Unreal Engine GPU Lightmass and Lightmap UVs: A Guide for Architectural Scenes

In Unreal Engine, GPU Lightmass precomputes complex interactions from Static or Stationary lights and stores them in lightmap textures applied to scene geometry. The workflow described in Epic’s official documentation uses the GPU to generate lighting data and lets you recalculate it while working in the scene. The same documentation stresses that Lightmap UVs require separate preparation to ensure baked lighting looks correct. GPU Lightmass is marked as beta, so it should be used with caution in projects intended for production. The documentation does not specify a particular Unreal Engine version; since the Lightmap UV page also includes references to UE4, the steps below are presented without assuming a version number.

1. Enabling GPU Lightmass

Before getting started, check the mobility settings for the scene’s lights. GPU Lightmass bakes data from lights set to Static or Stationary mobility. Projects using only dynamic lighting do not need lightmaps for static meshes.

1. Open the Plugins tab from Edit > Plugins. Enable the GPU Lightmass plugin under the Built-In > Editor category. If prompted to restart the editor, finish configuring the project before restarting.

2. In the Edit > Project Settings window, go to Engine > Rendering. Enable Hardware Ray Tracing > Support Hardware Ray Tracing and the Virtual Textures > Enable Virtual Texture Support and Virtual Textures > Enable Virtual Texture Lightmaps options.

3. Under Platforms > Windows, set Targeted RHIs > Default RHI to DirectX 12. Then restart the editor.

4. Open the Build menu in the level editor toolbar and select the GPU Lightmass panel. After configuring the settings, start baking with Build Lighting.

GPU Lightmass requires ray tracing, but additional real-time ray-tracing effects—such as shadows, ambient occlusion or reflections—are not required. These can be left off if they will not be used elsewhere in the project. The documentation provides the following console command to disable all ray-tracing effects:

r.RayTracing.ForceAllRayTracingEffects 0

To apply the same setting when the project opens, add this line to the [/Script/Engine.RendererSettings] section of the DefaultEngine.ini file:

[/Script/Engine.RendererSettings]
r.RayTracing.ForceAllRayTracingEffects=0

2. Memory requirements and preview workflow

GPU Lightmass needs enough GPU memory to keep the scene’s highest-quality LOD meshes in memory during baking; the system does not account for LODs while baking. The virtual texturing system can also use additional memory, depending on the scene’s size and complexity. CPU memory is also needed for all the lightmaps generated. DX12 can generally use more GPU memory than DX11, so it is important to review settings and content detail in scenes where memory is limited. If there is not enough memory, Epic’s documentation recommends reducing geometry or texture detail, or using a GPU with more memory.

Virtual texture lightmaps are not essential just to bake lighting. However, they are required to update lighting data in real time in the level viewport and to edit the scene while baking. With Viewport Realtime enabled, you can use this interactive workflow; with it disabled, GPU Lightmass bakes significantly faster.

There are two baking modes. Full Bake calculates every object in the scene at full lightmap resolution; results appear when the process is complete. Bake What You See (BWYS) prioritizes areas and objects in the viewport, working at the resolution of the parts currently visible on screen. When the real-time viewport is enabled, it allows recalculation as changes are made. BWYS can therefore be practical when adjusting lighting in a close-up corner of an interior, while Full Bake may be preferable when the completed data for the entire scene is needed. These are workflow interpretations of the modes described in the documentation.

3. Preparing Lightmap UVs correctly

Lightmap UVs define the layout used by static mesh surfaces to store baked lighting and shadow information. Unlike texture UVs, each face needs its own non-overlapping area. Overlapping surfaces may be acceptable in texture UVs, but they can mix lighting data in a lightmap. Spacing between islands is also important for reducing compression-related leakage artifacts.

Automatic Lightmap UV generation during import is available unless it was disabled in the FBX Import Options window. The system uses the UV channel from the texture layout to create a new lightmap layout, repacking the islands without overlaps or wrapping and leaving spacing based on the target resolution. This provides a quick starting point for most static meshes. The tool repacks islands but does not cut them into smaller pieces. If the geometry does not produce a suitable automatic result, create a custom unwrap during modeling or UV layout.

For custom UVs, keep meaningfully connected faces in the same island where possible so that lighting flows smoothly across surfaces. Deep recesses and cavities can be good places to separate islands. Complex or multipart meshes may need more islands and spacing; in turn, a higher lightmap resolution may be needed to maintain the same quality. Epic’s documentation gives a practical guideline of usually leaving at least four texels between islands. The calculation provided for the UV grid is the inverse of the target lightmap resolution; for a resolution of 64, the documentation shows a 1 / 62 calculation after allowing one pixel along each edge.

4. Architectural visualization workflows and common mistakes

GPU Lightmass may be of interest to architectural visualization teams that want to iterate on the look of scenes with baked lighting. However, hardware selection depends on more than GPU performance: GPU memory, CPU memory, scene complexity and virtual texture use also affect results. Multi-GPU support requires Windows 10 version 2004 or later and ray-tracing-capable NVIDIA GPUs connected with SLI. In Epic’s test example, two RTX-2080TI GPUs delivered an average speed increase of approximately 1.7x in medium-sized scenes that did not make intensive use of volumetric lightmaps; this is not guaranteed and may vary by scene.

Common mistakes

• Treating Lightmap UVs like texture UVs and overlapping islands: each face needs its own area in Lightmap UVs.

• Leaving too little space between islands: this can cause light or shadow leaks; the documentation recommends usually leaving at least four texels.

• Not enabling virtual texture lightmaps when expecting interactive previews: GPU Lightmass can bake lighting, but this system is needed for the real-time editing workflow.

• Ignoring memory use when switching to DX12: DX12 and virtual textures may require additional GPU memory.

• Taking GPU Lightmass directly into a production workflow while ignoring its beta warning: Epic advises caution when using it in production.