METIS-based meshlet generation (#16947)
# Objective Improve DAG building for virtual geometry ## Solution - Use METIS to group triangles into meshlets which lets us minimize locked vertices which improves simplification, instead of using meshopt which prioritizes culling efficiency. Also some other minor tweaks. - Currently most meshlets have 126 triangles, and not 128. Fixing this might involve calling METIS recursively ourselves to manually bisect the graph, not sure. Not going to attempt to fix this in this PR. ## Testing - Did you test these changes? If so, how? - Tested on bunny.glb and cliff.glb - Are there any parts that need more testing? - No - How can other people (reviewers) test your changes? Is there anything specific they need to know? - Download the new bunny asset, run the meshlet example. --- ## Showcase New  Old  --------- Co-authored-by: IceSentry <IceSentry@users.noreply.github.com>
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JMS55
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@ -1236,7 +1236,7 @@ setup = [
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"curl",
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"-o",
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"assets/models/bunny.meshlet_mesh",
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"https://raw.githubusercontent.com/JMS55/bevy_meshlet_asset/defbd9b32072624d40d57de7d345c66a9edf5d0b/bunny.meshlet_mesh",
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"https://raw.githubusercontent.com/JMS55/bevy_meshlet_asset/7a7c14138021f63904b584d5f7b73b695c7f4bbf/bunny.meshlet_mesh",
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],
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]
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@ -60,7 +60,7 @@ lz4_flex = { version = "0.11", default-features = false, features = [
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], optional = true }
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range-alloc = { version = "0.1.3", optional = true }
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half = { version = "2", features = ["bytemuck"], optional = true }
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meshopt = { version = "0.4", optional = true }
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meshopt = { version = "0.4.1", optional = true }
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metis = { version = "0.2", optional = true }
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itertools = { version = "0.13", optional = true }
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bitvec = { version = "1", optional = true }
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@ -16,7 +16,7 @@ use meshopt::{
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build_meshlets, ffi::meshopt_Meshlet, generate_vertex_remap_multi,
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simplify_with_attributes_and_locks, Meshlets, SimplifyOptions, VertexDataAdapter, VertexStream,
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};
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use metis::Graph;
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use metis::{option::Opt, Graph};
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use smallvec::SmallVec;
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use thiserror::Error;
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@ -67,12 +67,29 @@ impl MeshletMesh {
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// Validate mesh format
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let indices = validate_input_mesh(mesh)?;
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// Split the mesh into an initial list of meshlets (LOD 0)
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// Get meshlet vertices
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let vertex_buffer = mesh.create_packed_vertex_buffer_data();
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let vertex_stride = mesh.get_vertex_size() as usize;
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let vertices = VertexDataAdapter::new(&vertex_buffer, vertex_stride, 0).unwrap();
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let vertex_normals = bytemuck::cast_slice(&vertex_buffer[12..16]);
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let mut meshlets = compute_meshlets(&indices, &vertices);
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// Generate a position-only vertex buffer for determining triangle/meshlet connectivity
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let (position_only_vertex_count, position_only_vertex_remap) = generate_vertex_remap_multi(
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vertices.vertex_count,
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&[VertexStream::new_with_stride::<Vec3, _>(
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vertex_buffer.as_ptr(),
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vertex_stride,
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)],
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Some(&indices),
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);
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// Split the mesh into an initial list of meshlets (LOD 0)
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let mut meshlets = compute_meshlets(
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&indices,
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&vertices,
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&position_only_vertex_remap,
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position_only_vertex_count,
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);
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let mut bounding_spheres = meshlets
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.iter()
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.map(|meshlet| compute_meshlet_bounds(meshlet, &vertices))
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@ -92,16 +109,6 @@ impl MeshletMesh {
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.take(meshlets.len())
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.collect::<Vec<_>>();
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// Generate a position-only vertex buffer for determining what meshlets are connected for use in grouping
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let (position_only_vertex_count, position_only_vertex_remap) = generate_vertex_remap_multi(
