Headless renderer example has been added (#13006)

# Objective

Fixes #11457.
Fixes #22.

## Solution

Based on [another headless
application](https://github.com/richardanaya/headless/)

---

## Changelog

- Adopted to bevy 0.14

---------

Co-authored-by: BD103 <59022059+BD103@users.noreply.github.com>
Co-authored-by: François Mockers <francois.mockers@vleue.com>

Cargo.toml

@@ -1274,6 +1274,17 @@ description = "An application that runs with default plugins and displays an emp
 category = "Application"
 wasm = false
 
+[[example]]
+name = "headless_renderer"
+path = "examples/app/headless_renderer.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.headless_renderer]
+name = "Headless Renderer"
+description = "An application that runs with no window, but renders into image file"
+category = "Application"
+wasm = false
+
 [[example]]
 name = "without_winit"
 path = "examples/app/without_winit.rs"

examples/README.md

@@ -188,6 +188,7 @@ Example | Description
 [Empty](../examples/app/empty.rs) | An empty application (does nothing)
 [Empty with Defaults](../examples/app/empty_defaults.rs) | An empty application with default plugins
 [Headless](../examples/app/headless.rs) | An application that runs without default plugins
+[Headless Renderer](../examples/app/headless_renderer.rs) | An application that runs with no window, but renders into image file
 [Log layers](../examples/app/log_layers.rs) | Illustrate how to add custom log layers
 [Logs](../examples/app/logs.rs) | Illustrate how to use generate log output
 [No Renderer](../examples/app/no_renderer.rs) | An application that runs with default plugins and displays an empty window, but without an actual renderer

