Converting RGB to CMYK

[drummerpaco]

I have question on converting RGB images to CMYK.
I know that RGB color gamut is much wider than CMYK, however we like to eliminate color shifting as much as possible.

Currently we have tested that two RGB settings (Adobe RGB (1998) and sRGB) converting to CMYK (Coated GRACoL 2006) using Photoshop CS4.

The test result from Adobe RGB (1998) to CMYK (GRACoL 2006) and sRGB to CMYK (GRACoL 2006) shows different conversion.
Some of the details has been disappeared even though Adobe RGB (1998) has bigger color space, but sRGB to CMYK is still has the original details.

By searching many color management forum, we found many of professionals uses Adobe RGB (1998) convert to CMYK color space which by the test, we found sRGB is better solution.

What is your thoughts and suggestion for converting CMYK o RGB?
Also should we use 1)relative colormetric or 2)perceptual?

[Mike F]

colorometric for vector and perceptual for raster is what I was taught.

[photoplan]

Hello drummerpaco,

you should use a perceptual intent for RGB pictures.

You can read this :

Understanding Rendering Intents

Color Management: Color Space Conversion

[David Milisock]

I have question on converting RGB images to CMYK.
I know that RGB color gamut is much wider than CMYK, however we like to eliminate color shifting as much as possible.

Currently we have tested that two RGB settings (Adobe RGB (1998) and sRGB) converting to CMYK (Coated GRACoL 2006) using Photoshop CS4.

The test result from Adobe RGB (1998) to CMYK (GRACoL 2006) and sRGB to CMYK (GRACoL 2006) shows different conversion.
Some of the details has been disappeared even though Adobe RGB (1998) has bigger color space, but sRGB to CMYK is still has the original details.

By searching many color management forum, we found many of professionals uses Adobe RGB (1998) convert to CMYK color space which by the test, we found sRGB is better solution.

What is your thoughts and suggestion for converting CMYK o RGB?
Also should we use 1)relative colormetric or 2)perceptual?

Ther RGB color sace you use is a matter of requirements, for captures I use prophoto, convert to Adobe RGB or ECI RGB for print ans sRGB for web or presentation.

I use perceptual rendering for all RGB to CMYK conversions because the important thing to remember is that color is a perception and there is no correct way to convert something large to something small however a methode that maintins a persons perception is in my opinion the best.

[Lukas Engqvist]

The colour space and the colours used in the images are not allways the same.
To use perceptual rendering is like using an instamatic camera, you fit everything in the picture and get a reasonable representation, a fair compromise. It is also the most reasonable when there is a large difference in gamut size/shape. Synthetic images will have detail all over at the expense of having anything looking it's best. Compromise is sometimes the way to go though!

To use relative, is like making a decision on how to crop an image, what you get you get more of, but it is important to evaluate using proof colours so that you are not compromising in important details. Together with a good monitor and manual assessment/intervention this will allow for brightest and best colours. In preview see that no essential details are lost, if some bright coloured area needs more detail, usually taking down saturation ever so slightly will bring back those details. From Adobe RGB to GRACOL on coated relative colometric will give great results.

Which RGB space to use depends on your source, for untagged images sRGB is a better blind guess, but many photographers will use AdobeRGB. (I would not recommend ProPhoto RGB unless you are using 16 or 32 bit images, and that you are aware that you a large number of colours in that space can not be viewed on any monitor, so you run the potential risk of making edits that you cannnot evaluate. How one would capture in Prophoto I don't know...would that be from Camera Raw, and how would you evaluate it?)
When you are evaluating RGB what to look for in the sRGB is how Cyan is represented. sRGB is weak in representing Cyan, and in that area there are printed colours that you cannot represent in that space.

PS @photoplan ty for the links the one image with the magenta blobs was a nice clear example of the differrences.

[gordo]

From Adobe RGB to GRACOL on coated relative colometric will give great results.
[SNIP]

PS @photoplan ty for the links the one image with the magenta blobs was a nice clear example of the differrences.

Relative colormetric is what i've used for the majority of work that I've done for print.

