The Mask option in the Apply Image dialog box provides a method for you to import only a selected portion of the source image into the target image. Select the Mask check box and choose the image that contains the mask from the pop-up menu on the immediate right. As with the Source pop-up menu, the Mask menu lists only those images that are open and happen to be the exact same size as the target image. If necessary, select the layer on which the mask appears from the Layer pop-up menu. Then select the specific mask channel from the final pop-up menu. This doesn't have to be a mask channel; you can use any color channel as a mask.
After you select all the necessary options, the mask works like so: Where the mask is white, the source image shows through and mixes in with the target image, just as if it were a selected portion of the floating image. Where the mask is black, the source image is absent. Gray values in the mask mix the source and target with progressive emphasis on the target as the grays darken.
If you prefer to swap the masked and unmasked areas of the source image, select the Invert check box at the bottom of the dialog box. Now, where the mask is black, you see the source image; where the mask is white, you don't.
The first example in Color Plate 13-7 shows a mask viewed as a rubylith overlay with the image. To make the mask, I selected the background with the Color Range command, inversed the selection, and saved the result as a separate channel. In the other examples in the color plate, I again composited the RGB and Lab versions of the image — as in the previous section — using Photoshop's most outrageous blend modes. No matter how dramatically the Apply Image command affected the thinker, his background remained unscathed, thanks to the mask. If the Mask option had not been turned on, the background would have changed with the mode, turning light blue for Color Dodge, black for Color Burn, and deep red for Difference.
Tip You can even use a selection outline or layer as a mask. If you select some portion of the source image before switching to the target image and choosing Image S * Apply Image, you can access the selection by choosing Selection from the Channel pop-up menu at the very bottom of the dialog box. Those pixels from the source image that fall inside the selection remain visible; those that do not are transparent. Use the Invert check box to inverse the selection outline. To use the boundaries of a layer selected from the Layer pop-up menu as a mask, choose the Transparency option from the Channel menu. Where the layer is opaque, the source image is opaque (assuming that the Opacity option is set to 100 percent, of course); where the layer is transparent, so too is the source image.
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