examples/app/headless_renderer.rs

@@ -0,0 +1,529 @@
+//! This example illustrates how to make headless renderer
+//! derived from: <https://sotrh.github.io/learn-wgpu/showcase/windowless/#a-triangle-without-a-window>
+//! It follows this steps:
+//! 1. Render from camera to gpu-image render target
+//! 2. Copy form gpu image to buffer using `ImageCopyDriver` node in `RenderGraph`
+//! 3. Copy from buffer to channel using `receive_image_from_buffer` after `RenderSet::Render`
+//! 4. Save from channel to random named file using `scene::update` at `PostUpdate` in `MainWorld`
+//! 5. Exit if `single_image` setting is set
+
+use bevy::{
+    app::{AppExit, ScheduleRunnerPlugin},
+    core_pipeline::tonemapping::Tonemapping,
+    prelude::*,
+    render::{
+        camera::RenderTarget,
+        render_asset::RenderAssetUsages,
+        render_asset::RenderAssets,
+        render_graph::{self, NodeRunError, RenderGraph, RenderGraphContext, RenderLabel},
+        render_resource::{
+            Buffer, BufferDescriptor, BufferUsages, CommandEncoderDescriptor, Extent3d,
+            ImageCopyBuffer, ImageDataLayout, Maintain, MapMode, TextureDimension, TextureFormat,
+            TextureUsages,
+        },
+        renderer::{RenderContext, RenderDevice, RenderQueue},
+        texture::BevyDefault,
+        Extract, Render, RenderApp, RenderSet,
+    },
+};
+use crossbeam_channel::{Receiver, Sender};
+use std::{
+    ops::{Deref, DerefMut},
+    path::PathBuf,
+    sync::{
+        atomic::{AtomicBool, Ordering},
+        Arc,
+    },
+    time::Duration,
+};
+
+// To communicate between the main world and the render world we need a channel.
+// Since the main world and render world run in parallel, there will always be a frame of latency
+// between the data sent from the render world and the data received in the main world
+//
+// frame n => render world sends data through the channel at the end of the frame
+// frame n + 1 => main world receives the data
+//
+// Receiver and Sender are kept in resources because there is single camera and single target
+// That's why there is single images role, if you want to differentiate images
+// from different cameras, you should keep Receiver in ImageCopier and Sender in ImageToSave
+// or send some id with data
+
+/// This will receive asynchronously any data sent from the render world
+#[derive(Resource, Deref)]
+struct MainWorldReceiver(Receiver<Vec<u8>>);
+
+/// This will send asynchronously any data to the main world
+#[derive(Resource, Deref)]
+struct RenderWorldSender(Sender<Vec<u8>>);
+
+// Parameters of resulting image
+struct AppConfig {
+    width: u32,
+    height: u32,
+    single_image: bool,
+}
+
+fn main() {
+    let config = AppConfig {
+        width: 1920,
+        height: 1080,
+        single_image: true,
+    };
+
+    // setup frame capture
+    App::new()
+        .insert_resource(SceneController::new(
+            config.width,
+            config.height,
+            config.single_image,
+        ))
+        .insert_resource(ClearColor(Color::srgb_u8(0, 0, 0)))
+        .add_plugins(
+            DefaultPlugins
+                .set(ImagePlugin::default_nearest())
+                // Do not create a window on startup.
+                .set(WindowPlugin {
+                    primary_window: None,
+                    exit_condition: bevy::window::ExitCondition::DontExit,
+                    close_when_requested: false,
+                }),
+        )
+        .add_plugins(ImageCopyPlugin)
+        // headless frame capture
+        .add_plugins(CaptureFramePlugin)
+        .add_plugins(ScheduleRunnerPlugin::run_loop(
+            // Run 60 times per second.
+            Duration::from_secs_f64(1.0 / 60.0),
+        ))
+        .init_resource::<SceneController>()
+        .add_systems(Startup, setup)
+        .run();
+}
+
+/// Capture image settings and state
+#[derive(Debug, Default, Resource)]
+struct SceneController {
+    state: SceneState,
+    name: String,
+    width: u32,
+    height: u32,
+    single_image: bool,
+}
+
+impl SceneController {
+    pub fn new(width: u32, height: u32, single_image: bool) -> SceneController {
+        SceneController {
+            state: SceneState::BuildScene,
+            name: String::from(""),
+            width,
+            height,
+            single_image,
+        }
+    }
+}
+
+/// Capture image state
+#[derive(Debug, Default)]
+enum SceneState {
+    #[default]
+    // State before any rendering
+    BuildScene,
+    // Rendering state, stores the number of frames remaining before saving the image
+    Render(u32),
+}
+
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+    mut images: ResMut<Assets<Image>>,
+    mut scene_controller: ResMut<SceneController>,
+    render_device: Res<RenderDevice>,
+) {
+    let render_target = setup_render_target(
+        &mut commands,
+        &mut images,
+        &render_device,
+        &mut scene_controller,
+        // pre_roll_frames should be big enough for full scene render,
