Lightmap
Pre-baked indirect lighting
Also known as: Light baking · Baked lighting
Definition
A lightmap is a texture baked offline that stores the result of complex indirect lighting calculations (bounce light, ambient occlusion, area lights), applied at runtime as a tinted overlay on each surface. Lightmaps deliver film-quality lighting at zero runtime cost — but only for static geometry that doesn't move. UE5's Lumen has replaced lightmaps for most projects, but mobile titles and stylized games still rely on baking.
In production
Every mesh that receives baked lighting needs a second UV channel (UV1) without overlaps, sized appropriately for the bake resolution — typically generated automatically by Unreal's "Generate Lightmap UVs" import option or manually laid out in Blender/Maya for critical hero assets where auto-unwrapping wastes texel density. Forgetting this is a classic source of patchy or black-spotted lighting after bake, since overlapping lightmap UVs cause two different surfaces to share and corrupt the same baked light data. Unreal's Lightmass and Unity's Progressive Lightmapper are the standard offline bakers, both computing global illumination bounces that can take minutes to hours per level depending on bounce count and resolution. A common production pitfall is a lightmap resolution set too low on a large floor plane, producing visibly blurry, blocky shadows that only show up once the bake finishes — forcing a costly re-bake cycle.
