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Use RenderStartup for occlusion_culling example. #20184

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Jul 21, 2025
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Use RenderStartup for occlusion_culling example. #20184

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113 changes: 62 additions & 51 deletions examples/3d/occlusion_culling.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,15 +26,14 @@ use bevy::{
prelude::*,
render::{
batching::gpu_preprocessing::{
GpuPreprocessingMode, GpuPreprocessingSupport, IndirectParametersBuffers,
IndirectParametersIndexed,
GpuPreprocessingSupport, IndirectParametersBuffers, IndirectParametersIndexed,
},
experimental::occlusion_culling::OcclusionCulling,
render_graph::{self, NodeRunError, RenderGraphContext, RenderGraphExt, RenderLabel},
render_resource::{Buffer, BufferDescriptor, BufferUsages, MapMode},
renderer::{RenderContext, RenderDevice},
settings::WgpuFeatures,
Render, RenderApp, RenderDebugFlags, RenderPlugin, RenderSystems,
Render, RenderApp, RenderDebugFlags, RenderPlugin, RenderStartup, RenderSystems,
},
};
use bytemuck::Pod;
Expand Down Expand Up @@ -111,7 +110,7 @@ struct IndirectParametersStagingBuffers {
/// really care how up-to-date the counter of culled meshes is. If it's off by a
/// few frames, that's no big deal.
#[derive(Clone, Resource, Deref, DerefMut)]
struct SavedIndirectParameters(Arc<Mutex<SavedIndirectParametersData>>);
struct SavedIndirectParameters(Arc<Mutex<Option<SavedIndirectParametersData>>>);

/// A CPU-side copy of the GPU buffer that stores the indirect draw parameters.
///
Expand All @@ -138,27 +137,31 @@ struct SavedIndirectParametersData {
occlusion_culling_introspection_supported: bool,
}

impl FromWorld for SavedIndirectParameters {
fn from_world(world: &mut World) -> SavedIndirectParameters {
let render_device = world.resource::<RenderDevice>();
SavedIndirectParameters(Arc::new(Mutex::new(SavedIndirectParametersData {
data: vec![],
count: 0,
// This gets set to false in `readback_indirect_buffers` if we don't
// support GPU preprocessing.
occlusion_culling_supported: true,
// In order to determine how many meshes were culled, we look at the
// indirect count buffer that Bevy only populates if the platform
// supports `multi_draw_indirect_count`. So, if we don't have that
// feature, then we don't bother to display how many meshes were
// culled.
occlusion_culling_introspection_supported: render_device
.features()
.contains(WgpuFeatures::MULTI_DRAW_INDIRECT_COUNT),
})))
impl SavedIndirectParameters {
fn new() -> Self {
Self(Arc::new(Mutex::new(None)))
}
}

fn init_saved_indirect_parameters(
render_device: Res<RenderDevice>,
gpu_preprocessing_support: Res<GpuPreprocessingSupport>,
saved_indirect_parameters: Res<SavedIndirectParameters>,
) {
let mut saved_indirect_parameters = saved_indirect_parameters.0.lock().unwrap();
*saved_indirect_parameters = Some(SavedIndirectParametersData {
data: vec![],
count: 0,
occlusion_culling_supported: gpu_preprocessing_support.is_culling_supported(),
// In order to determine how many meshes were culled, we look at the indirect count buffer
// that Bevy only populates if the platform supports `multi_draw_indirect_count`. So, if we
// don't have that feature, then we don't bother to display how many meshes were culled.
occlusion_culling_introspection_supported: render_device
.features()
.contains(WgpuFeatures::MULTI_DRAW_INDIRECT_COUNT),
});
}

/// The demo's current settings.
#[derive(Resource)]
struct AppStatus {
Expand Down Expand Up @@ -210,12 +213,25 @@ fn main() {

impl Plugin for ReadbackIndirectParametersPlugin {
fn build(&self, app: &mut App) {
// Create the `SavedIndirectParameters` resource that we're going to use
// to communicate between the thread that the GPU-to-CPU readback
// callback runs on and the main application threads. This resource is
// atomically reference counted. We store one reference to the
// `SavedIndirectParameters` in the main app and another reference in
// the render app.
let saved_indirect_parameters = SavedIndirectParameters::new();
app.insert_resource(saved_indirect_parameters.clone());

