Converting RGB to CMYK , Using ICC Profile

Question

I\'m about to converting RGB color to CMYK for printing purpose. scale of this conversion is Adobe Photoshop ( Image -> Mode -> CMYK color )

I tried

Answer 1

As I see the theory…

The gamut of CMYK by itself is slightly smaller than RGB, so the difference after straight RGB->CMYK conversion will be easy visible. In order to compensate that diff Color Profiles are used. Color Profile inside is actually a set of color transforms which allows to enlarge the gamut. In this case image source is just a source for these transforms.

CMYK is almost always used with Color Profiles (I mean correct use cases) otherwise color loss will be significant.

In Photoshop try to convert here: Edit -> Convert to Profile. The main difference from your way - you can choose destination CMYK profile and color managment engine including Microsoft ICM, which could give you different results.

Here is a good article about CMYK: http://www.northlight-images.co.uk/article_pages/colour_management/cmyk_for_photographers.html

As for programming...

Maybe it will be easier to use some specialized image processing SDK. Color Management is a basic feature of any of them and should be available even for free.

If you need to estimate how your RGB color will be actually printed you need to use feature called “soft proofing”. This is RGB -> CMYK (with printer profile) -> RGB conversion. In Photoshop you can check it here: View -> Proof Color. You can do the same thing with SDKs.

Answer 2

May be this (untested) snippet helps a bit - it uses the the .NET api for ImageMagick.

MagickReadSettings settings = new MagickReadSettings();
settings.ColorSpace = ColorSpace.CMYK;
using (MagickImage image = new MagickImage())
{
  image.AddProfile(ColorProfile.CMYK);
  image.Read("image_rgb.tiff", settings);
  image.Write("image_cmyk.tiff");
}

If you can use the commandline this will also do the job:

convert image_rgb.tiff -profile "RGB.icc" -profile "CMYK.icc" image_cmyk.tiff

Answer 3

Let me try to help you sort it all out:

1) RGB is a format to describe a color value. i.e.: 255,0,0 for red.

2) HSV is another format to describe a color value. i.e.: 0,100,100 for red

Those two are the only ones that will give you the digitally accurate color,
you can think of them as digital representation of the color or it's real DNA and thanks god the monitor is able to present them - exactly for what they are.

Let's move on:

3) Lab is another format to describe a color value.

54,81,70 for red RGB which is (255,0,0) however..
54,81,70 can also apply to a different RGB.. (254,0,0)

why is that? because Lab format is designed to approximate human vision.
and for the human eye there is no difference between (255,0,0) and (254,0,0)
well.. not really accurate to say that for the human eye..
more accurate to say the model which is being used to get the Lab color which intends to..

4) CYMK is designed to tell the printer what mixes of Cyan, Yellow and Magenta to press to the paper and K (key or black) for how much dark to press into that mix.

So 0%,100%,100%,0% will give us the mixture for red..
and 0%,50%,50%,0% will give us pink.

Let's move on:

Your effort is to adjust the RGB which seen in the monitor to the CMYK of the printer.
God knows why you want to do that when it's been fully automatic and handled by the driver for years..
But I assume you have your reasons, So let's continue,
Each printer has a slightly different CMYK values to mix to get to that specific red..

And this is where ICC profiles come into action..
They give a standard for RED in example by providing table for (1) the original RGB-RED and (2) the CMYK red in a specific printer.

So converting from RGB(Red) to CMYK(red) according to an ICC profile is logical to wish for.

But if you try to convert back - you will notice that the CMYK for RED according to a specific profile can have multiple digital RED values..

This is because the digital color resolution is much more accurate than what comes out to a printed paper, another way to look at it is to say that a specific printer ICC could have been built upon Lab.

Now I know.. I know.. you probably already knew most of if not all of it.
(I wrote it just to be sure we both on same page in the book.)

So, when you say "none of them return the right value" (Assuming them = the conversions) what exactly do you mean?! All seems right to me, as In fact they do return the right value - for printing purposes.