From BlenderWiki

Mask Modifier

Description

The Mask modifier allows certain parts of an objects mesh to be hidden from view (masked off), in effect making the parts of the mesh that are masked act as if they were no longer there.

Cube before a Mask modifier is added to it.

The same cube with the Mask modifier added, with assigned vertex groups.

The screenshots above show a cube before and after a Mask modifier is added to it. In the second screenshot, the cube has had parts of its mesh assigned to a vertex group which the Mask modifier uses to determine which parts will be visible and which parts won’t.

Options

Mask Mode

The Mask modifier can hide parts of a mesh based on two different modes, selectable from this drop-down list:

The Mask modifier panel settings with the Vertex Group option highlighted in yellow.

Vertex Group

When the Vertex Group option is selected, the Mask modifier uses a vertex group, specified in the VGroup text field that shows up, to determine which parts of the mesh are masked by the modifier.

Once you have entered the desired name, the Mask modifier will update so that anywhere the vertices of the mesh are part of the named vertex group will be masked (which normally means they will be visible…), and anything not part of the named vertex group will be made non-visible.

Cube showing the masked parts of the mesh that are part of a selected vertex group (here highlighted in yellow).

The effect of the vertex group on the underlying cube with a Mask modifier active.

The two screenshots above show, first the underlying vertex groups assigned to the cube (highlighted in yellow), and then the effect on the cube when it has a Mask modifier applied to it.

Any of the methods for assigning vertex weights to a mesh work with the Mask modifier, however the actual weight value assigned to a vertex group is completely ignored. The Mask modifier only takes into account whether a set of vertices are part of a group or not, the weight is not taken into account. So having a vertex group weight of say 0.5 will not make a partially masked mesh. Just being part of the vertex group is enough for the Mask modifier, even if the weight is 0.0.

The Mask modifier panel settings with the Selected Bones option highlighted in yellow.

Selected Bones

Selected Bones is useful in Pose mode or when editing an armature. Enter the name of the armature object in the Ob field that appears. When working with bones in Pose mode, vertex groups not associated with the active bone are masked. The Inverse button can be useful to see how a bone affects the mesh down the chain of bones.

Inverse

Normally, when the Mask modifier is applied to areas of a mesh, the parts that are under the influence of the modifier are left visible while the parts that aren’t are hidden. The Inverse button reverses this behavior, in that now parts of the mesh that were not originally visible become visible, and the parts that were visible become hidden!

A cube with a Mask modifier, without the Inverse option.

A cube with a Mask modifier, with the Inverse option active.

Examples

Example Blend file using Mask modifier