Meshlet continuous LOD

crates/bevy_pbr/Cargo.toml

@@ -16,9 +16,15 @@ shader_format_glsl = ["bevy_render/shader_format_glsl"]
 trace = ["bevy_render/trace"]
 ios_simulator = ["bevy_render/ios_simulator"]
 # Enables the meshlet renderer for dense high-poly scenes (experimental)
-meshlet = []
+meshlet = [
+  "dep:lz4_flex",
+  "dep:serde",
+  "dep:bincode",
+  "dep:thiserror",
+  "dep:range-alloc",
+]
 # Enables processing meshes into meshlet meshes
-meshlet_processor = ["dep:meshopt", "dep:thiserror"]
+meshlet_processor = ["meshlet", "dep:meshopt", "dep:metis", "dep:itertools"]
 
 [dependencies]
 # bevy
@@ -37,18 +43,25 @@ bevy_utils = { path = "../bevy_utils", version = "0.14.0-dev" }
 bevy_window = { path = "../bevy_window", version = "0.14.0-dev" }
 bevy_derive = { path = "../bevy_derive", version = "0.14.0-dev" }
 
+
 # other
-meshopt = { version = "0.2", optional = true }
-thiserror = { version = "1", optional = true }
 bitflags = "2.3"
 fixedbitset = "0.5"
+# meshlet
+lz4_flex = { version = "0.11", default-features = false, features = [
+  "frame",
+], optional = true }
+serde = { version = "1", features = ["derive", "rc"], optional = true }
+bincode = { version = "1", optional = true }
+thiserror = { version = "1", optional = true }
+range-alloc = { version = "0.1", optional = true }
+meshopt = { version = "0.2", optional = true }
+metis = { version = "0.2", optional = true }
+itertools = { version = "0.12", optional = true }
 # direct dependency required for derive macro
 bytemuck = { version = "1", features = ["derive"] }
 radsort = "0.1"
 smallvec = "1.6"
-serde = { version = "1", features = ["derive", "rc"] }
-bincode = "1"
-range-alloc = "0.1"
 nonmax = "0.5"
 static_assertions = "1"
 

crates/bevy_pbr/src/lib.rs

@@ -1,7 +1,7 @@
 // FIXME(3492): remove once docs are ready
 #![allow(missing_docs)]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
-#![forbid(unsafe_code)]
+#![allow(unsafe_code)]
 #![doc(
     html_logo_url = "https://bevyengine.org/assets/icon.png",
     html_favicon_url = "https://bevyengine.org/assets/icon.png"

crates/bevy_pbr/src/meshlet/asset.rs

@@ -1,13 +1,17 @@
 use bevy_asset::{
-    io::{Reader, Writer},
+    io::{AsyncReadAndSeek, Reader, Writer},
     saver::{AssetSaver, SavedAsset},
     Asset, AssetLoader, AsyncReadExt, AsyncWriteExt, LoadContext,
 };
 use bevy_math::Vec3;
 use bevy_reflect::TypePath;
 use bytemuck::{Pod, Zeroable};
+use lz4_flex::frame::{FrameDecoder, FrameEncoder};
 use serde::{Deserialize, Serialize};
-use std::sync::Arc;
+use std::{io::Cursor, sync::Arc};
+
+/// The current version of the [`MeshletMesh`] asset format.
+pub const MESHLET_MESH_ASSET_VERSION: u64 = 0;
 
