1// A compute shader that unpacks bins.
2//
3// This shader runs before mesh preprocessing in order to generate
4// `PreprocessWorkItem`s from cached bins. A single dispatch of this shader
5// corresponds to a single batch set, and one invocation of this shader
6// corresponds to one binned entity. Each shader invocation builds the work item
7// for its entity by copying over the mesh input uniform index and calculating
8// the position of the command in the indirect parameters buffer that will draw
9// that entity.
10
11#import bevy_pbr::mesh_preprocess_types::PreprocessWorkItem
12
13// Information needed to unpack bins belonging to a single batch set.
14struct BinUnpackingMetadata {
15    // The index of the first `PreprocessWorkItem` that this compute shader
16    // dispatch is to write to.
17    base_output_work_item_index: u32,
18    // The index of the first GPU indirect parameters command for this batch
19    // set.
20    base_indirect_parameters_index: u32,
21    // The number of binned mesh instances in the `binned_mesh_instances`
22    // array.
23    binned_mesh_instance_count: u32,
24    // Padding.
25    pad_a: u32,
26    // Padding.
27    pad_b: array<vec4<u32>, 15>,
28};
29
30// One mesh instance in a bin.
31//
32// This corresponds to the CPU-side `GpuRenderBinnedMeshInstance` structure.
33//
34// Note that this structure isn't sorted within the
35// `binned_mesh_instances` buffer. Instances from the same bin aren't
36// guaranteed to be adjacent to one another.
37struct BinnedMeshInstance {
38    // The index of the `MeshInputUniform` corresponding to this mesh instance
39    // in the `MeshInputUniform` buffer.
40    input_uniform_index: u32,
41    // The index of the bin that this mesh instance belongs to.
42    bin_index: u32,
43};
44
45// Metadata for the entire batch set.
46@group(0) @binding(0) var<uniform> bin_unpacking_metadata: BinUnpackingMetadata;
47
48// The input array of `BinnedMeshInstance`s.
49//
50// Note that this array isn't sorted.
51@group(0) @binding(1) var<storage> binned_mesh_instances: array<BinnedMeshInstance>;
52
53// The output list of `PreprocessWorkItem`s.
54@group(0) @binding(2) var<storage, read_write> preprocess_work_items: array<PreprocessWorkItem>;
55
56// A mapping from each `bin_index` to the index of the GPU indirect parameters
57// for this bin, relative to the start of the indirect parameters for this batch
58// set.
59@group(0) @binding(3) var<storage> bin_index_to_indirect_parameters_offset: array<u32>;
60
61@compute
62@workgroup_size(64)
63fn main(@builtin(global_invocation_id) global_invocation_id: vec3<u32>) {
64    // Figure out which instance we're looking at.
65    let global_id = global_invocation_id.x;
66    if (global_id >= bin_unpacking_metadata.binned_mesh_instance_count) {
67        return;
68    }
69
70    // Unpack the `BinnedMeshInstance`.
71    let input_uniform_index = binned_mesh_instances[global_id].input_uniform_index;
72    let bin_index = binned_mesh_instances[global_id].bin_index;
73
74    // Look up the indirect parameters index for this bin, relative to the first
75    // indirect parameters offset for this batch set.
76    let indirect_parameters_offset = bin_index_to_indirect_parameters_offset[bin_index];
77
78    // Determine the location we should write the work item to.
79    let output_index = bin_unpacking_metadata.base_output_work_item_index + global_id;
80
81    // Write out the resulting work item.
82    preprocess_work_items[output_index].input_index = input_uniform_index;
83    preprocess_work_items[output_index].output_or_indirect_parameters_index =
84        bin_unpacking_metadata.base_indirect_parameters_index + indirect_parameters_offset;
85}