Selecting Non-Transparent Pixels
Photoshop CS3 Tutorial
How to Select the Non-Transparent Pixels in Photoshop
Layers are usually a combination of transparent and opaque pixels. For instance, you may have an image of a masked ball. The ball is opaque and the remainder of the layer is transparent. Using this method you can quickly make a selection of the ball.
Open an image of a single masked object in Photoshop (File > Open).
Open the Layers palette (Window > Layers).
Cmd (PC-Ctrl) click the image thumbnail in the Layers palette. Notice the marching ants. Your object is selected.