 /// A mesh that has been pre-processed into multiple small clusters of triangles called meshlets.
 ///
@@ -25,8 +29,8 @@ use std::sync::Arc;
 /// See also [`super::MaterialMeshletMeshBundle`] and [`super::MeshletPlugin`].
 #[derive(Asset, TypePath, Serialize, Deserialize, Clone)]
 pub struct MeshletMesh {
-    /// The total amount of triangles summed across all meshlets in the mesh.
-    pub total_meshlet_triangles: u64,
+    /// The total amount of triangles summed across all LOD 0 meshlets in the mesh.
+    pub worst_case_meshlet_triangles: u64,
     /// Raw vertex data bytes for the overall mesh.
     pub vertex_data: Arc<[u8]>,
     /// Indices into `vertex_data`.
@@ -35,8 +39,8 @@ pub struct MeshletMesh {
     pub indices: Arc<[u8]>,
     /// The list of meshlets making up this mesh.
     pub meshlets: Arc<[Meshlet]>,
-    /// A list of spherical bounding volumes, 1 per meshlet.
-    pub meshlet_bounding_spheres: Arc<[MeshletBoundingSphere]>,
+    /// Spherical bounding volumes.
+    pub bounding_spheres: Arc<[MeshletBoundingSpheres]>,
 }
 
 /// A single meshlet within a [`MeshletMesh`].
@@ -51,7 +55,19 @@ pub struct Meshlet {
     pub triangle_count: u32,
 }
 
-/// A spherical bounding volume used for culling a [`Meshlet`].
+/// Bounding spheres used for culling and choosing level of detail for a [`Meshlet`].
+#[derive(Serialize, Deserialize, Copy, Clone, Pod, Zeroable)]
+#[repr(C)]
+pub struct MeshletBoundingSpheres {
+    /// The bounding sphere used for frustum and occlusion culling for this meshlet.
+    pub self_culling: MeshletBoundingSphere,
+    /// The bounding sphere used for determining if this meshlet is at the correct level of detail for a given view.
+    pub self_lod: MeshletBoundingSphere,
+    /// The bounding sphere used for determining if this meshlet's parent is at the correct level of detail for a given view.
+    pub parent_lod: MeshletBoundingSphere,
+}
+
+/// A spherical bounding volume used for a [`Meshlet`].
 #[derive(Serialize, Deserialize, Copy, Clone, Pod, Zeroable)]
 #[repr(C)]
 pub struct MeshletBoundingSphere {
@@ -65,17 +81,24 @@ pub struct MeshletMeshSaverLoad;
 impl AssetLoader for MeshletMeshSaverLoad {
     type Asset = MeshletMesh;
     type Settings = ();
-    type Error = bincode::Error;
+    type Error = MeshletMeshSaveOrLoadError;
 
     async fn load<'a>(
         &'a self,
         reader: &'a mut Reader<'_>,
         _settings: &'a Self::Settings,
         _load_context: &'a mut LoadContext<'_>,
     ) -> Result<Self::Asset, Self::Error> {
+        let version = read_u64(reader).await?;
+        if version != MESHLET_MESH_ASSET_VERSION {
+            return Err(MeshletMeshSaveOrLoadError::WrongVersion { found: version });
+        }
+
         let mut bytes = Vec::new();
         reader.read_to_end(&mut bytes).await?;
-        bincode::deserialize(&bytes)
+        let asset = bincode::deserialize_from(FrameDecoder::new(Cursor::new(bytes)))?;
+
+        Ok(asset)
     }
 
     fn extensions(&self) -> &[&str] {
@@ -87,16 +110,44 @@ impl AssetSaver for MeshletMeshSaverLoad {
     type Asset = MeshletMesh;
     type Settings = ();
     type OutputLoader = Self;
-    type Error = bincode::Error;
+    type Error = MeshletMeshSaveOrLoadError;
 
