Although its options are nearly identical, the Calculations command performs a slightly different function than Apply Image. Rather than compositing a source image on top of the current target image, Image ^ Calculations combines two source channels and puts the result in a target channel. You can use a single image for both sources, a source and the target, or all three (both sources and the target). The target doesn't have to be the foreground image (although Photoshop previews the effect in the foreground image window). And the target can even be a new image. But the biggest difference is that instead of affecting entire full-color images, the Calculations command affects individual color channels only. Only one channel changes as a result of this command.
Choosing Image S Calculations displays the dialog box shown in Figure 13-27. Rather than explaining this dialog box option by option — I'd just end up wasting 35 pages and repeating myself every other sentence — I attack the topic in a less structured but more expedient fashion.
When you arrive inside the dialog box, you select your source images from the Source 1 and Source 2 pop-up menus. As with Apply Image, the images have to be exactly the same size. You can composite individual layers using the Layer menus.
Select the channels you want to mix together from the Channel options. In place of the full-color options — RGB, Lab, CMYK — each Channel menu offers a Gray option, which represents the grayscale composite of all channels in an image.
The Blending pop-up menu offers the same 14 blend modes — including Add and Subtract — found in the Apply Image dialog box. However, it's important to keep in mind how the Calculations dialog box organizes the source images when working with blend modes. The Source 1 image is equivalent to the source when using the Apply Image command (or the floating selection when compositing conventionally); the Source 2 image is equivalent to the target (or the underlying original). Therefore, choosing the Normal blend mode displays the Source 1 image. The Subtract command subtracts the Source 1 image from the Source 2 image.
Half of the blend modes perform identically regardless of which of the two images is Source 1 and which is Source 2. The other half — including Normal, Overlay, Soft Light, and Hard Light — produce different results based on the image you assign to each spot. But as long as you keep in mind that Source 1 is the floater — hey, it's at the top of the dialog box, right? — you should be okay.
Tip The only mode that throws me off is Subtract, because I see Source 1 at the top of the dialog box and naturally assume that Photoshop subtracts Source 2, which is underneath it. Unfortunately, this is exactly opposite to the way it really works. If you find yourself similarly confused and set up the equation backwards, you can reverse it by selecting both Invert options. Source 2 minus Source 1 results in the same effect as an inverted Source 1 minus an inverted Source 2. After all, the equation (255 - Source 1) - (255 - Source 2), which represents an inverted Source 1 minus an inverted Source 2, simplifies down to Source 2 - Source 1. If math isn't your strong point, don't worry. I was just showing my work.
As you can in the Apply Image dialog box, you can specify a mask using the Mask options in the Calculations dialog box. The difference here is that the mask applies to the first source image and protects the second one. So where the mask is white, the two sources mix together normally. Where the mask is black, you see the second source image only.
The Result option determines the target for the composited channels. If you select New Document from the Result pop-up menu, as in Figure 13-27, Photoshop creates a new grayscale image. Alternatively, you can stick the result of the composited channels in any channel inside any image that is the same size as the source images.
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