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Converting CMYK to RGB and back to CMYK.

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  #1  
Old 11-27-2008, 10:12 AM
xxxmen xxxmen is offline
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Converting CMYK to RGB and back to CMYK.

Hello all,

I'm aware that RGB is a larger gamut of colours then CMYK. If I had an original image that is given to me in "CMYK" when using Photoshop for colour correcting; is there any loss in quality if I convert the CMYK image to RGB.
The reason for converting to RGB is because there are filters and options that can be applied which cannot be used in CMYK mode. I would then convert the RGB image back to CMYK. Can someone explain to me if there is loss in quality if this is created? The final file will be printed. Also if I did a straight CMYK to RGB conversion without any changes, this would not put any colours in CMYK out of gamut when converting to RGB? Am I correct in this understanding?

Thanks.
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  #2  
Old 11-27-2008, 11:11 AM
Michel B Michel B is offline
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Re: Converting CMYK to RGB and back to CMYK.

First I would like to say I am not experienced at doing those conversions, but I thank you for your question which will give me the opportunity to test my theoretical understanding of those color conversions...
The big thing in my opinion is that if RGB has a wider gamut than CMYK, CMYK has a tiny part (blues?) wider than RGB, at least sRGB. So there could be a very small risk, if such colours are present in your pictures. It might be negligible, though.
If you do your CMYK to RGB conversion and back without changes I would be surprised if you could spot a visible change. However, anything can happen when editing in RGB, and there is real risk of out-of-gamut colours. I suppose softproofing would be necessary to avoid such situations.
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  #3  
Old 11-27-2008, 11:11 AM
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mistermonday mistermonday is offline
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Re: Converting CMYK to RGB and back to CMYK.

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.
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Old 11-27-2008, 11:31 AM
xxxmen xxxmen is offline
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Re: Converting CMYK to RGB and back to CMYK.

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?
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  #5  
Old 11-27-2008, 01:14 PM
pixelzombie pixelzombie is offline
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Re: Converting CMYK to RGB and back to CMYK.

it used to be a strict cmyk workflow, but that seems to have changed..there are time when i'll need to use a RGB only filter but usually for only a small part of the image, plastic wrap for example...another way around that is to run the filter on the individual cmyk channels...
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Old 11-27-2008, 07:52 PM
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TommyO TommyO is offline
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Re: Converting CMYK to RGB and back to CMYK.

You may be thinking backwards about the color spaces.
To answer your questions:
  • Loss of quality when converting CMYK to RGB ? quality, no; color, yes.
  • Convert the RGB image back to CMYK ? quality, no; color, yes.
  • A straight CMYK to RGB conversion without any changes, this would not put any colors in CMYK out of gamut ? not exactly, all colors are converted into the nearest RGB equal. But, these are not equal color spaces (see below); RGB cannot represent all the colors of CMYK. So, in your thinking, yes there will be out of gamut problems.


First, I’d like to expand on the term you used, “gamut”, and saying that RGB has a larger gamut than CMYK. Technically this is not correct. Assuming each color space uses 8-bits to represent each channel, each channel yields 256 different levels. For RGB, this results in 256x256x256, or 16 million colors. CMYK then might look like it would yield 256x256x256x256, or 4.3 billion colors – far more than RGB. But with CMYK, since the last channel represents black, it does not add significantly to the color spectrum. In fact there is some redundancy. So, the end result is much less than the 4.3 billion. The down side to CMYK has traditionally been the larger file sizes required to add that extra channel. Today though, this tends to be less of a concern.

So, this is also the relationship we consider when deciding to convert between color spaces, or not. If you try to take that larger higher-bit four channel CMYK file and dump it into a smaller lesser-bit three channel RGB file, something must get lost. It also depends on the type images you work with, as well as the type of output devices you work with. If your images are of undetermined destinations, meaning you may have photos that could end up on the web or just sitting in your portfolio, then there may be no need to convert at all from their original profile. On the other hand, if you receive files predestined for prepress work already in CMYK, then by all means leave them in CMYK. When to convert to RGB so you can use those filters is often a tough question. Only you can make that judgment based upon your images workflow.

Two other terms used in the industry related to this are: early binding workflows; and late binding workflows. The short version… “early” means making educated decisions on the type images and their final destination, then converting them to their output profile early in the game, if not upon receipt. “Late” means by not knowing the final destination, leaving them in their original profile or an intermediate one until much later. Many of us actually work somewhere in between, or make several different conversions later to see what final adjustments are needed. There are pro’s and con’s to each method. There are many books on that subject and I suggest you get one or two.

