what is it about raytraced animation that makes it more expensive to update the BVH mesh vertices every frame, compared to old-school bone animation where you send the info to the vertex shader every frame?
Massively simplifying here but when doing one rasterization pass you process a flat list of triangles, in order, once. When raytracing a camera view you query a database of triangles, at least once for each pixel. To avoid scanning the whole list of triangles for every pixel, the database needs a fancy 3D spatial index whose structure depends on the exact position of each vertex. When you modify the position of vertices you have to fix up or rebuild the fancy spatial index too.
it's a global visibility problem, unlike rasterization which is a mostly localized problem.
apart from all the updates and acceleration structures you have to do, you are also battling cache coherency during ray traversal, or as the saying goes: Primary rays cache, secondary trash.
https://youtu.be/h2V8YyfKCVs?t=4421
apart from all the updates and acceleration structures you have to do, you are also battling cache coherency during ray traversal, or as the saying goes: Primary rays cache, secondary trash.