     async fn save<'a>(
         &'a self,
         writer: &'a mut Writer,
         asset: SavedAsset<'a, Self::Asset>,
         _settings: &'a Self::Settings,
     ) -> Result<(), Self::Error> {
-        let bytes = bincode::serialize(asset.get())?;
+        writer
+            .write_all(&MESHLET_MESH_ASSET_VERSION.to_le_bytes())
+            .await?;
+
+        let mut bytes = Vec::new();
+        let mut sync_writer = FrameEncoder::new(&mut bytes);
+        bincode::serialize_into(&mut sync_writer, asset.get())?;
+        sync_writer.finish()?;
         writer.write_all(&bytes).await?;
+
         Ok(())
     }
 }
+
+#[derive(thiserror::Error, Debug)]
+pub enum MeshletMeshSaveOrLoadError {
+    #[error("expected asset version {MESHLET_MESH_ASSET_VERSION} but found version {found}")]
+    WrongVersion { found: u64 },
+    #[error("failed to serialize or deserialize asset data")]
+    SerializationOrDeserialization(#[from] bincode::Error),
+    #[error("failed to compress or decompress asset data")]
+    CompressionOrDecompression(#[from] lz4_flex::frame::Error),
+    #[error("failed to read or write asset data")]
+    Io(#[from] std::io::Error),
+}
+
+async fn read_u64(
+    reader: &mut (dyn AsyncReadAndSeek + Sync + Send + Unpin),
+) -> Result<u64, bincode::Error> {
+    let mut bytes = [0u8; 8];
+    reader.read_exact(&mut bytes).await?;
+    Ok(u64::from_le_bytes(bytes))
+}

crates/bevy_pbr/src/meshlet/cull_meshlets.wgsl

@@ -20,19 +20,34 @@
 @compute
 @workgroup_size(128, 1, 1) // 128 threads per workgroup, 1 instanced meshlet per thread
 fn cull_meshlets(@builtin(global_invocation_id) cluster_id: vec3<u32>) {
-    // Fetch the instanced meshlet data
+    // Fetch the instance data and check for instance culling
     if cluster_id.x >= arrayLength(&meshlet_thread_meshlet_ids) { return; }
     let instance_id = meshlet_thread_instance_ids[cluster_id.x];
     if should_cull_instance(instance_id) {
         return;
     }
+
+    // Fetch other meshlet data
     let meshlet_id = meshlet_thread_meshlet_ids[cluster_id.x];
-    let bounding_sphere = meshlet_bounding_spheres[meshlet_id];
     let instance_uniform = meshlet_instance_uniforms[instance_id];
     let model = affine3_to_square(instance_uniform.model);
     let model_scale = max(length(model[0]), max(length(model[1]), length(model[2])));
-    let bounding_sphere_center = model * vec4(bounding_sphere.center, 1.0);
-    let bounding_sphere_radius = model_scale * bounding_sphere.radius;
+    let bounding_spheres = meshlet_bounding_spheres[meshlet_id];
+
+    // Calculate view-space LOD bounding sphere for the meshlet
+    let lod_bounding_sphere_center = model * vec4(bounding_spheres.self_lod.center, 1.0);
+    let lod_bounding_sphere_radius = model_scale * bounding_spheres.self_lod.radius;
+    let lod_bounding_sphere_center_view_space = (view.inverse_view * vec4(lod_bounding_sphere_center.xyz, 1.0)).xyz;
+
+    // Calculate view-space LOD bounding sphere for the meshlet's parent
+    let parent_lod_bounding_sphere_center = model * vec4(bounding_spheres.parent_lod.center, 1.0);
+    let parent_lod_bounding_sphere_radius = model_scale * bounding_spheres.parent_lod.radius;
+    let parent_lod_bounding_sphere_center_view_space = (view.inverse_view * vec4(parent_lod_bounding_sphere_center.xyz, 1.0)).xyz;
+
+    // Check LOD cut (meshlet error imperceptible, and parent error not imperceptible)
+    let lod_is_ok = lod_error_is_imperceptible(lod_bounding_sphere_center_view_space, lod_bounding_sphere_radius);
+    let parent_lod_is_ok = lod_error_is_imperceptible(parent_lod_bounding_sphere_center_view_space, parent_lod_bounding_sphere_radius);
+    if !lod_is_ok || parent_lod_is_ok { return; }
 
     // In the first pass, operate only on the clusters visible last frame. In the second pass, operate on all clusters.
 #ifdef MESHLET_SECOND_CULLING_PASS
@@ -42,18 +57,22 @@ fn cull_meshlets(@builtin(global_invocation_id) cluster_id: vec3<u32>) {
     if !meshlet_visible { return; }
 #endif
 