// Fetch the render app.
let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
return;
};

render_app
// Insert another reference to the `SavedIndirectParameters`.
.insert_resource(saved_indirect_parameters)
// Setup the parameters in RenderStartup.
.add_systems(RenderStartup, init_saved_indirect_parameters)
.init_resource::<IndirectParametersStagingBuffers>()
.add_systems(ExtractSchedule, readback_indirect_parameters)
.add_systems(
Expand Down Expand Up @@ -245,26 +261,6 @@ impl Plugin for ReadbackIndirectParametersPlugin {
),
);
}

fn finish(&self, app: &mut App) {
// Create the `SavedIndirectParameters` resource that we're going to use
// to communicate between the thread that the GPU-to-CPU readback
// callback runs on and the main application threads. This resource is
// atomically reference counted. We store one reference to the
// `SavedIndirectParameters` in the main app and another reference in
// the render app.
let saved_indirect_parameters = SavedIndirectParameters::from_world(app.world_mut());
app.insert_resource(saved_indirect_parameters.clone());

// Fetch the render app.
let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
return;
};

render_app
// Insert another reference to the `SavedIndirectParameters`.
.insert_resource(saved_indirect_parameters);
}
}

/// Spawns all the objects in the scene.
Expand Down Expand Up @@ -550,6 +546,10 @@ fn update_status_text(
occlusion_culling_introspection_supported,
): (u32, bool, bool) = {
let saved_indirect_parameters = saved_indirect_parameters.lock().unwrap();
let Some(saved_indirect_parameters) = saved_indirect_parameters.as_ref() else {
// Bail out early if the resource isn't initialized yet.
return;
};
(
saved_indirect_parameters
.data
Expand Down Expand Up @@ -597,14 +597,15 @@ fn update_status_text(
fn readback_indirect_parameters(
mut indirect_parameters_staging_buffers: ResMut<IndirectParametersStagingBuffers>,
saved_indirect_parameters: Res<SavedIndirectParameters>,
gpu_preprocessing_support: Res<GpuPreprocessingSupport>,
) {
// If culling isn't supported on this platform, note that, and bail.
if gpu_preprocessing_support.max_supported_mode != GpuPreprocessingMode::Culling {
saved_indirect_parameters
.lock()
.unwrap()
.occlusion_culling_supported = false;
// If culling isn't supported on this platform, bail.
if !saved_indirect_parameters
.lock()
.unwrap()
.as_ref()
.unwrap()
.occlusion_culling_supported
{
return;
}

Expand All @@ -620,10 +621,20 @@ fn readback_indirect_parameters(
let saved_indirect_parameters_0 = (**saved_indirect_parameters).clone();
let saved_indirect_parameters_1 = (**saved_indirect_parameters).clone();
readback_buffer::<IndirectParametersIndexed>(data_buffer, move |indirect_parameters| {
saved_indirect_parameters_0.lock().unwrap().data = indirect_parameters.to_vec();
saved_indirect_parameters_0
.lock()
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Not a fan of those massive chains, but not sure how to solve it either 🙃

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One "solution" is to do what I did initially: rather than making the struct hold on Option, just default the inner struct (with false for the supported features), and then just reassign it later. I didn't want to do that though since it isn't very rusty and we now have a resource that has "wrong" values.

Another alternative I considered was initializing the resource in RenderStartup and then clone it over to the main world in ExtractSchedule. That would mean extraction is "going the other way" which is weird, and the main world would have to live with the resource being missing for a frame.

This example feels a little overcomplicated, but it's also hard to show users the effect of occlusion culling, so alas, we live with what we have.

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@nicopap nicopap Jul 18, 2025 •
edited
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On a purely esthetic level, it's possible to get rid of them with an assignment, I gave a patch in the expandable in my review (just click on > Click here to reveal diff)

.unwrap()
.as_mut()
.unwrap()
.data = indirect_parameters.to_vec();
});
readback_buffer::<u32>(batch_sets_buffer, move |indirect_parameters_count| {
saved_indirect_parameters_1.lock().unwrap().count = indirect_parameters_count[0];
saved_indirect_parameters_1
.lock()
.unwrap()
.as_mut()
.unwrap()
.count = indirect_parameters_count[0];
});
}

Expand Down
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