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This book excerpt is from "The Hidden Power of Photoshop CS: Advanced Techniques for Smarter,
Faster
Image Processing." ISBN 0-7821-4255-9. All rights reserved.
Chapter 2: Color Separations. , is posted with permission from Sybex.
Photoshop provides many tools that seem to produce some magic behind the scenes. Channels, one of the most prominent of these tools, allow you to work directly with components of a color model, such as RGB or CMYK. But, as we saw in the previous chapter, channels can actually be simulated using some simple light theory. Understanding what the channels represent can help you make more intelligent color and correction decisions, and can very much change the way you work with images.
Not only does learning about separation help you understand what is going on in an image, it also frees you up to use different tools and techniques—because you can apply them to get a more targeted result. Good use of separation can give you the opportunity to make selective changes to image areas based on color or tone, and opens the door to advanced manipulations, selections, and calculations using targeted masking.
Turning color to grayscale
Adding color to tone
Separating CMYK color
When I was learning to work with digital images, one of my first experiences was preparing images to be printed in black-and-white photography books. Problems often arose when photographers submitted their images as color scans. Converting RGB color scans to black-and-white was pretty easy—I could just choose Grayscale mode and was done with it. This approach resulted in a quick conversion so the images looked like black-and-white images, but it didn’t always produce the most predictable—and not nearly the best—result. Just doing the conversion wasn’t enough for a photography book; the goal was for the reader to see the best possible grayscale representation. Because it was extremely important how things looked when all was done and printed, I started looking for other solutions.
Grayscale images are quite a different animal from color. Because there is only tone—no color to worry about—it is natural to think that working with tone should be easier than working with color. There are fewer possibilities because images only have 256 shades of gray as opposed to 16 million–plus colors in an 8-bit image. However, the simplicity of the tone makes getting a good conversion important; with fewer ways for the difference in an image to be displayed, every tiny variation becomes critical.
You might think that converting an image to grayscale would be simple, since it is a simplification of the image. Besides that, Photoshop gives you several ways to make the conversion quickly, such as choosing Grayscale mode or selecting Image ‘ Adjustments ‘ Desaturate. However, there are hundreds of ways to convert images from color to grayscale. And just as there are various types of film with different-flavored results, users can control the flavor of their grayscale conversion. Clever use of information in an RGB image can even mimic other processes, such as infrared film.
Converting to grayscale is not only important for printing black-and-white images. Once images have color removed, you can create duotones and hand-colored images, or use the image information for repair by defining tone-based selections, for creating masks, and for adjusting and repairing channels. You may find different techniques and conversions to suit different situations. More important, you may find that dealing with tone is, in reality, easier to handle for getting good color results.
The easiest way to convert a color image to grayscale is to desaturate the image. Desaturating neutralizes pixel color by equalizing the red, green, and blue components to a single value. When the red, green, and blue values are the same measured value for a pixel, the pixel displays as gray. The calculation takes the highest and lowest measures and averages them, ignoring the middle value. Averaging may not be a good representation of actual tone because it doesn’t do much to account for color. For example, RGB values of 255, 255, 0 and 255, 0, 0 both have a desaturation result of 127, 127, 127.
The obvious way to desaturate is to use the Desaturate function. Just choose
Image ‘ Adjustments ‘ Desaturate. You can also open the Hue/Saturation
dialog box and slide the Saturation slider all the way to the left. If you try
creating a new image and filling different areas with pure RGB color (255, 0,
0 for red, 0, 255, 0 for green and 0, 0, 255 for blue), then convert to black-and-white
by desaturating, the color becomes indistinguishable as tone, because all convert
to a medium gray (127,127,127). Other methods of converting may salvage some
of that difference.
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Created: March 27, 2003
Revised: March 1, 2004
URL: http://webreference.com/graphics/ps1/1