+        // but the bigger it is, the longer example will run.
+        // To visualize stages of scene rendering change this param to 0
+        // and change AppConfig::single_image to false in main
+        // Stages are:
+        // 1. Transparent image
+        // 2. Few black box images
+        // 3. Fully rendered scene images
+        // Exact number depends on device speed, device load and scene size
+        40,
+        "main_scene".into(),
+    );
+
+    // Scene example for non black box picture
+    // circular base
+    commands.spawn(PbrBundle {
+        mesh: meshes.add(Circle::new(4.0)),
+        material: materials.add(Color::WHITE),
+        transform: Transform::from_rotation(Quat::from_rotation_x(-std::f32::consts::FRAC_PI_2)),
+        ..default()
+    });
+    // cube
+    commands.spawn(PbrBundle {
+        mesh: meshes.add(Cuboid::new(1.0, 1.0, 1.0)),
+        material: materials.add(Color::srgb_u8(124, 144, 255)),
+        transform: Transform::from_xyz(0.0, 0.5, 0.0),
+        ..default()
+    });
+    // light
+    commands.spawn(PointLightBundle {
+        point_light: PointLight {
+            shadows_enabled: true,
+            ..default()
+        },
+        transform: Transform::from_xyz(4.0, 8.0, 4.0),
+        ..default()
+    });
+
+    commands.spawn(Camera3dBundle {
+        transform: Transform::from_xyz(-2.5, 4.5, 9.0).looking_at(Vec3::ZERO, Vec3::Y),
+        tonemapping: Tonemapping::None,
+        camera: Camera {
+            // render to image
+            target: render_target,
+            ..default()
+        },
+        ..default()
+    });
+}
+
+/// Plugin for Render world part of work
+pub struct ImageCopyPlugin;
+impl Plugin for ImageCopyPlugin {
+    fn build(&self, app: &mut App) {
+        let (s, r) = crossbeam_channel::unbounded();
+
+        let render_app = app
+            .insert_resource(MainWorldReceiver(r))
+            .sub_app_mut(RenderApp);
+
+        let mut graph = render_app.world_mut().resource_mut::<RenderGraph>();
+        graph.add_node(ImageCopy, ImageCopyDriver);
+        graph.add_node_edge(bevy::render::graph::CameraDriverLabel, ImageCopy);
+
+        render_app
+            .insert_resource(RenderWorldSender(s))
+            // Make ImageCopiers accessible in RenderWorld system and plugin
+            .add_systems(ExtractSchedule, image_copy_extract)
+            // Receives image data from buffer to channel
+            // so we need to run it after the render graph is done
+            .add_systems(Render, receive_image_from_buffer.after(RenderSet::Render));
+    }
+}
+
+/// Setups render target and cpu image for saving, changes scene state into render mode
+fn setup_render_target(
+    commands: &mut Commands,
+    images: &mut ResMut<Assets<Image>>,
+    render_device: &Res<RenderDevice>,
+    scene_controller: &mut ResMut<SceneController>,
+    pre_roll_frames: u32,
+    scene_name: String,
+) -> RenderTarget {
+    let size = Extent3d {
+        width: scene_controller.width,
+        height: scene_controller.height,
+        ..Default::default()
+    };
+
+    // This is the texture that will be rendered to.
+    let mut render_target_image = Image::new_fill(
+        size,
+        TextureDimension::D2,
+        &[0; 4],
+        TextureFormat::bevy_default(),
+        RenderAssetUsages::default(),
+    );
+    render_target_image.texture_descriptor.usage |=
+        TextureUsages::COPY_SRC | TextureUsages::RENDER_ATTACHMENT | TextureUsages::TEXTURE_BINDING;
+    let render_target_image_handle = images.add(render_target_image);
+
+    // This is the texture that will be copied to.
+    let cpu_image = Image::new_fill(
+        size,
+        TextureDimension::D2,
+        &[0; 4],
+        TextureFormat::bevy_default(),
+        RenderAssetUsages::default(),
+    );
+    let cpu_image_handle = images.add(cpu_image);
+
+    commands.spawn(ImageCopier::new(
+        render_target_image_handle.clone(),
+        size,
+        render_device,
+    ));
+
+    commands.spawn(ImageToSave(cpu_image_handle));
+
+    scene_controller.state = SceneState::Render(pre_roll_frames);
+    scene_controller.name = scene_name;
+    RenderTarget::Image(render_target_image_handle)
+}
+
+/// Setups image saver
+pub struct CaptureFramePlugin;
+impl Plugin for CaptureFramePlugin {
+    fn build(&self, app: &mut App) {
+        info!("Adding CaptureFramePlugin");
+        app.add_systems(PostUpdate, update);
+    }
+}
+
+/// `ImageCopier` aggregator in `RenderWorld`
+#[derive(Clone, Default, Resource, Deref, DerefMut)]
+struct ImageCopiers(pub Vec<ImageCopier>);
+
+/// Used by `ImageCopyDriver` for copying from render target to buffer
+#[derive(Clone, Component)]
+struct ImageCopier {
+    buffer: Buffer,
+    enabled: Arc<AtomicBool>,
+    src_image: Handle<Image>,
+}
+
+impl ImageCopier {
+    pub fn new(
+        src_image: Handle<Image>,
+        size: Extent3d,
+        render_device: &RenderDevice,
