Tony Cooper wrote:
I was working through some tutorials on the subject of colorizing a black & white image. One made a particular point of converting the image from grayscale to CMYK. Since I normally convert to RGB, I started looking up the two terms.
There’s quite a bit of information on what RGB is, and what CMYK is, but no information that I found on why to choose which or which projects require which.
Under what circumstances is CMYK preferable to RGB and vice-versa?
As output spaces, RGB is designed for CRT’s, CMYK is designed for commercial color printing. Due to historical driver issues, and marketing segmentation, consumer inkjets happen to be RGB devices, but this will hopefully change in the future to give people like us more control over the final result.
But, in addition to their roles as output spaces, both RGB and CMYK may also be thought of as color correction spaces, and each as their own advantages depending on your judgement of the subject matter of the image and what needs to be done to the image to improve it.
CMYK is good for images with dark shadows, or black objects, in which you wish to retain detail. It is also good for controlling texture and detail in red, green, and blue objects, and for increasing color differentiation between an object and a similarly colored background. It is a relatively subtle color space – meaning that a large curve move is possible wth relatively small impact on the color being adjusted and on the overall brightness of the image.
RGB is good for removing a medium to large color cast, particularly a color cast that is dependent on illumination. RGB is also good for controlling textures on cyan, magenta, and yellow objects.
Then there’s Lab, and HSB, which also have their specialties. Lab is good for very large color casts, setting overall brightness without changing color hues, and for setting neutrals to get rid of color casts in images that don’t suffer from mixed illumination. HSB is good for increasing saturation in images with a mixture of saturated and non-saturated colors, for example a landscape with saturated colors in the foreground, and haze for objects that are further away.
For a tutorial on color correctiion in different color spaces, download the Curvemeister plugin for windows demo and follow the tutorials, which are fully functional. This plugin allows simultaneous correction in four different color spaces, and adds some improvements to the curve interface: www.curvemeister.com
I am also starting a yahoo group devoted to using curves for color correction. Although it is aimed at Curvemeister customers, I will be happy to answer questions about curves and color spaces in general in this group:
http://groups.yahoo.com/group/Curvemeister/ —
Mike Russell
www.curvemeister.com
www.geigy.2y.net