A layer mask conceals a portion of a layer without actually deleting it, allowing the lower layers to show through. Erasing the layer mask restores the layer's original appearance so that nothing is lost. The general approach to applying a layer mask is to brush it on using any of the paint tools, controlling the visibility and transparency with grayscale values. As you saw Part 3, Task 4, "How to Use Quick Mask," a black value conceals the layer image, white reveals it, and a grayscale value dictates the transparency.
Open the File
Choose File, Open and select the image file you want to modify. For this task, I've opened an image with two layers. The lower layer is of an arrangement of foliage, and the top layer is of a museum statue. By "painting" in a mask, I'll make it appear as if the statue is surrounded by the foliage and not sitting in the corridor of a museum.
Open the Layers Palette
Choose Window, Layers to open the Layers palette.
Select the Mask Layer
Select the layer to be masked by clicking its name in the Layers palette. In this example, select the layer containing the statue image.
Add the Layer Mask
Click the Add A Mask button at the bottom of the Layers palette to add a layer mask to the current layer. The layer mask is indicated in the image tile by a white thumbnail next to the image thumbnail. The border of the new thumbnail is bold, indicating that it is selected.
Paint the Mask
Select the Paintbrush tool from the toolbox. Move the cursor into the image area and paint the mask. As you do so, you'll see the current layer disappearing and the lower layer showing through; the thumbnail mask icon is updated to show the current mask. In this example, notice that the museum area behind the statue is being "erased" and the woodsy Background layer is showing through.
Delete the Mask
To erase or fade part of the mask, change the foreground color to white and repaint the mask. The image layer reappears as you work. To paint in transparency, select a gray foreground color or reduce the Opacity setting in the Options bar for the paint tool you are using. Note that pressing the X key enables you to quickly toggle between the foreground and background colors. This is helpful while adjusting the edges of the mask.
Apply the Mask
Because layer masks add significantly to file size, you should consider applying them to the layer when you're confident that you will not be making any further changes. To apply the mask to the layer, choose Layer, Remove Layer Mask, Apply.
Make Sure That You Paint the Mask
When applying a layer mask, make sure that the mask thumbnail is highlighted in the Layers palette before you make any changes. As you're working, it's easy to forget that the image thumbnail should be selected; if it's not selected, you'll be painting directly into the image.