+    ) -> ImageCopier {
+        let padded_bytes_per_row =
+            RenderDevice::align_copy_bytes_per_row((size.width) as usize) * 4;
+
+        let cpu_buffer = render_device.create_buffer(&BufferDescriptor {
+            label: None,
+            size: padded_bytes_per_row as u64 * size.height as u64,
+            usage: BufferUsages::MAP_READ | BufferUsages::COPY_DST,
+            mapped_at_creation: false,
+        });
+
+        ImageCopier {
+            buffer: cpu_buffer,
+            src_image,
+            enabled: Arc::new(AtomicBool::new(true)),
+        }
+    }
+
+    pub fn enabled(&self) -> bool {
+        self.enabled.load(Ordering::Relaxed)
+    }
+}
+
+/// Extracting `ImageCopier`s into render world, because `ImageCopyDriver` accesses them
+fn image_copy_extract(mut commands: Commands, image_copiers: Extract<Query<&ImageCopier>>) {
+    commands.insert_resource(ImageCopiers(
+        image_copiers.iter().cloned().collect::<Vec<ImageCopier>>(),
+    ));
+}
+
+/// `RenderGraph` label for `ImageCopyDriver`
+#[derive(Debug, PartialEq, Eq, Clone, Hash, RenderLabel)]
+struct ImageCopy;
+
+/// `RenderGraph` node
+#[derive(Default)]
+struct ImageCopyDriver;
+
+// Copies image content from render target to buffer
+impl render_graph::Node for ImageCopyDriver {
+    fn run(
+        &self,
+        _graph: &mut RenderGraphContext,
+        render_context: &mut RenderContext,
+        world: &World,
+    ) -> Result<(), NodeRunError> {
+        let image_copiers = world.get_resource::<ImageCopiers>().unwrap();
+        let gpu_images = world
+            .get_resource::<RenderAssets<bevy::render::texture::GpuImage>>()
+            .unwrap();
+
+        for image_copier in image_copiers.iter() {
+            if !image_copier.enabled() {
+                continue;
+            }
+
+            let src_image = gpu_images.get(&image_copier.src_image).unwrap();
+
+            let mut encoder = render_context
+                .render_device()
+                .create_command_encoder(&CommandEncoderDescriptor::default());
+
+            let block_dimensions = src_image.texture_format.block_dimensions();
+            let block_size = src_image.texture_format.block_copy_size(None).unwrap();
+
+            let padded_bytes_per_row = RenderDevice::align_copy_bytes_per_row(
+                (src_image.size.x as usize / block_dimensions.0 as usize) * block_size as usize,
+            );
+
+            let texture_extent = Extent3d {
+                width: src_image.size.x,
+                height: src_image.size.y,
+                depth_or_array_layers: 1,
+            };
+
+            encoder.copy_texture_to_buffer(
+                src_image.texture.as_image_copy(),
+                ImageCopyBuffer {
+                    buffer: &image_copier.buffer,
+                    layout: ImageDataLayout {
+                        offset: 0,
+                        bytes_per_row: Some(
+                            std::num::NonZeroU32::new(padded_bytes_per_row as u32)
+                                .unwrap()
+                                .into(),
+                        ),
+                        rows_per_image: None,
+                    },
+                },
+                texture_extent,
+            );
+
+            let render_queue = world.get_resource::<RenderQueue>().unwrap();
+            render_queue.submit(std::iter::once(encoder.finish()));
+        }
+
+        Ok(())
+    }
+}
+
+/// runs in render world after Render stage to send image from buffer via channel (receiver is in main world)
+fn receive_image_from_buffer(
+    image_copiers: Res<ImageCopiers>,
+    render_device: Res<RenderDevice>,
+    sender: Res<RenderWorldSender>,
+) {
+    for image_copier in image_copiers.0.iter() {
+        if !image_copier.enabled() {
+            continue;
+        }
+
+        // Finally time to get our data back from the gpu.
+        // First we get a buffer slice which represents a chunk of the buffer (which we
+        // can't access yet).
+        // We want the whole thing so use unbounded range.
+        let buffer_slice = image_copier.buffer.slice(..);
+
+        // Now things get complicated. WebGPU, for safety reasons, only allows either the GPU
+        // or CPU to access a buffer's contents at a time. We need to "map" the buffer which means
+        // flipping ownership of the buffer over to the CPU and making access legal. We do this
+        // with `BufferSlice::map_async`.
+        //
+        // The problem is that map_async is not an async function so we can't await it. What
+        // we need to do instead is pass in a closure that will be executed when the slice is
+        // either mapped or the mapping has failed.
+        //
+        // The problem with this is that we don't have a reliable way to wait in the main
+        // code for the buffer to be mapped and even worse, calling get_mapped_range or
+        // get_mapped_range_mut prematurely will cause a panic, not return an error.
