Masks hide things; layer masks in Photoshop hide layers, or parts of layers, making them invisible.
Layer masks are probably the most underused tool in Photoshop. The typical Photoshop beginner fears and avoids layer masks. Don’t be that guy. Layer masks are super-amazing. Layer masks are fun and easy to use. Layer masks are your friends. Really. Learn to love them and your love will be rewarded.
Layer masks are like magical cloaks of invisibility. Do you remember those old cartoons where one character would find a can of ‘vanishing cream’ or ‘invisible ink’ and, upon applying it, vanish? That’s what a layer mask does. Painting a layer mask is like painting over a layer with that mythical vanishing cream: whatever you paint, disappears. It looks like you’re erasing, even though you’re not. In fact, a layer mask can do anything the eraser tool can do.
If you can’t tell one from the other, then why, you might wonder, are you supposed to care? Why am I prattling on? Why not just use the Eraser tool and be done with it? I’ll tell you why. A layer mask is infinitely more versatile than the Eraser. When you erase something, it’s gone; but when you mask something, your layer is still intact; part of it is just invisible. You can unmask it again at any time, painting in the hidden portions.
Layer masks contain black and white and shades of gray. Where a layer mask is white, that layer is visible. Where the mask is black, that layer is invisible. Ah, you ask, but what about where the mask is grey? Shades of gray render the layer is partially invisible. The lighter the shade of gray, the more the layer shows.
Contrary to what you might expect, it’s actually easier to mask things with a layer mask than with the Eraser tool. Mask allow you to be sloppy, you can use a big, broad brushstrokes, make mistakes, then go back with a smaller brush, swap the colors, and clean things up a bit.
Granted, Photoshop allows you to undo almost any action. You can undo a stroke of the eraser; you can undo the last 20 strokes of the eraser … before you save the file. But, once you save your file, all those undos are lost. Forever. Really.
But … layer masks are forever, if you want them to be. Because layer masks are saved with the file. When you open your image tomorrow, or next year, the layer mask is still there.
You can always go back and change it. Always.
You can move a layer mask around, to hide, or reveal, different parts of the layer.
Unlock the layer from the layer mask by clicking the chain link button between them in the Layers palette, then click on the layer mask thumbnail in the Layers palette.
Use the Move tool to drag the layer mask to a new position.
Or
Use your arrow keys to nudge the layer mask to a new position.
Or
Use Shift+arrow keys to move the layer mask 10 pixels at a time.
The mask moves, but the images themselves stay in place. Lock the layer and mask again, by clicking between them to restore the chain link.
You can make a layer mask bigger or smaller, squash it and stretch it, spin it around or flip it over.
Unlock the layer from the layer mask by clicking the chain link button between them, then click on the layer mask thumbnail in the Layers palette.
Lock the layer and mask again, by clicking between them to restore the chain link.
Ctrl/Command+click on the layer mask in the Layers palette to create a selection that exactly matches the layer mask. Visible portions of the layer will be selected. The hidden portions of the layer won’t. That is, the layer will be selected wherever the mask was white. They won’t be selected where the mask was black. And anywhere they mask is gray, the image will be partially selected.
You can convert a selection into a layer mask with a single mouse click. With a selection active, click the Add layer mask button at the bottom of the Layers palette (add layer mask icon). Photoshop turns your selection into a layer mask for the selected layer.
In the photo below, note that the layer mask on the top layer reveals the girl but hides everything but the girl. (It's white over the girl and the fence near her - black everywhere else.) So the girl from the top layer/photo shows, but everywhere else you see the bottom photo. Thus I use one photo/pose of the girl and another photo/pose of the boy.
