Lightmap UVs

Lightmap UVs are a separate UV set used only to store baked lighting. Each face needs its own space in the texture: Epic says "each face of the model needs to have its own unique space on the lightmap with no overlapping faces" (Unreal Engine documentation: Understanding Lightmapping in Unreal Engine). They usually live in the second UV channel, next to the texture UVs in channel 0.

#Why a separate set

Texture UVs can overlap, mirror and tile, because many faces may share the same pixels. Lighting cannot be shared: the shadow under a table is not on its top. Unreal notes that for normal textures "Triangles in the UV mapping can overlap", but lightmap UVs are a special case (Unreal Engine documentation: Using UV Channels with Static Meshes in…).

#The rules

1. No overlap. Unreal: "no two triangles in the mesh can overlap in the 2D texture space" (Unreal Engine documentation: Using UV Channels with Static Meshes in…).
2. Inside 0–1. Unreal: "all UV coordinate values must fall between 0 and 1" (Unreal Engine documentation: Using UV Channels with Static Meshes in…).
3. Padding. Epic says "a minimum of four texels is usually required to avoid all bleeding artifacts" (Unreal Engine documentation: Understanding Lightmapping in Unreal Engine). The UE3 guide explains why lightmaps need so much: block compression works on 4 × 4 texels, so at a lightmap resolution of 32 the padding "should be 12.5% of the entire UV space" (Unreal Development Kit documentation: Unwrapping UVs for Lightmaps).
4. Even density. "It's important to have an even density across your scene to get consistent lighting when using baked lighting" (Unreal Engine documentation: Understanding Lightmapping in Unreal Engine).
5. Split where light changes sharply. Split "at sections where there are deep recesses or crevices", but remember "any edge split will break the smoothness of the lighting result" (Unreal Engine documentation: Understanding Lightmapping in Unreal Engine).

#Generating them automatically

Unreal Engine. With Generate Lightmap UVs enabled, "a lightmap UV will be generated for any static mesh by default" (Unreal Engine documentation: Generating Lightmap UVs in Unreal Engine). The FBX importer describes it as "a set of unique, non-overlapping, UVs for use with static lighting" (Unreal Engine documentation: FBX Import Options Reference). Key settings (Unreal Engine documentation: Generating Lightmap UVs in Unreal Engine):

• Source Lightmap Index: the UV channel to generate from (your texture UVs).
• Destination Lightmap Index: where to store the result.
• Min Lightmap Resolution: "Dictates the minimum padding needed for UV charts when packing the UVs". Epic advises setting it "to an ideal power of 2", ideally the lowest resolution the mesh will use.
• Lightmap Resolution: the default resolution for instances placed in a level; keep it higher than the minimum.

Unreal repacks the existing UV charts rather than re-cutting them: it "will not split UV chart edges to create separate UV islands" (Unreal Engine documentation: Generating Lightmap UVs in Unreal Engine). A badly cut texture layout therefore gives a badly cut lightmap.

Unity. The model importer's Generate Lightmap UVs option "Creates a second UV channel for the lightmap" (Unity Manual: Model tab).

Godot. Lightmaps need UV2, where "Faces must not share pixels in the texture" (Godot documentation: Using Lightmap global illumination). You can generate it on import by setting the mesh's Light Baking option to static lightmaps (Godot documentation: Using Lightmap global illumination).

Blender. Lightmap Pack "takes each of a mesh's faces, or selected faces, and packs them into the UV bounds" (Blender Manual: UV tools).

#Do you still need them?

It depends on the lighting method. See baked vs. dynamic lighting.

• Fully dynamic lighting. "If your game or project only uses dynamic lighting, there is no need to set up lightmaps for each static mesh" (Unreal Engine documentation: Understanding Lightmapping in Unreal Engine).
• Unreal with Lumen. When Lumen is enabled, "precomputed static lighting contributions are disabled and all lightmaps are hidden" (Unreal Engine documentation: Lumen Global Illumination and Reflections in…). You can disable static lighting for the whole project in the rendering settings (Unreal Engine documentation: Lumen Global Illumination and Reflections in…). Lightmap UVs are then unused.
• Baked lighting, mobile and low-end. Lightmaps are still the cheap option here. Godot says baked lightmaps suit low-end PCs and mobile because they "consume almost no resources at runtime" (Godot documentation: Using Lightmap global illumination).

#Common mistakes

• Overlapping lightmap UVs, which produce shadows in the wrong place.
• Too little padding for small resolutions: light and shadow bleed across islands (Unreal Engine documentation: Generating Lightmap UVs in Unreal Engine).
• Lightmap resolution below the Min Lightmap Resolution used to pack the UVs (Unreal Engine documentation: Generating Lightmap UVs in Unreal Engine).
• Uneven density between neighbouring meshes. Unreal's Lightmap Density view mode shows it; Epic stresses "even texel density across your scene" (Unreal Engine documentation: Viewport Modes in Unreal Engine).

#Related terms

UV mapping · UV packing and padding · baked vs. dynamic lighting · global illumination · texel density

#Further reading and watching