+        //
+        // Using channels solves this as awaiting the receiving of a message from
+        // the passed closure will force the outside code to wait. It also doesn't hurt
+        // if the closure finishes before the outside code catches up as the message is
+        // buffered and receiving will just pick that up.
+        //
+        // It may also be worth noting that although on native, the usage of asynchronous
+        // channels is wholly unnecessary, for the sake of portability to WASM
+        // we'll use async channels that work on both native and WASM.
+
+        let (s, r) = crossbeam_channel::bounded(1);
+
+        // Maps the buffer so it can be read on the cpu
+        buffer_slice.map_async(MapMode::Read, move |r| match r {
+            // This will execute once the gpu is ready, so after the call to poll()
+            Ok(r) => s.send(r).expect("Failed to send map update"),
+            Err(err) => panic!("Failed to map buffer {err}"),
+        });
+
+        // In order for the mapping to be completed, one of three things must happen.
+        // One of those can be calling `Device::poll`. This isn't necessary on the web as devices
+        // are polled automatically but natively, we need to make sure this happens manually.
+        // `Maintain::Wait` will cause the thread to wait on native but not on WebGpu.
+
+        // This blocks until the gpu is done executing everything
+        render_device.poll(Maintain::wait()).panic_on_timeout();
+
+        // This blocks until the buffer is mapped
+        r.recv().expect("Failed to receive the map_async message");
+
+        // This could fail on app exit, if Main world clears resources (including receiver) while Render world still renders
+        let _ = sender.send(buffer_slice.get_mapped_range().to_vec());
+
+        // We need to make sure all `BufferView`'s are dropped before we do what we're about
+        // to do.
+        // Unmap so that we can copy to the staging buffer in the next iteration.
+        image_copier.buffer.unmap();
+    }
+}
+
+/// CPU-side image for saving
+#[derive(Component, Deref, DerefMut)]
+struct ImageToSave(Handle<Image>);
+
+// Takes from channel image content sent from render world and saves it to disk
+fn update(
+    images_to_save: Query<&ImageToSave>,
+    receiver: Res<MainWorldReceiver>,
+    mut images: ResMut<Assets<Image>>,
+    mut scene_controller: ResMut<SceneController>,
+    mut app_exit_writer: EventWriter<AppExit>,
+    mut file_number: Local<u32>,
+) {
+    if let SceneState::Render(n) = scene_controller.state {
+        if n < 1 {
+            // We don't want to block the main world on this,
+            // so we use try_recv which attempts to receive without blocking
+            let mut image_data = Vec::new();
+            while let Ok(data) = receiver.try_recv() {
+                // image generation could be faster than saving to fs,
+                // that's why use only last of them
+                image_data = data;
+            }
+            if !image_data.is_empty() {
+                for image in images_to_save.iter() {
+                    // Fill correct data from channel to image
+                    let img_bytes = images.get_mut(image.id()).unwrap();
+                    img_bytes.data.clone_from(&image_data);
+
+                    // Create RGBA Image Buffer
+                    let img = match img_bytes.clone().try_into_dynamic() {
+                        Ok(img) => img.to_rgba8(),
+                        Err(e) => panic!("Failed to create image buffer {e:?}"),
+                    };
+
+                    // Prepare directory for images, test_images in bevy folder is used here for example
+                    // You should choose the path depending on your needs
+                    let images_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("test_images");
+                    info!("Saving image to: {images_dir:?}");
+                    std::fs::create_dir_all(&images_dir).unwrap();
+
+                    // Choose filename starting from 000.png
+                    let image_path = images_dir.join(format!("{:03}.png", file_number.deref()));
+                    *file_number.deref_mut() += 1;
+
+                    // Finally saving image to file, this heavy blocking operation is kept here
+                    // for example simplicity, but in real app you should move it to a separate task
+                    if let Err(e) = img.save(image_path) {
+                        panic!("Failed to save image: {}", e);
+                    };
+                }
+                if scene_controller.single_image {
+                    app_exit_writer.send(AppExit::Success);
+                }
+            }
+        } else {
+            // clears channel for skipped frames
+            while receiver.try_recv().is_ok() {}
+            scene_controller.state = SceneState::Render(n - 1);
+        }
+    }
+}