ce433955e6
Currently, when a mesh slab overflows, we recreate the allocator and reinsert all the meshes that were in it in an arbitrary order. This can result in the meshes moving around. Before `MeshInputUniform`s were retained, this was slow but harmless, because the `MeshInputUniform` that contained the positions of the vertex and index data in the slab would be recreated every frame. However, with mesh retention, there's no guarantee that the `MeshInputUniform`, which could be cached from the previous frame, will reflect the new position of the mesh data within the buffer if that buffer happened to grow. This manifested itself as seeming mesh data corruption when adding many meshes dynamically to the scene. There are three possible ways that I could have fixed this that I can see: 1. Invalidate and rebuild all the `MeshInputUniform`s belonging to all meshes in a slab when that mesh grows. 2. Introduce a second layer of indirection so that the `MeshInputUniform` points to a *mesh allocation table* that contains the current locations of the data of each mesh. 3. Avoid moving meshes when reallocating the buffer. To be efficient, option (1) would require scanning meshes to see if their positions changed, a la `mark_meshes_as_changed_if_their_materials_changed`. Option (2) would add more runtime indirection and would require additional bookkeeping on the part of the allocator. Therefore, this PR chooses option (3), which was remarkably simple to implement. The key is that the offset allocator happens to allocate addresses from low addresses to high addresses. So all we have to do is to *conceptually* allocate the full 512 MiB mesh slab as far as the offset allocator is concerned, and grow the underlying backing store from 1 MiB to 512 MiB as needed. In other words, the allocator now allocates *virtual* GPU memory, and the actual backing slab resizes to fit the virtual memory. This ensures that the location of mesh data remains constant for the lifetime of the mesh asset, and we can remove the code that reinserts meshes one by one when the slab grows in favor of a single buffer copy. Closes #17766. |
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Bevy Render
