What is a Mask in Blender?

In Blender, a **mask** is typically a way to selectively hide or show parts of an object, commonly a mesh, allowing you to work on specific areas without affecting others. One primary tool for achieving this is the **Mask Modifier**.

## Understanding the Blender Mask Modifier

The Mask Modifier is a powerful non-destructive tool applied to a mesh object. Its core function is to control the visibility of vertices or faces based on certain criteria.

According to the reference:
*   The Mask Modifier can **hide parts of a mesh based on two different modes**.
*   These modes are selectable from a menu within the modifier settings.
*   One of the modes is **Vertex Group**. This mode **hides all vertices not included in the chosen vertex group**.

This means you can paint weights onto different parts of your mesh (creating a vertex group) and then use the Mask Modifier to only display the areas within that group, or hide the areas outside of it.

## How Masks are Used

Masking in Blender offers flexibility for various tasks:

*   **Targeted Editing:** Focus on sculpting or editing complex areas of a mesh without interference from surrounding geometry.
*   **Performance:** Temporarily hide heavy parts of a mesh to improve viewport performance while working on lighter sections.
*   **Rendering:** While the Mask Modifier primarily affects the viewport, other masking techniques (like using textures for material masks) can control how different parts of an object render or interact with effects.

## Modes of the Mask Modifier

The Mask Modifier offers flexibility through its modes. While the reference specifically details the "Vertex Group" mode, understanding there are different options is key.

| Mode          | Description (Based on Reference/Standard Use)                       |
| :------------ | :------------------------------------------------------------------ |
| **Vertex Group** | Hides geometry based on inclusion/exclusion in a selected vertex group. |
| *Other Mode(s)* | Controls masking using different criteria (e.g., Armature bones).   |

Using the **Vertex Group** mode is straightforward:

1.  Assign vertices to a vertex group and paint weights if desired.
2.  Add a Mask Modifier to your object.
3.  Select the Vertex Group mode.
4.  Choose the specific Vertex Group you created.
5.  Use the 'Inverse' option if you want to hide *the selected* group instead of *everything else*.

This creates a non-destructive mask, meaning you can easily enable or disable the modifier or change its settings without permanently altering your mesh data.

In summary, a mask in Blender, particularly via the Mask Modifier, is a system for controlling the visibility of mesh components, often based on vertex groups, to streamline workflows and improve editing precision.