# [Reduce Acceleration Structure Run-time Update Cost](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/7-run-time-update/)

## In this learning path

- [Introduction](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/)
- [Lumen and Ray Tracing](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/1-ray-tracing/)
- [Acceleration Structure](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/2-acceleration-structure/)
- [Only Add Important Objects into Ray Tracing](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/4-add-object/)
- [Take Full Advantage of Instancing](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/5-instancing/)
- [Optimize Acceleration Structure](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/6-optimize-as/)
- [Reduce Acceleration Structure Run-time Update Cost](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/7-run-time-update/)
- [Lumen General Setting Optimizations](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/8-lumen-general/)
- [Next Steps](https://learn.arm.com/learning-paths/mobile-graphics-and-gaming/best-practices-for-hwrt-lumen-performance/_next-steps/)

We now turn to run-time. The acceleration structure for static meshes is built offline and does not require updates at run-time. However, the acceleration structure for skinned meshes needs to be updated at run-time, which incurs a performance cost due to the need to update geometry data on the GPU and rebuild the acceleration structure. When the polygon count of a skinned mesh is high, the update cost can be too high for real-time applications. To reduce this cost, you can use a higher LOD (Level of Detail) level for skinned meshes in ray tracing, resulting in fewer polygons needing updates at run-time, as shown below:

Note that using a higher LOD level for skinned meshes in ray tracing may cause self-shadowing artifacts, if ray tracing shadows are enabled. These artifacts are caused by differences between the rendering mesh and the ray tracing mesh, as illustrated below: