Mesh projection

Mesh projection is a method of generating UVs to allow one mesh to share the lightmap of another mesh in close proximity. One mesh is designated the target, and the other mesh the detail mesh. The mesh projection produces a set of UVs for the detail mesh that share the same space in the lightmap as the target mesh.

The mesh projection process:

1. generates simplified lightmap UVs for the target mesh
2. forms charts for the detail mesh, but doesn't generate lightmap UVs
3. chooses the best target mesh chart for each detail mesh chart 
4. projects each chart onto its target to obtain the lightmap UVs for the detail mesh

Chart alignment

To make sure the detail charts have the correct lighting, the target chart should be large enough to contain the projected charts, as shown below:

To prevent incorrect lighting, a boundary between charts in the target mesh should roughly correspond with a chart boundary in the detail mesh. 

The best target chart

For each projected mesh chart, the mesh projection process identifies the closest target mesh chart. Closeness is determined by the distance between the projected vertex and a vertex in the target chart. The relative orientations of the detail and target surfaces are ignored.

Projection issues 

when a detail mesh chart is projected to a poor choice of target mesh chart, the resulting distorted lightmap UVs cause incorrect lighting.

Use the GeoRadiosity Mesh projection debugging tools to check for distortion. It should be evident if any areas have unacceptable amounts of distortion.

The GeoRadiosity Projection Issues visualisation service shows problem areas are shown in bright colors. 

To investigate further, click on the Filter tab in GeoRadiosity and toggle Show Target Geometry on and off.