Preparing Lightmap UVs in Unreal Engine: A Step-by-Step Guide to Clean Bakes

When preparing a scene with baked lighting in Unreal Engine, static meshes need a separate Lightmap UV channel to store lighting and shadow information. Epic Games’ official documentation recommends either generating this channel automatically or creating it manually in a UV editing tool. As described in the documentation, the workflow draws on Unreal Engine’s general lightmapping documentation; the FBX import setting example is from Unreal Python 4.27, the Alembic setting from 5.0, and the static mesh UV channel tools from 5.3.

Why keep Lightmap UVs separate?

Texture UVs determine how a material is mapped onto surfaces. They can use repeating or overlapping islands; for example, UVs for facade surfaces that use the same texture can overlap to use space more efficiently. Lightmap UVs serve a different purpose: baked lighting and shadow information must be written separately for each surface. Islands therefore must not overlap or extend over one another, and there must be enough space between them to limit light leaks and visual artifacts.

Lightmaps are needed only when static meshes will be lit using baked, or precomputed, lighting. If a project uses dynamic lighting only, preparing Lightmap UVs for every mesh is not required. Making this distinction early can avoid unnecessary UV work.

When is automatic generation enough?

1. During FBX import, check the option to generate Lightmap UVs in the FBX Import Options window. According to the official documentation, Lightmap UVs can be generated during import unless automatic generation is disabled.

2. Keep in mind that the automatic unwrap uses the texture UV channel as its source, then repacks the islands. Its goal is to prevent overlap and wrapping while leaving enough space for the target Lightmap resolution.

3. Check the result. Automatic generation works for many static meshes with little manual intervention, but it does not guarantee an ideal result for every model.

After Lightmap UVs have been generated, you can change the settings in the Build Settings section of the Details panel in the Static Mesh Editor. These tools can generate a new Lightmap UV or repack existing islands. The source and destination channel options determine which UV channel the islands are taken from and written to. Automatic unwrapping repacks islands but does not cut or split them in the process. If the model’s existing islands are unsuitable, it may be better to edit them in your modeling or UV tool before import.

Custom unwrapping, island layout, and padding

If the automatic result isn’t good enough, unwrap the UVs manually in your modeling software or a separate UV editing tool. Where possible, keeping connected surfaces together in logical, continuous groups helps produce smooth lighting transitions. For a facade element, one approach is to put the front and side surfaces in the same island and make the top surface a separate island. Deep recesses and cavities can be suitable boundaries for separating UV islands.

Epic’s documentation notes that DXT compression works in 4×4 texel blocks, so at least four texels of space are generally needed to prevent light and shadow bleeding. For custom unwrapping, assess the spacing against the target resolution. For the 64-resolution example, the documentation gives a calculation of 1/62 and an approximate value of 0.0161290323 after subtracting one pixel from each edge. Automatic generation can place appropriate padding itself.

On complex models with thin, fragmented parts and many gaps, padding can consume a significant portion of the UV space. A higher Lightmap resolution may be needed in that case; even so, use the lowest resolution that delivers the required quality. For organic shapes, it can help to flatten the surface, relax the UVs as needed, create seams along natural boundaries, and preserve continuous lighting across visible areas. Keeping islands at a uniform scale isn’t always the most important goal; the documentation notes that an unwrap with unevenly scaled islands that use more UV space may produce better lighting.

Applying this in architectural and archviz workflows

For architectural facade modules, interior elements, and repeated static meshes, automatic generation can save time at the start, but the bake result should be checked in a representative scene. Reusing texture UVs as Lightmap UVs can cause problems if the texture islands overlap. Custom unwrapping provides more control, particularly for complex railings, recessed surfaces, and organic objects. A higher Lightmap resolution may reduce some artifacts, but its impact on the memory budget must also be considered. The documentation doesn’t specify particular hardware requirements or licensing conditions, so no further assumptions can be made on those topics.

The import pipeline can vary depending on the file type and engine version. The Unreal Python 4.27 documentation includes the generate_lightmap_u_vs property for FBX static mesh import data. The experimental Alembic settings in Unreal Python 5.0 also include the generate_lightmap_u_vs option. Unreal Python 5.3 documents functions for adding UV channels to static meshes and generating box, cylinder, or planar UV mappings. These are documents for different versions; confirm separately that the relevant option and function are available in your project’s version.

Common mistakes

• Using texture UVs as Lightmap UVs without checking their overlap.

• Placing islands too close together, causing light or shadow leaks.

• Assuming automatic packing will split islands on complex meshes; the tool does not cut existing islands.

• Increasing the resolution more than necessary to fix the problem; first review the UV layout and the lowest sufficient resolution.