AdobeRGB does encompass all of those values but you are still limited to what your actual output device can reproduce..i don't know if there is an output device that can reproduce the AdobeRGB spectrum...
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  #19  
Old 11-02-2010, 04:46 PM
bakerser bakerser is offline
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Re: Converting CMYK to RGB and back to CMYK.

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Originally Posted by pixelzombie View Post
i don't know if there is an output device that can reproduce the AdobeRGB spectrum...
Most of the high end monitors coming out now can get almost if not all of AdobeRGB; it'll probably be a while before they get near ProPhotoRGB (and hopefully they'll be 12+bit by then).

Some of the highend photo printers can equally get almost all of AdobeRGB. I don't know of any presses which come close to it, though.

I admire that you stick to a space closer to your output, though - it seems that everyone wants to use big spaces for everything lately.
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  #20  
Old 11-02-2010, 05:38 PM
pixelzombie pixelzombie is offline
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Re: Converting CMYK to RGB and back to CMYK.

it all depends on what the client is willing to sign off on, i have shown them 2 different c-prints in the past, one with colormatch as the working and one with adobeRGB and in all cases they shave said the aRGB print is far too saturated...
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