Using Channel Operation Commands

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Image ^ Apply Image and Image ^ Calculations provide access to Photoshop's channel operations, which composite one or more channels with others according to predefined mathematical calculations. Although once hailed as Photoshop's most powerful capabilities, channel operations have been eclipsed by the standard and more accessible functions available from the Layers and Channels palettes. One day, I suspect Adobe will scrap Apply Image and Calculations altogether. But until that day, I will dutifully document them both.

The Apply Image and Calculations commands allow you to merge one or two identically sized images using 12 of the 17 blend modes discussed earlier plus 2 additional modes, Add and Subtract. In a nutshell, the commands duplicate the process of dragging and dropping one image onto another (or cloning an image onto a new layer) and then using the blend mode and the Opacity settings in the Layers palette to mix the two images together.

Although Apply Image and Calculations are more similar than different, each command fulfills a specific — if not entirely unique — function:

♦ Apply Image: This command takes an open image and merges it with the foreground image (or takes the foreground image and composites it onto itself). You can apply the command to either the full-color image or one or more of the individual channels.

♦ Calculations: The Calculations command works on individual channels only. It takes a channel from one image, mixes it with a channel from another (or the same) image, and puts the result inside an open image or in a new image window.

High Pass filter
Color Dodge, Underlying Layer: 0, 80/200

Duplicate layer, Color Burn, Underlying Layer: 100/150, 180/255

Figure 13-23: After combining a High Pass effect with the radical Color Dodge and Color Burn blend modes, I used the Underlying Layer slider bar to force through pixels from the background so that the sunset and ocean didn't get lost.

Duplicate layer, Color Burn, Underlying Layer: 100/150, 180/255

Figure 13-23: After combining a High Pass effect with the radical Color Dodge and Color Burn blend modes, I used the Underlying Layer slider bar to force through pixels from the background so that the sunset and ocean didn't get lost.

The primary advantage of these commands over other, more straightforward compositing methods is that they allow you to access and composite the contents of individual color channels without a lot of selecting, copying and pasting, cloning, floating, and layering. You also get two extra blend modes, Add and Subtract, which may prove useful on a rainy day.

The Apply Image and Calculations commands provide previewing options, so you can see how an effect will look in the image window. But thanks to the sheer quantity of unfriendly options offered by the two commands, I suggest that you use them on only an occasional basis. The Calculations command can be a handy way to combine masks and layer transparencies to create precise selection outlines. Apply Image is good for compositing images in different color models, such as RGB and Lab (as I explain in the "Mixing images in different color modes" section later in this chapter).

But if your time is limited and you want to concentrate your efforts on learning Photoshop's most essential features, feel free to skip Apply Image and Calculations. I assure you, you won't be missing much.

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