RGB vs CMYK

TC
Posted By
Tony Cooper
Jan 17, 2004
Views
374
Replies
4
Status
Closed
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?

I

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edjh
Jan 17, 2004
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?
I
CMYK is needed for printing on offset printing presses which is what’s used for most magazines, books, newspapers, etc. RGB for inkjets and web.


Comic book sketches and artwork:
http://www.sover.net/~hannigan/edjh.html
MR
Mike Russell
Jan 17, 2004
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
BV
Branko Vukelic
Jan 17, 2004
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?
I

RGB and CMYK record color values differently. You can see in the PS manual that rounding of values occur every time you convert from one mode to the another, so inevitable loss of color data occurs. CMYK is for print, RGB is multipurpose. (Most Inkjet printers also accept RGB and not CMYK.) Most filters would work only in RGB mode. So, it’s advised that you work with RGB as long as you can and convert to CMYK later in the process for fine-tuning.

There’s also an issue of gamuts. RBG and CMYK reproduce colors differently (RGB by emitting light, CMYK by reflecting it). RGB can reproduce a fairly larger number of unique colors compared to CMYK, but CMYK can still reproduce color that RGB can’t (pastel colors, most of them in brown hue). Since RGB can reproduce so many colors that’s one more reson to keep the document in that mode until a conversion is required. Lab color mode has the gamut that encompasses all colors visible (therefore, it includes both CMYK and RGB). It’s notation (Using L, a and b components, where L is the grayscale value of the color, a for amount of either green or red, b for amount of either yellow or blue) is extremely useful for precisely correcting color and very useful for correcting the dark areas of the image (also switching to L channel yields best results with Unsharp Mask effect). However, it’s notation is also such that you’d still loose color data (because of the rounding) when converting to and from Lab mode.

When working with images (but not reproductions) I usually switch to Lab early on, and turn on "Proof Colors" (naturally, you’d have to color manage your system) to see how it looks in CMYK space. I work with Lab right until the end, and then switch to CMYK prior to "Save". OTOH, if you work with filters, you’d want to remain in RGB space for as long as possible. If you work with the WWW, you don’t need to switch to anything.

Once you get a better understanding of curves, you’ll quickly discover that you work more in one mode than the others. So that should be your choice. I’ve heard people complaining about RGB because the lack of yellow channel was confusing to them, but there were also people who found RGB more intuitive to work with. The former group were mostly designers that were in business before the PC era, so CMYK was natural to them. Today, people work more in RGB mode, since it has more advantages over the CMYK.

All in all, you’d have to decide between various modes twice: once while you’re editing the image, and once before you output it. The second time, you can’t be picky. Print requires CMYK, RGB is for WWW and Inkjets. While you’re working, as I said before, it’s a matter of your choice. Only be sure to soft-proof your image if you’re printing it on a CMYK priter…


Branko Vukelic ()
H
Hecate
Jan 18, 2004
On Sat, 17 Jan 2004 16:32:54 GMT, edjh wrote:

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?
I
CMYK is needed for printing on offset printing presses which is what’s used for most magazines, books, newspapers, etc. RGB for inkjets and web.

And if that’s what you need make sure you profile images in Adobe 1998 RGB and not sRGB as the Adobe settings have a different/wider gamut than sRGB and convert better to CMYK.



Hecate

veni, vidi, reliqui

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