For lightmap lighting, a set of lightmap UV coordinates assign a location in the 2D lightmap to each vertex of a mesh.

The UV layout below shows the way each chart is unwrapped and arranged in 2D space.

To make sure no two charts share the same lightmap pixel in a UV, Enlighten moves and scales them so there's at least half a pixel of unused space around each. 

When a mesh has many small charts (relative to the lightmap pixel size), the unused space around each chart wastes lots of lightmap space. To solve this, Enlighten intelligently merges charts to optimize the UV layout.

It's sometimes important to preserve the boundary between charts to preserve the hard edge in indirect lighting, such as where two surfaces meet at 90 degrees.

Nearby parts of a mesh can share a single chart in the Enlighten lightmap without a significant loss of lighting accuracy. Triangles merged into the same chart share the same lightmap area, and have the same indirect lighting.

The images below show the same mesh before and after merging charts.

Settings

You can sometimes produce better results by tweaking the Auto UV merging parameters. We recommend using the default values unless you have a good understanding of the Auto UV algorithm.

After changing Auto UV properties, you can run Enlighten Parameterize and use the Enlighten Charts visualization to see the result.

To control how Enlighten creates UVs for a mesh, in the Static Mesh Editor, under Details > General Settings > Enlighten, use the Enlighten Auto UV properties.

Simplification mode

There are three simplification modes:

Automatic 

(Default) Enlighten automatically merges charts only when it produces a better result than the existing lightmap UVs.

Override

Enlighten automatically merges charts.

Disable

Enlighten doesn't merge charts.

Max Distance

The maximum distance (in Output Pixel Size pixels) allowed between input charts for Enlighten to consider merging them.

Max Initial Normal Deviation

The maximum angular deviation in degrees allowed between input charts for Enlighten considers them candidates for projection plane creation.

Max General Normal Deviation

The maximum angular deviation in degrees allowed between input charts and projection planes.

Expansion Factor

The bounding box expansion when detecting which input charts overlap.

Significant Area Ratio

The ratio of the chart area within which normals point at more than 90 degrees from the chart principal normal to the area of the chart above which the chart is considered curved. Also a ratio of the chart area to the area of the merged group below which the chart is considered insignificant.

Disable Auto UV globally

You can disable Auto UVs globally in World Settings > Enlighten.

Use manually authored UVs

The automated tools generally produce simplified lightmap UVs with a reasonable balance between quality and cost. In the rare case where the automated result is poor, and the mesh can still be efficiently lit using lightmaps, you can provide a manually simplified UV mapping. Enlighten uses the UV channel specified in the Light Map UV Index property of the mesh in the Static Mesh Editor.

When you create a simplified UV mapping, you can control which parts of the mesh are merged into a single chart.

The image below shows the 3ds Max Unwrap UVW modifier. 3ds Max highlights boundaries between charts in the selected UV channel in green.

When a manually simplified UV mapping is available, configure Enlighten to Disregard Vertex Normals and turn off Auto UV chart merging.

1. Open the mesh in the Static Mesh Editor.
2. Under General Settings > Enlighten Settings, enable Disregard Vertex Normals.

3. Under Auto UV Settings, set Simplification Mode to Disable.