In UV mapping, triangles in meshes are grouped into connected regions known as charts. 3D modelling tools use various names for this concept, including pelts, shells and islands.

In the image below, each colored surface is a chart. For example, the blue wall is one chart.

There's usually a hard edge in the Enlighten lighting where charts meet, such as the edge between the blue and orange walls in this image.

Visualize charts

To see how a meshes are split into charts in your level, use the Lightmap Charts visualization. This shows each chart in a different color.

To view the charts for a single mesh, you can also use the Charts button in the StaticMesh Viewer toolbar.

Lighting seams

Seams (discontinuities) in indirect lighting are sometimes visible at the boundaries between charts. This occurs because the adjacent triangles don't occupy the same chart, and may have different lighting values.

Visualization: defaults	Visualization: Lighting only	Visualization: Enlighten Charts

Notice the seam, highlighted.	The Lighting only visualization makes seams easy to identify.	The seam occurs at the boundaries between charts.

Seams are desirable in some scenarios, such as where two surfaces meet at a right angle – for example, the adjacent brick walls in the image above.

In other situations, the boundary between charts may cause unwanted seams when using lightmaps, such as on the surface of a sphere.

How Enlighten forms charts

The images below show two ways to group triangles into charts (separated by bold lines):

Four triangles grouped into a chart	Four triangles grouped into two charts

Enlighten groups triangles connected by at least one shared identical vertex into a single chart. By default, Enlighten considers vertices identical if they have the same position, normal, and input UV values. This results in chart boundaries where:

there is a (possibly tiny) gap in the surface of the mesh
two triangles meet at a hard edge (different vertex normals)
there's a split between charts in the Lightmap UV channel

Control how charts are formed

When the Auto UVs method is used to generate lightmap UVs, charts are automatically merged. You can control the merging process for a mesh in the Static Mesh Editor under General Settings > Enlighten Settings.

To split charts at a specific location, turn off Auto UV and provide a manually authored UV set. Enlighten won't group triangles into a chart unless they're connected in the UV set you provide.

To ignore normals when forming charts, in the Static Mesh Editor, enable Disregard Vertex Normals.

Open the mesh in the Static Mesh Editor.
Under General Settings > Enlighten Settings, enable Disregard Vertex Normals.
Under Auto UV Settings, set Simplification Mode to Disable.

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