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?
> compared to old-school bone animation where you send the info to the vertex shader every frame?
If I understand correctly this really isn't a thing anymore. Instead most modern engines copy the original vertices to a temp or skinned vertex buffer with a compute shader and then use normal non-skinned rendering techniques to display them. This means you don't need 2 copies of every shader, one with skinning and one without so less combinations.
That doesn't change the other answers to your question.
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
If I understand correctly this really isn't a thing anymore. Instead most modern engines copy the original vertices to a temp or skinned vertex buffer with a compute shader and then use normal non-skinned rendering techniques to display them. This means you don't need 2 copies of every shader, one with skinning and one without so less combinations.
That doesn't change the other answers to your question.
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.