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vertices.vertex_count,
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&[VertexStream::new_with_stride::<Vec3, _>(
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vertex_buffer.as_ptr(),
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vertex_stride,
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)],
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Some(&indices),
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);
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let mut vertex_locks = vec![false; vertices.vertex_count];
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// Build further LODs
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@ -163,6 +170,8 @@ impl MeshletMesh {
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let new_meshlets_count = split_simplified_group_into_new_meshlets(
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&simplified_group_indices,
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&vertices,
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&position_only_vertex_remap,
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position_only_vertex_count,
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&mut meshlets,
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);
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@ -243,8 +252,103 @@ fn validate_input_mesh(mesh: &Mesh) -> Result<Cow<'_, [u32]>, MeshToMeshletMeshC
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}
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}
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fn compute_meshlets(indices: &[u32], vertices: &VertexDataAdapter) -> Meshlets {
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build_meshlets(indices, vertices, 255, 128, 0.0) // Meshoptimizer won't currently let us do 256 vertices
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fn compute_meshlets(
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indices: &[u32],
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vertices: &VertexDataAdapter,
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position_only_vertex_remap: &[u32],
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position_only_vertex_count: usize,
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) -> Meshlets {
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// For each vertex, build a list of all triangles that use it
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let mut vertices_to_triangles = vec![Vec::new(); position_only_vertex_count];
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for (i, index) in indices.iter().enumerate() {
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let vertex_id = position_only_vertex_remap[*index as usize];
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let vertex_to_triangles = &mut vertices_to_triangles[vertex_id as usize];
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vertex_to_triangles.push(i / 3);
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}
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// For each triangle pair, count how many vertices they share
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let mut triangle_pair_to_shared_vertex_count = <HashMap<_, _>>::default();
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for vertex_triangle_ids in vertices_to_triangles {
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for (triangle_id1, triangle_id2) in vertex_triangle_ids.into_iter().tuple_combinations() {
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let count = triangle_pair_to_shared_vertex_count
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.entry((
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triangle_id1.min(triangle_id2),
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triangle_id1.max(triangle_id2),
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))
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.or_insert(0);
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*count += 1;
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}
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}
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// For each triangle, gather all other triangles that share at least one vertex along with their shared vertex count
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let triangle_count = indices.len() / 3;
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let mut connected_triangles_per_triangle = vec![Vec::new(); triangle_count];
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for ((triangle_id1, triangle_id2), shared_vertex_count) in triangle_pair_to_shared_vertex_count
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{
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// We record both id1->id2 and id2->id1 as adjacency is symmetrical
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connected_triangles_per_triangle[triangle_id1].push((triangle_id2, shared_vertex_count));
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connected_triangles_per_triangle[triangle_id2].push((triangle_id1, shared_vertex_count));
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}
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// The order of triangles depends on hash traversal order; to produce deterministic results, sort them
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for list in connected_triangles_per_triangle.iter_mut() {
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list.sort_unstable();
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}
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let mut xadj = Vec::with_capacity(triangle_count + 1);
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let mut adjncy = Vec::new();
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let mut adjwgt = Vec::new();
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for connected_triangles in connected_triangles_per_triangle {
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xadj.push(adjncy.len() as i32);
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for (connected_triangle_id, shared_vertex_count) in connected_triangles {
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adjncy.push(connected_triangle_id as i32);
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adjwgt.push(shared_vertex_count);
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// TODO: Additional weight based on triangle center spatial proximity?
