Real-time lightmap UVs

Real-time lightmapA pre-rendered texture that contains the effects of light sources on static objects in the scene. Lightmaps are overlaid on top of scene geometry to create the effect of lighting. More info
See in Glossary UVs are per-Mesh Renderer: all instances of the same meshThe main graphics primitive of Unity. Meshes make up a large part of your 3D worlds. Unity supports triangulated or Quadrangulated polygon meshes. Nurbs, Nurms, Subdiv surfaces must be converted to polygons. More info
See in Glossary share the same input data, but different Mesh RendererA mesh component that takes the geometry from the Mesh Filter and renders it at the position defined by the object’s Transform component. More info
See in Glossary instances can use different real-time lightmap UVs at runtime. Unity calculates the UVs for the Realtime Global IlluminationA group of techniques that model both direct and indirect lighting to provide realistic lighting results.
See in Glossary system during the precompute stage. This calculation takes per-mesh UVs as input and produces per-Mesh Renderer UVs. Unity can generate the input per-mesh UVs when you import a model, or you can provide your own data.

This works as follows:

• Unity can use data in the Mesh.uv3 channel as input for the real-time lightmap UV calculations. Mesh.uv3 maps to the TEXCOORD2 shaderA program that runs on the GPU. More info
  See in Glossary semantic and is commonly called UV2.
• If there’s no data in Mesh.uv3, but there’s data in Mesh.uv2, Unity falls back to using the data in Mesh.uv2 as input for the real-time lightmap UV calculations. Unity uses Mesh.uv2 for baked lightmap UVs. It’s common to use the baked lightmap UVs as input data for the real-time lightmap UVs.
• Unity stores the results of the calculations per-Mesh Renderer, in MeshRenderer.enlightenVertexStream. When you enable Realtime Global Illumination and a given Mesh Renderer contributes to global illumination and receives its global illumination from lightmaps, Unity automatically passes the data in MeshRenderer.enlightenVertexStream to TEXCOORD2 in shaders, instead of the data in Mesh.uv3.

Note: If you want to use Mesh.uv3 for another purpose in a mesh that uses Realtime Global Illumination, you must ensure that all Mesh Renderer components that use the mesh receive global illumination from Light ProbesLight probes store information about how light passes through space in your scene. A collection of light probes arranged within a given space can improve lighting on moving objects and static LOD scenery within that space. More info
See in Glossary rather than lightmaps. Change this with the Mesh Renderer component Inspector, or the MeshRenderer.receiveGI API.

For more information, refer to Generating lightmap UVs.

How Unity calculates real-time lightmap UVs

This is what happens when Unity takes the per-mesh input UVs and processes them into per-Mesh Renderer output UVs.

Merging

Optionally, you can instruct Unity to merge UV charts where possible during this process. This reduces the size of the lightmaps and can improve runtime memory usage and performance.

Enable this optimization on any GameObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary with a Mesh Renderer component. In the Mesh Renderer Inspector, navigate to the Lightmapping section and enable Optimize Realtime UVs.

Note: This feature can misinterpret discontinuities in the original UV mapping. For example, Unity might treat an intentionally sharp edge as a continuous surface. If this happens, disable this feature.