Auto generated LODs doubling draw calls compared to imported LOD meshes

Unreal Engine Platform & Builds Debugging, Optimization, & Profiling
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#1

Hello,

I’m currently working on foliage models and whilst making some performance tests I’ve noticed that if I use the automatic LoD generation I’m getting around double the draw calls compared to using imported LOD meshes.

I set up two versions of my model, one using my custom LOD models and the other using generated LODs. I set them up identically otherwise (same LOD distances, materials etc.) and tuned the generation settings of the LODs to have a similar tri / vertex count as the custom made LODs. In a simple test map, I swapped between the meshes in the foliage settings I could see the auto generated version was doubling the draw calls and saw a drop in the frame rate.

Has anyone else experienced this issue?

Thanks

#2

What about material slots? Like does your LOD0 have several and then the lower LODs only have one? Each material slot is an extra draw call on a mesh. Auto generating LODs would likely keep the same slot count for the ones it generates.

#3

Okay, I did some more testing this morning and it seems the culprit was a slight difference in vertex count on the generated LODs. I’d matched the triangle count but the vertex count was a bit higher. When I matched the vertex count the draw calls were identical.

So my guess is that the foliage instances are divided into batches with a fixed memory size, so the increased vertex count meant there were less instances per batch leading to more batches and the extra draw calls.