Layer Masks give you precise control over what’s seen or not seen in a layer. You literally can paint that layer in or out of the image. They come automatically with Adjustment Layers in most programs and can be added to other types of layers with advanced software such as Adobe Photoshop or Corel Paint Shop Pro.

A Layer Mask appears in the Layers palette as a box on a particular layer. If it’s white, it’s on (the layer acts normally); if it’s black, it’s off (the layer’s effect disappears). Think of the following analogies: A light switch turns on the lights, allowing you to see the contents of a room, just as white turns on the layer, allowing you to see its contents. Black is like turning off that switch so you can’t see the layer’s contents. Gray is in between, sort of like putting a dimmer on the switch.

The real power of Layer Masks comes when you paint parts of the layer on and off. Click on the Layer Mask box to be sure you’re working in it, then paint black over parts of the image where you want to remove the layer’s effect, or white where you want it to show. Go back and forth as much as you like to refine your work. You even can use the Gradient tool for large areas that need a gradual change from on to off. A useful technique for refined changes is to fill the Layer Mask with black (look for a Fill command), then paint back just what’s needed in white.

Possibilities include selective sharpening (copy your image to a new layer, sharpen it, add a Layer Mask, fill it with black, then paint in just what needs to be sharpened); darkening a sky (add an Adjustment Layer to darken the sky, then use the Gradient tool to create a blended Layer Mask from white for the darkened sky to black for the unaffected ground); and opening shadows (add an Adjustment Layer to lighten shadows, fill the Layer Mask with black, then paint in the lightened shadows).