You can reduce the number of pixels in a lightmap by having two or more static meshes share the same area of the lightmap. This is done through mesh projection. One of the meshes is designated as the target and the other the projected mesh.

For the target mesh, Enlighten creates lightmap UVs in the usual way. The target mesh is always included in the radiosity computation (see How actors interact with Enlighten).

For the projected mesh, Enlighten creates lightmap UVs by projecting its vertices (mesh projection) onto the surface of the target mesh. The projected mesh is excluded and has no effect on the indirect lighting result.

You can do this for any actor which uses a static mesh asset, including foliage and spline meshes.

Meshes in close proximity

A small mesh can share the lightmap of a nearby larger mesh. This is perfect for:

flat objects attached to surfaces, such as decals or posters
small objects, such as debris
grass and roads on terrain

For best results when sharing a lightmap, the entirety of the mesh should extend less than one lightmap pixel's width from the surface.

Enlighten automatically shares lightmaps between certain kinds of meshes. By default, Instanced Foliage shares the lightmap of the surface onto which you place it. Landscape Grass always shares the lightmap of the Landscape.

Share lightmaps

To share a lightmap between meshes:

Select the smaller mesh.
In Details > Enlighten, choose Set Asset Lighting Mode > Detail.

Enlighten automatically chooses which nearby lightmap to share with this mesh by using mesh projection.

With the Lightmap Charts visualization, you can see that the mesh shares its lightmap with another mesh.

When you move a mesh with Detail lighting mode, the lightmap UVs update automatically to match the position.

Before moving mesh	After

By default, a mesh that has Detail lighting mode may share the lightmap of a nearby surface in a different streaming level. In World Settings when Share Lightmaps Across Levels is disabled, only surfaces in the same levels are considered for sharing.

When Share Lightmaps Across Levels is disabled, make sure the mesh with Detail lighting mode is assigned to the same streaming level as the nearby surfaces.

Mesh LOD

Each static mesh asset may contain multiple level of detail (LOD) versions of the mesh containing different numbers of triangles. Enlighten doesn't generate separate lightmaps for each LOD. Instead, all LODs share the same lightmap space.

In the Static Mesh Editor, you can choose to use a specific LOD as the target mesh with the Enlighten Target Lod property. When the Enlighten Target Lod is -1, Enlighten chooses the target LOD automatically.