Added the ability to build an octree, and an example of using the built octree to speed up ray detection

[bigbigbig2]

[bigbigbig2]

I added an example comparison. The effect is very obvious for large data. Due to data permission reasons, the code I submitted did not use large data, but used the original small data, so the effect is not as obvious as in my video.

20250721_103749.-.Compressed.with.FlexClip.mp4

[asundqui]

This is tremendous work @bigbigbig2 , thank you for volunteering this code! Please give us some time to review and discuss this... there's great merit here. I wonder a little bit about how this will interact with the "dynamic" nature of Spark... I have some other thoughts about how to do raycast that involve the GPU but are async, maybe this is the right way to get good performance in a "sync" fashion. Just wanted to leave this note so you know this PR hasn't gone unnoticed!

[soundform]

It would be good to make a compact representation of this octree available in the vertex shader, as this will pave the way to many advanced rendering features (https://research.nvidia.com/labs/toronto-ai/3DGRT). For example, simple GI (https://iquilezles.org/articles/simplegi) can be used to properly illuminate splats with an extra light source.

[bigbigbig2]

This is tremendous work @bigbigbig2 , thank you for volunteering this code! Please give us some time to review and discuss this... there's great merit here. I wonder a little bit about how this will interact with the "dynamic" nature of Spark... I have some other thoughts about how to do raycast that involve the GPU but are async, maybe this is the right way to get good performance in a "sync" fashion. Just wanted to leave this note so you know this PR hasn't gone unnoticed!这项工作非常出色，感谢您自愿贡献这段代码！请给我们一些时间来审查和讨论这个……这里有很多价值。我稍微有点好奇这个将如何与 Spark 的“动态”特性交互……我有一些关于如何进行 raycast 的想法，这些想法涉及 GPU 但异步，也许这是以一种“同步”方式获得良好性能的正确方法。只是想留下这个备注，让您知道这个 PR 没有被忽视！

I am very happy to receive your reply. Because this function is a function I have applied in actual production, I am also constantly maintaining and optimizing it. My subsequent updates will be synchronized in the fork warehouse. If there is a need later, I can merge it.

[soundform]

There's also an actively developed https://github.com/gkjohnson/three-mesh-bvh.

[bigbigbig2]

There's also an actively developed https://github.com/gkjohnson/three-mesh-bvh.还有一个活跃开发中的 https://github.com/gkjohnson/three-mesh-bvh。

I also tried this at the beginning, but it does not support instanceBufferGeomtry. I also saw that the author has explained the support in this regard, so I turned to the existing algorithm for constructing acceleration space structure for Gaussian.