1 Open the source image or images.
Note: If you are using more than one source image, the images must have the same pixel dimensions.
2 Choose Image > Calculations.
3 Select Preview to preview the results in the image window.
4 Choose the first source image, layer, and channel. To use all the layers in the source image, choose Merged for Layer.
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 277. For information on other blending modes, see "Selecting a blending mode" on page 241.
8 Enter an opacity to specify the effect's strength.
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