You mentioned the “final file will be printed”, but you don’t mention to what type print device. 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). The prepress environment is about the only reason to convert your final output to CMYK.

So, you need to determine… “what is the basis or reason someone sent the original file in CMYK ? Is it necessary for my final output ?
  • If not, convert to a suitable RGB based profile (i.e. AdobeRGB or ProPhoto) and stay there. There is no need to convert back unless going to prepress.
  • If so, then pick the right time to convert so you can use that special RGB only filter; then convert back and do all other color correction and edits in CMYK.
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Old 11-28-2008, 09:41 AM
xxxmen xxxmen is offline
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Re: Converting CMYK to RGB and back to CMYK.

Thank you Tommy,

I was visually mislead by the gamuts of both RGB and CMYK. This information mentioned above has helped me understand the process more. This was very informative. Thanks.
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Old 09-07-2010, 04:31 PM
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Re: Converting CMYK to RGB and back to CMYK.

Quote:
Originally Posted by TommyO View Post
You mentioned the “final file will be printed”, but you don’t mention to what type print device. 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). The prepress environment is about the only reason to convert your final output to CMYK.

So, you need to determine… “what is the basis or reason someone sent the original file in CMYK ? Is it necessary for my final output ?
  • If not, convert to a suitable RGB based profile (i.e. AdobeRGB or ProPhoto) and stay there. There is no need to convert back unless going to prepress.
Lots of great information from TommyO. A daunting wall of text, but worth the read. Especially the last parts where he points out that for most printers you do not need to convert to CMYK (it might even be a disadvantage)...

When I receive CMYK photos that needs to be color adjusted the first thing I do is convert it to AdobeRGB. Then I never convert back.

Personally I see no reason to ever convert an image to CMYK unless there are some specific requirements for this (not just "this is the way we learnt do do it but we don't know why") or you need detailed control of the separations. Stay in RGB (Adobe RGB is often preferred if the final output will be print) to get the advantage of the smaller file size, the extra colors and powerful tools.

Soft-proof if you know it will be printed so you know what will happen in the conversion later and can compensate for this (important if you have strong saturated colors; strong red/green easily clip, and blues can be extra troublesome as they can shift towards purple). One nice thing about just soft-proofing instead of actually converting is that if you cross-publish the same image for screen/web then that version will keep the shiny colors lost in the print (without having to keep multiple copies of the image)!

If the image is going to be used in e.g. an InDesign document and then exported to PDF, the CMYK-conversion can take place in the PDF generation (you get the same result but you won't have to worry about any manual converting of every image). I've heard of teachers that still teach their students to convert all image files to CMYK first but this is simply because those teachers have outdated knowledge of the software they are using.
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  #9  
Old 10-28-2010, 11:33 AM
Morph1 Morph1 is offline
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Re: Converting CMYK to RGB and back to CMYK.

I only work in RGB space, I do large format prints and what I see on the screen most likely prints that way on my CMYK large format printer, now when you need to color match something its easier to work with 3 spinners oppose to 4 CMYK spinners,
you monitor works in RGB space not CMYK space, even with "Soft proof mode" the cmyk colors look way too rich for any printer to replicate them.....
CMYK was mainly used for process printing "Screen printing" where you print each color separatelly overleying CMYK to obtain the final print/image....,
this technology is slowly being replaced by large format printers...
I will never select my space as CMYK unless client wants to print T-Shirts or something that require process printing...

Hope this helps.

Andrew
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  #10  
Old 10-28-2010, 12:53 PM
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Thumbs up Re: Converting CMYK to RGB and back to CMYK.

Quote:
Originally Posted by Morph1 View Post
(...) my CMYK large format printer,
Also, many large format printers don't even use CMYK-inks, but also light cyan, light magenta, several different blacks... In that case, there is absolutely no point to CMYK at all!

Quote:
Originally Posted by Morph1 View Post
you monitor works in RGB space not CMYK space, even with "Soft proof mode" the cmyk colors look way too rich for any printer to replicate them.....
Using Soft Proof requires a well calibrated monitor and a accurate color profile for the equipment you are proofing for. Also, consider turning on «simulate paper color» and «simulate black ink» in the custom soft proof options.

Due to the radically different mediums it will never match perfectly, but a good soft proof will give you good idea of how it will turn out, and most importantly (even with cheap calibration), what colors will be out of gamut (and thus can't be reproduced)!
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