—by Amy Palermo
Adobe Photoshop CS/CS2/CS3
Microsoft Windows, Macintosh
I was editing an image and I saved a selection as an alpha channel. However, when I saved the alpha channel, the selected area displayed as black and the rest of the image displayed as white. I was expecting the selected area to display as white. Do you know why this happened and how can I fix it?
You’re correct in your expectations: When you save a selection as an alpha channel, Photoshop displays the selected area as white, and the non-selected, or masked area, as black. It sounds like you inadvertently selected the inverse of what you intended to select.
This is a common mistake when you need to select an object that has a single-colored background, such as the racecar shown in Figure A. The easiest way to select the racecar is to select the white area, and then inverse the selection. However, if you select the white background, forget to inverse the selection, and then make an alpha channel based on your selection, the selected area will still display as white in the alpha channel, as shown in Figure B—it’s just not the selection you wanted.
Tip: Check your selection before you make an alpha channel. If you notice the marching ants selection around your image’s perimeter, as shown in Figure C, you probably selected the negative space, not the object.
In a case like this, you would have to inverse the selection before you make your alpha channel. To do this, simply choose Select > Inverse or press [shift][command]I ([Shift][Ctrl]I in Windows).
You can also reverse an alpha channel after the fact. If you’ve already saved the alpha channel but realized that you should have inversed your selection before you saved the channel, no worries. Simply select the alpha channel in the Channels palette, then choose Image > Adjustments > Invert, or press [command]I ([Ctrl]I in Windows) to invert the alpha channel, as shown in Figure D.
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