Adobe 23101335 User Guide - Page 206

196 CHAPTER 7 Using Channels and Masks 5 Select Invert to use the negative of the channel contents in the calculation. For Channel, choose Gray to get the same effect as would be obtained by converting the image to a grayscale image. 6 Choose the second source image, layer, and channel, specifying further options as described in step 5. 7 For Blending, choose a blending mode. For information on the Add and Subtract modes, see "About the Add and Subtract blending modes" on page 196. For information on other blending modes, see "Selecting a blending mode" in online Help. 8 Enter an opacity to specify the effect's strength. 9 Select Mask if you want to apply the blending through a mask. Then choose the image and layer containing the mask. For Channel, you can choose any color or alpha channel to use as the mask. You can also use a mask based on the active selection or the boundaries of the chosen layer (Transparency). Select Invert to reverse the masked and unmasked areas of the channel. 10 For Result, specify whether to place the blending results in a new document, or in a new channel or selection in the active image. 11 Click OK. About the Add and Subtract blending modes The Add and Subtract blending modes are available only for the Apply Image and Calculations commands. Add Adds the pixel values in two channels. This is a good way to combine nonoverlapping images in two channels. Because higher pixel values represent lighter colors, adding channels with overlapping pixels lightens the image. Black areas in both channels remain black (0 + 0 = 0). White in either channel results in white (255 + any value = 255 or greater). Add mode divides the sum of the pixel values by the Scale amount and then adds the Offset value to the sum. For example, if you wanted to find the average of the pixels in two channels, you would add them, divide by 2, and enter no Offset value. The Scale factor may be any number between 1.000 and 2.000. Entering a higher Scale value darkens the image. The Offset value lets you lighten or darken the pixels in the destination channel by any brightness value between +255 and -255. Negative values darken the image; positive values lighten the image. Subtract Subtracts the pixel values in the source channel from the corresponding pixels in the target channel. As with Add mode, the result is then divided by the Scale factor and added to the Offset value. The Scale factor may be any number between 1.000 and 2.000. The Offset value lets you lighten or darken the pixels in the destination channel by any brightness value between +255 and -255.