## Mesh Basics & Terminology

Meshmixer operates on **triangle meshes,** which are objects comprised of three types of elements: vertices, edges, and triangles.

**Vertices****Edges**are connections between pairs of vertices.**Triangles**

In Meshmixer we also use the term **faces** to refer to triangles (though really triangles are only one type of face)*. ***Edges** connect the corners of triangles, and **vertices** are points where two or more edges meet (corners of the triangles).

We use the term **neighbours** to refer to two triangles that share an edge, two edges that share a vertex, or two vertices that share an edge. We use the term **incident** to refer to an element that is part of another. For example, an edge is “incident upon” one or more triangles.

A mesh **boundary** is one or more edges incident upon only one triangle. A mesh is called **open** if it has one or more boundaries, and **closed** if it has no boundary edges.

A **shell** is a connected set of triangles. In other words, starting from one triangle, you can walk to any other triangle in the shell without leaving the surface. This is also sometimes referred to as a **connected component***.*

**Surface normal and tangent**: you can think of a **tangent plane at a point** on the surface as the plane that is parallel to the surface

*at that point*. The

**normal**at a point

*vector**is the vector that is perpendicular to this tangent plane. You can think of the normal vector as the vector that points “straight out” of the surface, and the tangent as the plane that fits “along” the surface at a given point.*

**UVs/Texture coordinates**: UV mapping associates with each vertex in the mesh a 2D coordinate. This allows for **texture mapping**, which "wraps" a 2D image onto a 3D surface. The **texture coordinates** or **UVs** are the set of 2D coordinates for all triangle vertices.

**Manifold** vs **non-manifold**: A **manifold** surface has either one or two triangles neighbouring every edge in the mesh. A **non-manifold** surface can have three or more triangles on a single edge. Usually there are only a small number of non-manifold edges in a non-manifold mesh. See the section on **Complexes** for more details.

## Face Groups

Meshmixer allows you to assign a tag to any triangle in the mesh. These tags are represented by colors in the application. A set of triangles with the same color (or tag) is called a **Face Group**.

**NOTE:** **To see face groups, you must choose Group Color (Default) in Mesh Color Mode in the preferences window.**

To assign a set of triangles to a new Face Group, select them using the **Select** tool, then choose **Modify > Create FaceGroup**. To remove the Face Group identifier from a set of triangles, select the triangles and click **Modify > Clear FaceGroup**.

Once you have one or more Face Groups you can do things with them. In **Select** mode, double click inside a group to select the entire group. Some tools are also aware of Face Groups. For example, in the **Remesh** tool, you can check the option **Preserve Group Borders**:

This will make it so that the shape of the Face Group is preserved, even if we add or remove triangles. In this example, notice that the outline of the green face group remains the same, even as many triangles are added inside and outside of the group.

The **Stamp** tool creates shapes in the mesh by refining the mesh (inserting vertices, edges, and triangles), and assigning triangles to Face Groups.