Occluder Meshes

A companion to Shadow Proxies — another manual mesh optimization technique that helps the engine discard work it doesn't need to do.

Purpose

As shaders become more complex and heavier, optimization becomes more important. If a shader doesn't write to the depth pass, it won't contribute to the pre-pass that culls unnecessary pixels further down the pipeline.

Systems like backface culling (which discards triangles facing away from the camera) or occlusion culling middleware like Umbra handle some of this automatically, but they don't take care of the pixel shader cost for geometry that's technically "in view" but hidden behind something else.

By creating a custom occluder mesh that writes to the depth buffer, you're helping the Z-prepass cull pixels that are definitely not visible from a given angle. The key constraint: all triangles of the occluder mesh must stay within the bounds of the visual mesh. If the occluder extends beyond the visual geometry, it will incorrectly kill pixel shader information behind it, causing rendering artifacts where visible pixels get culled.

Maximum draw distance is also worth considering. If an occluder mesh is too aggressive at large distances, it can cause issues due to reduced vertex precision over distance; depth buffers become less precise at far ranges because of how depth is distributed non-linearly (more precision near the camera, less far away). An overly tight occluder at distance can start incorrectly culling pixels that should be visible.

Rule of thumb: I tend to target walls or props that cover most of a character's body, or roughly 10–15% of the screen, at least for third-person games. Anything smaller and the occluder drawcall costs more than it saves.

Engine Support

Unreal Engine does support custom occluder meshes. In UE4 and UE5, you can enable Software Occlusion Culling and mark specific simplified meshes as occluders. UE5 also handles much of this automatically through Nanite's software rasterizer for opaque geometry, but for non-Nanite assets and translucent materials, custom occluders remain relevant.

Note: not all engines support this level of per-asset control. Some proprietary engines expose it explicitly, others handle occlusion at a higher level with less artist input.

When It Helps

• Large opaque props that block a meaningful portion of the screen.
• Interior walls, pillars, architectural elements in dense scenes.
• Non-Nanite assets in a UE5 pipeline.

When It Doesn't

• Small props: the occluder drawcall will cost more than it saves.
• Translucent or alpha-blended geometry: these don't write to depth and can't act as occluders.
• Nanite-enabled opaque geometry in UE5: Nanite handles this automatically.

© 2026 Stefan Groenewoud — All views are my own, not those of my employer.