Layer Mask

A layer is typically a rectangular grid of pixels which includes an alpha channel defining which pixels are visible from the layer below.

A layer mask allows the overall shape of the layer to be non-rectangular while still allowing you to work with the alpha channel within the layer. You can almost think of it as a secondary alpha channel.

Creating a mask

A mask can be created from a selection – either via freeform selection or the Magic Wand. Converting this type of selection to a layer will automatically set the layer's mask.

You can also create a blank mask by right-clicking on the layer, going to Mask, and then choosing Create New Empty Mask. By default this mask will be fully permissive (i.e. allow all pixels through).

Editing a mask

There are several ways to edit a mask once it has been created.

Swap Alpha and Mask

This command swaps the alpha and mask channels, allowing you to edit the mask as if it were the alpha. (Note that you will need to create an empty mask first if you don't have one.) This will allow you to, for example, take an image that already has alpha information and then create new alpha information to overwrite the layer below it while preserving the shape defined by the previous alpha channel.

Mask to Layer

This command will extract the mask from your layer and create a new grayscale layer with just a mask, allowing you to edit it with any drawing tools you want. Black regions are fully transparent and white regions are fully opaque.

When you are done, either click "Mask from Layer Above" or simply merge the layer down (you will be prompted to use it as a mask if the dimensions and color mode are correct).

The mask must always be the exact size of the layer, and the layer for mask editing must be in Grayscale mode. If you modify the mask using this command, make sure you don't accidentally resize it; otherwise you won't be able to merge it back down until you resize it again.

Disabling a mask

You can disable a mask (without deleting it) by unchecking "Enable Mask".