Where is "photoplan ty"?

best, gordo

[David Milisock]

The colour space and the colours used in the images are not allways the same.
To use perceptual rendering is like using an instamatic camera, you fit everything in the picture and get a reasonable representation, a fair compromise. It is also the most reasonable when there is a large difference in gamut size/shape. Synthetic images will have detail all over at the expense of having anything looking it's best. Compromise is sometimes the way to go though!

To use relative, is like making a decision on how to crop an image, what you get you get more of, but it is important to evaluate using proof colours so that you are not compromising in important details. Together with a good monitor and manual assessment/intervention this will allow for brightest and best colours. In preview see that no essential details are lost, if some bright coloured area needs more detail, usually taking down saturation ever so slightly will bring back those details. From Adobe RGB to GRACOL on coated relative colometric will give great results.

Which RGB space to use depends on your source, for untagged images sRGB is a better blind guess, but many photographers will use AdobeRGB. (I would not recommend ProPhoto RGB unless you are using 16 or 32 bit images, and that you are aware that you a large number of colours in that space can not be viewed on any monitor, so you run the potential risk of making edits that you cannnot evaluate. How one would capture in Prophoto I don't know...would that be from Camera Raw, and how would you evaluate it?)
When you are evaluating RGB what to look for in the sRGB is how Cyan is represented. sRGB is weak in representing Cyan, and in that area there are printed colours that you cannot represent in that space.

PS @photoplan ty for the links the one image with the magenta blobs was a nice clear example of the differrences.

This is a great discussion!!

The problem with any of the colorimetrics is this and is acerbated as the size of the gamut of the sourse RGB file increases. Example a prophoto image that utilizes a large portion of the gamut may have 70% of the colors out of gamut for the destination space. If converted using a colorimetric 30% of the colors are converted with accurate equivalents but 70% are radically changed altering th perception of the overall image.

This can also be seen using sRGB and Adobe RGB if one uses a scene that has several different but close hue greens in it. A capture that hase the underside of trees is another example.

[Stephen Marsh]

Also should we use 1)relative colormetric or 2)perceptual?

As a "general rule" Rel Col. Say 90% of images that I converted might have been slightly better with Rel Col, while say 8% of images might have been better with Perceptual, while say 2% of exceptional images may have used "other" methods to get to an acceptable CMYK version of the RGB original. Of course, in most cases, a Rel Col transform will need BPC turned on in the colour conversion when using the Adobe CMM.

That being said, the wider the gamut the RGB space, the more so that one may need perceptual over Rel Col (presuming that the image actually uses the wider gamut).

On an image by image basis, it is easy to check all four rendering intents when going from RGB to CMYK using a common table based CMYK output/printer profile. If one was doing a blind conversion and wanted a default intent with no user input or feedback, then it is all a matter of trade offs.


Best,

Stephen Marsh

[meddington]

Suffice to say that rendering intent choice is gamut and content dependent. As far as larger gamut size goes such as prophoto, theres a choice to make between mapping color to preserve the perceptual relationship (and perhaps sacrificing overall color accuracy and dynamic range), and using relative colorimetric, perhaps clipping colors that the monitor could not display nor could be reproduced via the print process on hand. Each rendering intent has a place. I personally find the vast majority of RGB imagery tagged with AdobeRGB or sRGB coverts quite well with rel. col. One things certain, rendering intent use can be subjective and a one size fits all approach will have caveats.

[David Milisock]

As a "general rule" Rel Col. Say 90% of images that I converted might have been slightly better with Rel Col, while say 8% of images might have been better with Perceptual, while say 2% of exceptional images may have used "other" methods to get to an acceptable CMYK version of the RGB original. Of course, in most cases, a Rel Col transform will need BPC turned on in the colour conversion when using the Adobe CMM.

That being said, the wider the gamut the RGB space, the more so that one may need perceptual over Rel Col (presuming that the image actually uses the wider gamut).

On an image by image basis, it is easy to check all four rendering intents when going from RGB to CMYK using a common table based CMYK output/printer profile. If one was doing a blind conversion and wanted a default intent with no user input or feedback, then it is all a matter of trade offs.


Best,

Stephen Marsh

>Of course, in most cases, a Rel Col transform will need BPC turned on in the colour conversion when using the Adobe CMM.


If you do extensive testing Rel COl with BPC on is very much like perceptual just NOT ICC compliant.