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}
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}
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xadj.push(adjncy.len() as i32);
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let mut options = [-1; metis::NOPTIONS];
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options[metis::option::Seed::INDEX] = 17;
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options[metis::option::UFactor::INDEX] = 1; // Important that there's very little imbalance between partitions
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let mut meshlet_per_triangle = vec![0; triangle_count];
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let partition_count = triangle_count.div_ceil(126); // Need to undershoot to prevent METIS from going over 128 triangles per meshlet
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Graph::new(1, partition_count as i32, &xadj, &adjncy)
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.unwrap()
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.set_options(&options)
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.set_adjwgt(&adjwgt)
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.part_recursive(&mut meshlet_per_triangle)
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.unwrap();
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let mut indices_per_meshlet = vec![Vec::new(); partition_count];
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for (triangle_id, meshlet) in meshlet_per_triangle.into_iter().enumerate() {
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let meshlet_indices = &mut indices_per_meshlet[meshlet as usize];
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let base_index = triangle_id * 3;
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meshlet_indices.extend_from_slice(&indices[base_index..(base_index + 3)]);
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}
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// Use meshopt to build meshlets from the sets of triangles
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let mut meshlets = Meshlets {
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meshlets: Vec::new(),
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vertices: Vec::new(),
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triangles: Vec::new(),
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};
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for meshlet_indices in &indices_per_meshlet {
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let meshlet = build_meshlets(meshlet_indices, vertices, 255, 128, 0.0);
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let vertex_offset = meshlets.vertices.len() as u32;
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let triangle_offset = meshlets.triangles.len() as u32;
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meshlets.vertices.extend_from_slice(&meshlet.vertices);
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meshlets.triangles.extend_from_slice(&meshlet.triangles);
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meshlets
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.meshlets
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.extend(meshlet.meshlets.into_iter().map(|mut meshlet| {
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meshlet.vertex_offset += vertex_offset;
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meshlet.triangle_offset += triangle_offset;
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meshlet
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}));
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}
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meshlets
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}
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fn find_connected_meshlets(
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@ -315,15 +419,19 @@ fn group_meshlets(
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}
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xadj.push(adjncy.len() as i32);
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let mut options = [-1; metis::NOPTIONS];
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options[metis::option::Seed::INDEX] = 17;
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options[metis::option::UFactor::INDEX] = 200;
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let mut group_per_meshlet = vec![0; simplification_queue.len()];
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let partition_count = simplification_queue
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.len()
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.div_ceil(TARGET_MESHLETS_PER_GROUP); // TODO: Nanite uses groups of 8-32, probably based on some kind of heuristic
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Graph::new(1, partition_count as i32, &xadj, &adjncy)
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.unwrap()
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.set_option(metis::option::Seed(17))
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.set_options(&options)
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.set_adjwgt(&adjwgt)
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.part_kway(&mut group_per_meshlet)
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.part_recursive(&mut group_per_meshlet)
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.unwrap();
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let mut groups = vec![SmallVec::new(); partition_count];
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@ -462,9 +570,16 @@ fn compute_lod_group_data(
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fn split_simplified_group_into_new_meshlets(
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simplified_group_indices: &[u32],
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vertices: &VertexDataAdapter<'_>,
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position_only_vertex_remap: &[u32],
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position_only_vertex_count: usize,
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meshlets: &mut Meshlets,
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) -> usize {
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let simplified_meshlets = compute_meshlets(simplified_group_indices, vertices);
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let simplified_meshlets = compute_meshlets(
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simplified_group_indices,
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vertices,
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position_only_vertex_remap,
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position_only_vertex_count,
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);
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let new_meshlets_count = simplified_meshlets.len();
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let vertex_offset = meshlets.vertices.len() as u32;
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@ -610,7 +725,7 @@ fn pack2x16snorm(v: Vec2) -> u32 {
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pub enum MeshToMeshletMeshConversionError {
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#[error("Mesh primitive topology is not TriangleList")]
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WrongMeshPrimitiveTopology,
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#[error("Mesh attributes are not {{POSITION, NORMAL, UV_0}}")]
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#[error("Mesh vertex attributes are not {{POSITION, NORMAL, UV_0}}")]
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WrongMeshVertexAttributes,
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#[error("Mesh has no indices")]
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MeshMissingIndices,
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@ -17,7 +17,7 @@ use camera_controller::{CameraController, CameraControllerPlugin};
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use std::{f32::consts::PI, path::Path, process::ExitCode};
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const ASSET_URL: &str =
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"https://raw.githubusercontent.com/JMS55/bevy_meshlet_asset/defbd9b32072624d40d57de7d345c66a9edf5d0b/bunny.meshlet_mesh";
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"https://raw.githubusercontent.com/JMS55/bevy_meshlet_asset/7a7c14138021f63904b584d5f7b73b695c7f4bbf/bunny.meshlet_mesh";
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fn main() -> ExitCode {
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if !Path::new("./assets/models/bunny.meshlet_mesh").exists() {
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