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Meanwhile, the a channel contains colors ranging from deep green (low-brightness values) to gray (medium-brightness values) to vivid pink (high-brightness values). The b channel ranges from bright blue (low-brightness values) to gray to burnt yellow (high-brightness values). As in the RGB model, these colors mix together to produce lighter colors. Only the brightness values in the luminosity channel darken the colors. So you can think of Lab as a two-channel RGB with brightness thrown on top.
To get a glimpse of how it works, try the following simple experiment.STEPS: Testing Out the Lab Mode
Create a new image in the Lab mode say, 300 300 pixels, setting the Background Contents option to White.
Press D to return the default colors to the toolbox. The foreground color is now black and the background color is white.
Press Ctrl+2 ( z -2 on the Mac). This takes you to the a channel.
Click the gradient tool in the toolbox. In the Options bar, select the Foreground to Background
option from the gradient pop-up menu, select the Linear gradient style, and select Normal from the Mode pop-up menu. (See Chapter 6 if you need help using these controls in the Options bar.)
Shift -drag with the gradient tool from the top to the bottom of the window. This creates a vertical black-to-white gradation.
Press Ctrl+3 ( z -3 on the Mac). This takes you to the b channel.
Shift -drag from left to right with the gradient tool. Photoshop paints a horizontal gradation.
Press Ctrl+tilde (~) ( z -tilde on the Mac) to return to the composite display. Now you can see all
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Part II: Painting and Retouching
channels at once. If you're using a 24-bit monitor, you should be looking at a window filled with an incredible array of super bright colors. In theory, these are the brightest shades of all the colors you can see. In practice, however, the colors are limited by the display capabilities of your RGB monitor.Using Lab
Because the Lab mode is device independent, you can use it to edit any image. Editing in the Lab mode is as fast as editing in the RGB mode and several times faster than editing in the CMYK mode. If you plan on printing your image to color separations, you may want to experiment with using the Lab mode instead of RGB, because Lab ensures no colors are altered when you convert the image to CMYK, except to change colors that fall outside the CMYK range. In fact, any time you convert an image from RGB to CMYK, Photoshop automatically converts the image to the Lab mode as an intermediate step.Tip
If you work with Photo CDs often, open the scans directly from the Photo CD format as Lab images. Kodak's proprietary YCC color model is nearly identical to Lab, so you can expect an absolute minimum of data loss; some people claim that no loss whatsoever occurs. Indexed Color Choose Image ® Mode ® Indexed Color to display the dialog box shown in Figure 4-5. This command permits you to strip an image of all but its most essential colors, a necessary step when saving GIF images and other graphics for display on the Web. Photoshop then generates a color look-up table (LUT), which describes the few remaining colors in the image. The LUT serves as an index, which is
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why the process is called indexing .
Figure 4-5: Use the Palette option to select the kinds of colors that remain in the image. Use the Colors option to specify how many colors remain.
For some reason, Photoshop doesn't let you apply the Indexed Color command to Lab or CMYK images. And although you can apply Indexed Color to a grayscale image, you don't get any control over the indexing process; Photoshop doesn't let you reduce the image to fewer than 256 colors, for example. So if you want to index a Lab or CMYK image or custom-prepare a grayscale image, choose Image ® Mode ® RGB to convert the image to the RGB mode and then choose Image ® Mode ® Indexed Color.Note
Note that you also cannot index an image that you've converted to the 16 Bits/ Channel mode, which I discuss shortly. If you want to index such an image, you must first choose Image ® Mode ® 8 Bits/Channel.Tip
Don't expect to be able to edit your image after indexing it. Most of Photoshop's functions including the gradient tool, all the edit tools, and the filters will refuse to work. Others, such as feathering and the brush tool, produce undesirable effects. If you plan on editing an 8-bit image much in Photoshop, convert it to the RGB mode, edit it as desired, and then switch back to the indexed color mode when you finish.
Now that I have all the warnings and special advice out of the way, the following list provides a brief rundown of the options inside the Indexed Color dialog box, along with some recommended settings for Web graphics:
Palette: This pop-up menu tells Photoshop how to compute the colors in the look-up table. You have lots of options here, but only a handful are really useful. If your image already contains fewer than 256 colors, the Exact option appears by default, in which case you should just press Enter or Return and let the command do its stuff. The Web option converts your image to the 216 so-called "Web-safe" colors. The Adaptive option selects the most frequently used colors in your image, which typically delivers the best possible results. The Perceptual and Selective options are variations on Adaptive. But where Adaptive maintains the most popular colors, Perceptual is more intelligent, sampling the colors that produce the best transitions. The Selective option tries to maintain key colors, including those in the