What Is a Mask?

In the days before digital photography, a physical mask (such as a sheet of paper) was placed over unexposed photographic film in a darkroom to prevent stray or unwanted light from reaching specific areas of an image. This technique allowed the photographer to darken or completely hide a portion of the image. In Photoshop, you can achieve the same effect by adding variations of gray to a layer mask. To apply a layer mask to an image layer, you must first select the layer, and then click the Layer Mask button at the bottom of the Layers palette.

When you add a layer mask to a selected layer, Photoshop displays a new blank thumbnail next to the image thumbnail in the Layers palette. Any time you’d like to work in the mask, you must click on this thumbnail. When you first add the mask, it is selected automatically. Photoshop tells you whether you are working in the image area or in the mask by displaying an outline around either thumbnail in the Layers palette. When you are through working in the mask, click on the image thumbnail or select a different layer altogether.

You can also use layer masks to control which areas of an image are affected by adjustment layer effects (such as Levels, Hue/Saturation, and so forth). As you’ve seen throughout the earlier chapters, Photoshop automatically creates a layer mask along with every adjustment layer that you add to a composition.

Black Conceals, White Reveals
When you’re working with layer masks, it’s important to understand that black conceals and white reveals. Any area of a layer mask containing 100% black conceals those portions of the image. When using layer masks with adjustment layers, this means that  the layers below it in the Layers palette are not affected by the adjustment. Any area containing 100% white reveals those portions of the image, and when working with adjustment layers, the layers below are affected by the adjustment. Shades of gray in a layer mask partially conceal those areas of the image. When applied to adjustment layers, the layers below are affected in proportion to the grayscale value.

What this means is that you can control which areas of your image are affected by an adjustment layer by painting in the companion layer mask with the Brush tool, or by filling selections in the mask with a color or gradient. This is an extremely powerful and nondestructive way to enhance print images that you intend to use in a print design or layout.