Painting black on channels

[tranquil222]

Ok, heres a somewhat tough question. I was trying to figure out what black and white meant in both colorspaces RGB and CMYK, so i painted a black line in the cyan channel. This gave me a cyan line, with no other channels affected. But then when I switched it to RGB, it put some of that black line in the cyan channel in the green channel. Why is that? I thought that cyan magenta yellow and red green and blue were the exact same and affecting the black in cmyk was the only thing that would affect more than one channel in a colorspace conversion from CMYK to RGB. [confused]

confused,

 

[Rantin Al]

The very very basics:

RGB is termed additive. As you add them together you get white.
CMY is termed subtractive. You need to reduce CMY to zero to get white.

100% CMY gives black in theory. In reality it gives a dirty grey brown.
The Black channel is an extra which is required for the CMYK print process to 'boost the black'.

To see the relationship between RGB and CMY, open an image and go to Image>Adjust>Variations. Have a look at the main part of the window with the six colours arranged around the 'Current View'.
See how the colours are split.

It's a heck of a subject to explain in a couple of lines, but I hope this gives you a start.

Cheers Al

 

[tranquil222]

I know....

Yeah, i'm pretty versed in the basics (additive & subtractive colorspace)sand some advanced topics of color correction(ive read michael kieran's and dan margulis's books on that)but where i get baffled is understanding channels and how they interact with each other and their differences. thanks for the help

 

[Rantin Al]

Each Channel is a greyscale representation of each colour component.
When you 'paint' with black on a single channel (say Cyan) the black area defines the part in the image which will contain cyan.

When you convert to RGB, there is no direct conversion to a single equivalent channel. Therefore the original cyan must be 'blended' from the available RGB components.

The reverse is also true when you convert from say a solid Red in RGB. The CMYK equivalent needs to be a blend of magenta and yellow.

Is this any help?
Cheers Al