Adobe Color System RGB and CMYK mutual conversion principle

In order to illustrate the process of Adobe's RGB conversion CMYK, one has to first touch on some aspects of the color conversion process.

CMYK_ADOBE_COLORSPACE:
r = (k*c)/maxval;
g = (k*m)/maxval;
b = (k*y)/maxval;

The above is Adobe's CMYK to RGB conversion formula, let's take a look at how RGB is converted to CMYK The following statement is a rewritten Actionscript for Flash:

// RGB to CMY color conversion
Function rgbtocmy () {
Mycyan = 255 - myred;
Mymagenta = 255 - mygreen;
Myyellow = 255 - myblue;
correctCMYK();
}
// CMY to RGB color conversion
Function cmytorgb () {
Myred = 255 - mycyan;
Mygreen = 255 - mymagenta;
Myblue = 255 - myyellow;
correctCMYK();
}
// CMYK color correction
Function correctCMYK () {
If (rep_v != "ERR") {
Var temp = Math.min(Math.min(mycyan,mymagenta),myyellow);
If (temp != 0) {
Temp2 = Math.round((rep_v / 100) * temp);
Rep_k = Math.round((temp2 / 255) * 100);
Rep_c = Math.round(((mycyan - temp2) / 255) * 100);
Rep_m = Math.round(((mymagenta - temp2) / 255) * 100);
Rep_y = Math.round(((myyellow - temp2) / 255) * 100);
} Else {
Rep_c = Math.round((mycyan / 255) * 100);
Rep_m = Math.round((mymagenta / 255) * 100);
Rep_y = Math.round((myyellow / 255) * 100);
Rep_k = 0;
}
}
}

We know that RGB and CMYK do not have a one-to-one correspondence. There is a problem with the replacement value of the black part.

However, the RGB can all correspond to the CMY format, and its range is 0-255. From the color wheel, we can easily understand the complementary color calculations. Therefore, the exchange of CMY and RGB can be said to be "lossless, reversible".

After getting the corresponding CMY from RGB, if we want to get the CMYK value, we must define a "black replacement value", that is to say, extract some components in each color channel to form a black channel. The CMYK value is 0-100.

The value of this "black replacement value" is different, and the resulting CMYK component distribution is also different. That is, we can choose CMYK's color separation scheme as "UCR" or "GCR" in Photoshop.

The larger the "black replacement value," the less likely the color cast in the CMYK image is when printing, but the corresponding condition is that the color ink component of the image is thin in the dark area and is not sufficiently saturated. This separation method is suitable for occasions where the printed material is thin or coarse and the printing accuracy is not high.

Note: Why does the direct color reduction and direct conversion of grayscale appear to be worse than that of Lab's L channel?

We still compare using the color conversion formula to speak:

L = 0.3R+0.59G+0.11B

Gray = (R+G+B)/3

It's easy to see the difference. The average gray value and the value blended according to the color brightness gain ratio are completely different.

Source: HC Network Printing Industry Channel