Clean up specialized_mesh_pipeline (#20107)

# Objective

- The example doesn't work on webgl2
- It tries to reimplement logic for batching instead of using existing
abstractions which breaks on webgl2
- It only uses batching

## Solution

- Use existing abstractions and remove code related to batching
	- This fixes the webgl2 issue
	- It also makes it use multi draw indirect instead of just batching

## Testing

- Tested the example with the bevy cli for webgl and wegpu and also ran
the example locally

Author: IceSentry

examples/shader_advanced/specialized_mesh_pipeline.rs

@@ -8,24 +8,20 @@
 
 use bevy::{
     core_pipeline::core_3d::{Opaque3d, Opaque3dBatchSetKey, Opaque3dBinKey, CORE_3D_DEPTH_FORMAT},
-    ecs::{component::Tick, system::StaticSystemParam},
+    ecs::component::Tick,
     math::{vec3, vec4},
     pbr::{
         DrawMesh, MeshPipeline, MeshPipelineKey, MeshPipelineViewLayoutKey, RenderMeshInstances,
         SetMeshBindGroup, SetMeshViewBindGroup, SetMeshViewEmptyBindGroup,
     },
     prelude::*,
     render::{
-        batching::{
-            gpu_preprocessing::{
-                self, PhaseBatchedInstanceBuffers, PhaseIndirectParametersBuffers,
-                PreprocessWorkItem, UntypedPhaseBatchedInstanceBuffers,
-            },
-            GetBatchData, GetFullBatchData,
-        },
-        experimental::occlusion_culling::OcclusionCulling,
+        batching::gpu_preprocessing::GpuPreprocessingSupport,
         extract_component::{ExtractComponent, ExtractComponentPlugin},
-        mesh::{Indices, MeshVertexBufferLayoutRef, PrimitiveTopology, RenderMesh},
+        mesh::{
+            allocator::MeshAllocator, Indices, MeshVertexBufferLayoutRef, PrimitiveTopology,
+            RenderMesh,
+        },
         render_asset::{RenderAssetUsages, RenderAssets},
         render_phase::{
             AddRenderCommand, BinnedRenderPhaseType, DrawFunctions, SetItemPipeline,
@@ -37,7 +33,6 @@ use bevy::{
             RenderPipelineDescriptor, SpecializedMeshPipeline, SpecializedMeshPipelineError,
             SpecializedMeshPipelines, TextureFormat, VertexState,
         },
-        view::NoIndirectDrawing,
         view::{self, ExtractedView, RenderVisibleEntities, ViewTarget, VisibilityClass},
         Render, RenderApp, RenderStartup, RenderSystems,
     },
@@ -262,7 +257,6 @@ impl SpecializedMeshPipeline for CustomMeshPipeline {
                 count: mesh_key.msaa_samples(),
                 ..default()
             },
-
             ..default()
         })
     }
@@ -278,75 +272,32 @@ fn queue_custom_mesh_pipeline(
         Res<DrawFunctions<Opaque3d>>,
     ),
     mut specialized_mesh_pipelines: ResMut<SpecializedMeshPipelines<CustomMeshPipeline>>,
-    views: Query<(
-        &RenderVisibleEntities,
-        &ExtractedView,
-        &Msaa,
-        Has<NoIndirectDrawing>,
-        Has<OcclusionCulling>,
-    )>,
+    views: Query<(&RenderVisibleEntities, &ExtractedView, &Msaa)>,
     (render_meshes, render_mesh_instances): (
         Res<RenderAssets<RenderMesh>>,
         Res<RenderMeshInstances>,
     ),
-    param: StaticSystemParam<<MeshPipeline as GetBatchData>::Param>,
-    mut phase_batched_instance_buffers: ResMut<
-        PhaseBatchedInstanceBuffers<Opaque3d, <MeshPipeline as GetBatchData>::BufferData>,
-    >,
-    mut phase_indirect_parameters_buffers: ResMut<PhaseIndirectParametersBuffers<Opaque3d>>,
     mut change_tick: Local<Tick>,
+    mesh_allocator: Res<MeshAllocator>,
+    gpu_preprocessing_support: Res<GpuPreprocessingSupport>,
 ) {
-    let system_param_item = param.into_inner();
-
-    let UntypedPhaseBatchedInstanceBuffers {
-        ref mut data_buffer,
-        ref mut work_item_buffers,
-        ref mut late_indexed_indirect_parameters_buffer,
-        ref mut late_non_indexed_indirect_parameters_buffer,
-        ..
-    } = phase_batched_instance_buffers.buffers;
-
     // Get the id for our custom draw function
-    let draw_function_id = opaque_draw_functions
+    let draw_function = opaque_draw_functions
         .read()
         .id::<DrawSpecializedPipelineCommands>();
 
     // Render phases are per-view, so we need to iterate over all views so that
     // the entity appears in them. (In this example, we have only one view, but
     // it's good practice to loop over all views anyway.)
-    for (view_visible_entities, view, msaa, no_indirect_drawing, gpu_occlusion_culling) in
-        views.iter()
-    {
+    for (view_visible_entities, view, msaa) in views.iter() {
         let Some(opaque_phase) = opaque_render_phases.get_mut(&view.retained_view_entity) else {
             continue;
         };
 
