To apply tools and effects only to areas of a raster layer with data, use the Lock Transparency feature on the Layers palette. Transparent areas remain protected when you paint, apply effects, paste selections, or perform other edits.
Note: This feature applies to raster layers only. You cannot lock the transparency of vector, art media, adjustment, mask, or group layers. Also, background layers do not support transparency.
When transparency is locked, the Lock Transparency button displays . When a transparency is not locked, the Lock Transparency button is grey .
To lock or unlock the transparency:
Do one of the following:
On the Layers palette, click the Lock transparency button of the appropriate raster layer.
Double-click the layer name to display the Layer Properties dialog, mark or clear the Lock transparency check box, and click OK.
Changing the Opacity of Layers
Cleaning up the Edges of Layers Created from Selections