Commits on Mar 23, 2024

1. Implement minimal GPU culling for cameras. …

   This commit introduces a new component, `GpuCulling`, which, when
   present on a camera, skips the CPU visibility check in favor of doing
   the frustum culling on the GPU. This trades off potentially-increased
   CPU work and drawcalls in favor of cheaper culling and doesn't improve
   the performance of any workloads that I know of today. However, it opens
   the door to significant optimizations in the future by taking the
   necessary first step toward *GPU-driven rendering*.
   
   Enabling GPU culling for a view puts the rendering for that view into
   *indirect mode*. In indirect mode, CPU-level visibility checks are
   skipped, and all visible entities are considered potentially visible.
   Bevy's batching logic still runs as usual, but it doesn't directly
   generate mesh instance indices. Instead, it generates *instance
   handles*, which are indices into an array of real instance indices.
   Before any rendering is done, for each view, a compute shader,
   `cull.wgsl`, maps instance handles to instance indices, discarding any
   instance handles that represent meshes that are outside the visible
   frustum. Draws are then done using the *indirect draw* feature of
   `wgpu`, which instructs the GPU to read the number of actual instances
   from the output of that compute shader.
   
   Essentially, GPU culling works by adding a new level of indirection
   between the CPU's notion of instances (known as instance handles) and
   the GPU's notion of instances.
   
   A new `--gpu-culling` flag has been added to the `many_foxes`,
   `many_cubes`, and `3d_shapes` examples.
   
   Potential follow-ups include:
   
   * Split up `RenderMeshInstances` into CPU-driven and GPU-driven parts.
     The former, which contain fields like the transform, won't be
     initialized at all in when GPU culling is enabled. Instead, the
     transform will be directly written to the GPU in `extract_meshes`,
     like `extract_skins` does for joint matrices.
   
   * Implement GPU culling for shadow maps.
   
     - Following that, we can treat all cascades as one as far as the CPU
       is concerned, simply replaying the final draw commands with
       different view uniforms, which should reduce the CPU overhead
       considerably.
   
   * Retain bins from frame to frame so that they don't have to be rebuilt.
     This is a longer term project that will build on top of bevyengine#12453 and
     several of the tasks in bevyengine#12590, such as main-world pipeline
     specialization.

   

   pcwalton committed Mar 23, 2024
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