Adobe 23101335 User Guide - Page 204
Using channel calculations to blend layers and channels (Photoshop)
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194 CHAPTER 7 Using Channels and Masks Use a printed sample of the overprinted inks to adjust your screen display to help you predict how colors will look when printed. Note: In some cases, such as varnish and bump plates, you may want colors to overprint. To adjust overlapping spot colors: 1 In the Channels palette, select the spot channel with the color you want to print. 2 Choose Select > Load Selection. To quickly select an image in a channel, hold down Ctrl (Windows) or Command (Mac OS), and click the channel in the Channels palette. 3 For Channel, choose the spot channel from step 1, and click OK. 4 To create a trap when knocking out the underlying color, choose Select > Modify > Expand or Contract, depending on whether the overlapping spot color is darker or lighter than the spot color beneath it. For more information on trapping, see "Creating color traps" on page 384. 5 In the Channels palette, select the underlying spot channel that contains areas you want to knock out. Press Backspace (Windows) or Delete (Mac OS). This method can be used to knock out areas from any channels under a spot color, such as the CMYK channels. 6 If a spot color in one channel overlaps more than one other spot color, repeat this process for each channel that contains areas you want removed. Using channel calculations to blend layers and channels (Photoshop) You can use the blending effects associated with layers to combine channels within and between images into new images using the Apply Image command (on single and composite channels) and the Calculations command (on single channels). These commands offer two additional blending modes not available in the Layers palette-Add and Subtract. Although it's possible to create new combinations of channels by copying channels into layers in the Layers palette, you may find it quicker to use the calculation commands to blend channel information. The calculation commands perform mathematical operations on the corresponding pixels of two channels (the pixels with identical locations on the image) and then combine the results in a single channel. Two concepts are fundamental to understanding how the calculation commands work. • Each pixel in a channel has a brightness value from 0 (off or black) to 255 (on or white). The Calculations and Apply Image commands manipulate these values to produce the resulting composite pixels. • These commands overlay the pixels in two or more channels. Thus, the images used for calculations must have the same pixel dimensions. (See "Changing image size and resolution" on page 95 for information on adjusting an image's pixel dimensions.)