-        // Create *work item buffers* if necessary. Work item buffers store the
-        // indices of meshes that are to be rendered when indirect drawing is
-        // enabled.
-        let work_item_buffer = gpu_preprocessing::get_or_create_work_item_buffer::<Opaque3d>(
-            work_item_buffers,
-            view.retained_view_entity,
-            no_indirect_drawing,
-            gpu_occlusion_culling,
-        );
-
-        // Initialize those work item buffers in preparation for this new frame.
-        gpu_preprocessing::init_work_item_buffers(
-            work_item_buffer,
-            late_indexed_indirect_parameters_buffer,
-            late_non_indexed_indirect_parameters_buffer,
-        );
-
         // Create the key based on the view. In this case we only care about MSAA and HDR
         let view_key = MeshPipelineKey::from_msaa_samples(msaa.samples())
             | MeshPipelineKey::from_hdr(view.hdr);
 
-        // Set up a slot to hold information about the batch set we're going to
-        // create. If there are any of our custom meshes in the scene, we'll
-        // need this information in order for Bevy to kick off the rendering.
-        let mut mesh_batch_set_info = None;
-
         // Find all the custom rendered entities that are visible from this
         // view.
         for &(render_entity, visible_entity) in
@@ -363,34 +314,14 @@ fn queue_custom_mesh_pipeline(
                 continue;
             };
 
+            let (vertex_slab, index_slab) = mesh_allocator.mesh_slabs(&mesh_instance.mesh_asset_id);
+
             // Specialize the key for the current mesh entity
             // For this example we only specialize based on the mesh topology
             // but you could have more complex keys and that's where you'd need to create those keys
             let mut mesh_key = view_key;
             mesh_key |= MeshPipelineKey::from_primitive_topology(mesh.primitive_topology());
 
-            // Initialize the batch set information if this was the first custom
-            // mesh we saw. We'll need that information later to create the
-            // batch set.
-            if mesh_batch_set_info.is_none() {
-                mesh_batch_set_info = Some(MeshBatchSetInfo {
-                    indirect_parameters_index: phase_indirect_parameters_buffers
-                        .buffers
-                        .allocate(mesh.indexed(), 1),
-                    is_indexed: mesh.indexed(),
-                });
-            }
-            let mesh_info = mesh_batch_set_info.unwrap();
-
-            // Allocate some input and output indices. We'll need these to
-            // create the *work item* below.
-            let Some(input_index) =
-                MeshPipeline::get_binned_index(&system_param_item, visible_entity)
-            else {
-                continue;
-            };
-            let output_index = data_buffer.add() as u32;
-
             // Finally, we can specialize the pipeline based on the key
             let pipeline_id = specialized_mesh_pipelines
                 .specialize(
@@ -399,8 +330,8 @@ fn queue_custom_mesh_pipeline(
                     mesh_key,
                     &mesh.layout,
                 )
-                // This should never with this example, but if your pipeline specialization
-                // can fail you need to handle the error here
+                // This should never happen with this example, but if your pipeline
+                // specialization can fail you need to handle the error here
                 .expect("Failed to specialize mesh pipeline");
 
             // Bump the change tick so that Bevy is forced to rebuild the bin.
@@ -410,59 +341,29 @@ fn queue_custom_mesh_pipeline(
             // Add the mesh with our specialized pipeline
             opaque_phase.add(
                 Opaque3dBatchSetKey {
-                    draw_function: draw_function_id,
+                    draw_function,
                     pipeline: pipeline_id,
                     material_bind_group_index: None,
-                    vertex_slab: default(),
-                    index_slab: None,
+                    vertex_slab: vertex_slab.unwrap_or_default(),
+                    index_slab,
                     lightmap_slab: None,
                 },
-                // The asset ID is arbitrary; we simply use [`AssetId::invalid`],
-                // but you can use anything you like. Note that the asset ID need
-                // not be the ID of a [`Mesh`].
+                // For this example we can use the mesh asset id as the bin key,
+                // but you can use any asset_id as a key
                 Opaque3dBinKey {
-                    asset_id: AssetId::<Mesh>::invalid().untyped(),
+                    asset_id: mesh_instance.mesh_asset_id.into(),
                 },
                 (render_entity, visible_entity),
                 mesh_instance.current_uniform_index,
-                // This example supports batching, but if your pipeline doesn't
-                // support it you can use `BinnedRenderPhaseType::UnbatchableMesh`
-                BinnedRenderPhaseType::BatchableMesh,
+                // This example supports batching and multi draw indirect,
+                // but if your pipeline doesn't support it you can use
+                // `BinnedRenderPhaseType::UnbatchableMesh`
+                BinnedRenderPhaseType::mesh(
+                    mesh_instance.should_batch(),
+                    &gpu_preprocessing_support,
+                ),
                 *change_tick,
             );
-
-            // Create a *work item*. A work item tells the Bevy renderer to
-            // transform the mesh on GPU.
-            work_item_buffer.push(
-                mesh.indexed(),
-                PreprocessWorkItem {
-                    input_index: input_index.into(),
-                    output_or_indirect_parameters_index: if no_indirect_drawing {
-                        output_index
-                    } else {
-                        mesh_info.indirect_parameters_index
-                    },
-                },
-            );
-        }
-
-        // Now if there were any meshes, we need to add a command to the
-        // indirect parameters buffer, so that the renderer will end up
-        // enqueuing a command to draw the mesh.
-        if let Some(mesh_info) = mesh_batch_set_info {
-            phase_indirect_parameters_buffers
-                .buffers
-                .add_batch_set(mesh_info.is_indexed, mesh_info.indirect_parameters_index);
         }
     }
 }
-
-// If we end up having any custom meshes to draw, this contains information
-// needed to create the batch set.
-#[derive(Clone, Copy)]
-struct MeshBatchSetInfo {
-    /// The first index of the mesh batch in the indirect parameters buffer.
-    indirect_parameters_index: u32,
-    /// Whether the mesh is indexed (has an index buffer).
-    is_indexed: bool,
-}