Converting CMYK to RGB and back to CMYK.

[xxxmen]

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.

[Michel B]

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.

[mistermonday]

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.

[xxxmen]

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?

[pixelzombie]

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...

[TommyO]

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.

[xxxmen]

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.