# Lumen GI and Reflections feedback thread **URL:** \ **Category:** Rendering\ **Tags:** unreal-engine, Lumen\ **Created:** [May 27, 2021, 2:39am UTC](https://forums.unrealengine.com/t/lumen-gi-and-reflections-feedback-thread/231403 "2021-05-27T02:39:31Z")\ **Posts on this page:** 1\ **Showing post:** 10
**Author:** ![Patrick.Kelly](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/patrick.kelly/32/746131_2.png) [@Patrick.Kelly](https://forums.unrealengine.com/u/Patrick.Kelly)\ **Post date:** [May 27, 2021, 1:46pm UTC](https://forums.unrealengine.com/t/lumen-gi-and-reflections-feedback-thread/231403/10 "2021-05-27T13:46:18Z")
I took a similar look at the runtime cost. The conclusion is still the same: ray-tracing cost for direct lighting in Lumen is too high. The cost, highlighted by Krzysztof, is due to the ray-gen dispatch across the entire surface cache atlas. This is currently done per-light, and the cost stacks up quickly. I’m actively working in this area so we can perform sparse updates and significantly lower the expected runtime. To give a brief example of in-progress work, when looking at Sun Temple, the current cost per-light in Early-Access is about 0.4ms while some preliminary work moves it down to 0.05ms. This isn’t a final report, however, as we expect batched evaluation to push the overhead down to something like 0.05ms per 16 lights, instead. Lumen’s use of ray-traced direct lighting is tied to the use of ray-traced shadows in the scene. You can decouple this by setting the following CVar: `r.Lumen.DirectLighting.HardwareRayTracing 0` However, you might opt to work without ray-traced shadows, since virtual shadow maps are the preferred method to use with Nanite assets. Patrick --- _[View the full topic](https://forums.unrealengine.com/t/lumen-gi-and-reflections-feedback-thread/231403)._