+    // Calculate world-space culling bounding sphere for the cluster
+    let culling_bounding_sphere_center = model * vec4(bounding_spheres.self_culling.center, 1.0);
+    let culling_bounding_sphere_radius = model_scale * bounding_spheres.self_culling.radius;
+
     // Frustum culling
     // TODO: Faster method from https://vkguide.dev/docs/gpudriven/compute_culling/#frustum-culling-function
     for (var i = 0u; i < 6u; i++) {
         if !meshlet_visible { break; }
-        meshlet_visible &= dot(view.frustum[i], bounding_sphere_center) > -bounding_sphere_radius;
+        meshlet_visible &= dot(view.frustum[i], culling_bounding_sphere_center) > -culling_bounding_sphere_radius;
     }
 
 #ifdef MESHLET_SECOND_CULLING_PASS
     // In the second culling pass, cull against the depth pyramid generated from the first pass
     if meshlet_visible {
-        let bounding_sphere_center_view_space = (view.inverse_view * vec4(bounding_sphere_center.xyz, 1.0)).xyz;
-        let aabb = project_view_space_sphere_to_screen_space_aabb(bounding_sphere_center_view_space, bounding_sphere_radius);
+        let culling_bounding_sphere_center_view_space = (view.inverse_view * vec4(culling_bounding_sphere_center.xyz, 1.0)).xyz;
+        let aabb = project_view_space_sphere_to_screen_space_aabb(culling_bounding_sphere_center_view_space, culling_bounding_sphere_radius);
 
         // Halve the AABB size because the first depth mip resampling pass cut the full screen resolution into a power of two conservatively
         let depth_pyramid_size_mip_0 = vec2<f32>(textureDimensions(depth_pyramid, 0)) * 0.5;
@@ -71,11 +90,11 @@ fn cull_meshlets(@builtin(global_invocation_id) cluster_id: vec3<u32>) {
         let occluder_depth = min(min(depth_quad_a, depth_quad_b), min(depth_quad_c, depth_quad_d));
         if view.projection[3][3] == 1.0 {
             // Orthographic
-            let sphere_depth = view.projection[3][2] + (bounding_sphere_center_view_space.z + bounding_sphere_radius) * view.projection[2][2];
+            let sphere_depth = view.projection[3][2] + (culling_bounding_sphere_center_view_space.z + culling_bounding_sphere_radius) * view.projection[2][2];
             meshlet_visible &= sphere_depth >= occluder_depth;
         } else {
             // Perspective
-            let sphere_depth = -view.projection[3][2] / (bounding_sphere_center_view_space.z + bounding_sphere_radius);
+            let sphere_depth = -view.projection[3][2] / (culling_bounding_sphere_center_view_space.z + culling_bounding_sphere_radius);
             meshlet_visible &= sphere_depth >= occluder_depth;
         }
     }
@@ -86,6 +105,16 @@ fn cull_meshlets(@builtin(global_invocation_id) cluster_id: vec3<u32>) {
     atomicOr(&meshlet_occlusion[cluster_id.x / 32u], occlusion_bit);
 }
 
+// https://stackoverflow.com/questions/21648630/radius-of-projected-sphere-in-screen-space/21649403#21649403
+fn lod_error_is_imperceptible(cp: vec3<f32>, r: f32) -> bool {
+    let d2 = dot(cp, cp);
+    let r2 = r * r;
+    let sphere_diameter_uv = view.projection[0][0] * r / sqrt(d2 - r2);
+    let view_size = f32(max(view.viewport.z, view.viewport.w));
+    let sphere_diameter_pixels = sphere_diameter_uv * view_size;
+    return sphere_diameter_pixels < 1.0;
+}
+
 // https://zeux.io/2023/01/12/approximate-projected-bounds
 fn project_view_space_sphere_to_screen_space_aabb(cp: vec3<f32>, r: f32) -> vec4<f32> {
     let inv_width = view.projection[0][0] * 0.5;