mod.rs

//! Per-object, per-pixel motion blur.
//!
//! Add the [`MotionBlur`] component to a camera to enable motion blur.

use crate::{
    bloom::bloom,
    motion_blur::pipeline::{MotionBlurPipeline, MotionBlurPipelineId},
};
use bevy_app::{App, Plugin};
use bevy_asset::embedded_asset;
use bevy_camera::Camera;
use bevy_core_pipeline::{
    prepass::{DepthPrepass, MotionVectorPrepass, ViewPrepassTextures},
    schedule::{Core3d, Core3dSystems},
};
use bevy_ecs::{
    component::Component,
    query::{QueryItem, With},
    reflect::ReflectComponent,
    schedule::IntoScheduleConfigs,
    system::Res,
};
use bevy_reflect::{std_traits::ReflectDefault, Reflect};
use bevy_render::{
    diagnostic::RecordDiagnostics,
    extract_component::{
        ComponentUniforms, ExtractComponent, ExtractComponentPlugin, UniformComponentPlugin,
    },
    globals::GlobalsBuffer,
    render_resource::{
        BindGroupEntries, Operations, PipelineCache, RenderPassColorAttachment,
        RenderPassDescriptor, ShaderType, SpecializedRenderPipelines,
    },
    renderer::{RenderContext, ViewQuery},
    sync_component::SyncComponent,
    view::{Msaa, ViewTarget},
    Render, RenderApp, RenderStartup, RenderSystems,
};

pub mod pipeline;

/// A component that enables and configures motion blur when added to a camera.
///
/// Motion blur is an effect that simulates how moving objects blur as they change position during
/// the exposure of film, a sensor, or an eyeball.
///
/// Because rendering simulates discrete steps in time, we use per-pixel motion vectors to estimate
/// the path of objects between frames. This kind of implementation has some artifacts:
/// - Fast moving objects in front of a stationary object or when in front of empty space, will not
///   have their edges blurred.
/// - Transparent objects do not write to depth or motion vectors, so they cannot be blurred.
///
/// Other approaches, such as *A Reconstruction Filter for Plausible Motion Blur* produce more
/// correct results, but are more expensive and complex, and have other kinds of artifacts. This
/// implementation is relatively inexpensive and effective.
///
/// # Usage
///
/// Add the [`MotionBlur`] component to a camera to enable and configure motion blur for that
/// camera.
///
/// ```
/// # use bevy_post_process::motion_blur::MotionBlur;
/// # use bevy_camera::Camera3d;
/// # use bevy_ecs::prelude::*;
/// # fn test(mut commands: Commands) {
/// commands.spawn((
///     Camera3d::default(),
///     MotionBlur::default(),
/// ));
/// # }
/// ````
#[derive(Reflect, Component, Clone)]
#[reflect(Component, Default, Clone)]
#[require(DepthPrepass, MotionVectorPrepass)]
pub struct MotionBlur {
    /// The strength of motion blur from `0.0` to `1.0`.
    ///
    /// The shutter angle describes the fraction of a frame that a camera's shutter is open and
    /// exposing the film/sensor. For 24fps cinematic film, a shutter angle of 0.5 (180 degrees) is
    /// common. This means that the shutter was open for half of the frame, or 1/48th of a second.
    /// The lower the shutter angle, the less exposure time and thus less blur.
    ///
    /// A value greater than one is non-physical and results in an object's blur stretching further
    /// than it traveled in that frame. This might be a desirable effect for artistic reasons, but
    /// consider allowing users to opt out of this.
    ///
    /// This value is intentionally tied to framerate to avoid the aforementioned non-physical
    /// over-blurring. If you want to emulate a cinematic look, your options are:
    ///   - Framelimit your app to 24fps, and set the shutter angle to 0.5 (180 deg). Note that
    ///     depending on artistic intent or the action of a scene, it is common to set the shutter
    ///     angle between 0.125 (45 deg) and 0.5 (180 deg). This is the most faithful way to
    ///     reproduce the look of film.
    ///   - Set the shutter angle greater than one. For example, to emulate the blur strength of
    ///     film while rendering at 60fps, you would set the shutter angle to `60/24 * 0.5 = 1.25`.
    ///     Note that this will result in artifacts where the motion of objects will stretch further
    ///     than they moved between frames; users may find this distracting.
    pub shutter_angle: f32,
    /// The quality of motion blur, corresponding to the number of per-pixel samples taken in each
    /// direction during blur.
    ///
    /// Setting this to `1` results in each pixel being sampled once in the leading direction, once
    /// in the trailing direction, and once in the middle, for a total of 3 samples (`1 * 2 + 1`).
    /// Setting this to `3` will result in `3 * 2 + 1 = 7` samples. Setting this to `0` is
    /// equivalent to disabling motion blur.
    pub samples: u32,
}

impl Default for MotionBlur {
    fn default() -> Self {
        Self {
            shutter_angle: 0.5,
            samples: 1,
        }
    }
}

impl SyncComponent for MotionBlur {
    type Out = MotionBlurUniform;
}

impl ExtractComponent for MotionBlur {
    type QueryData = &'static Self;
    type QueryFilter = With<Camera>;

    fn extract_component(item: QueryItem<Self::QueryData>) -> Option<Self::Out> {
        Some(MotionBlurUniform {
            shutter_angle: item.shutter_angle,
            samples: item.samples,
            #[cfg(all(feature = "webgl", target_arch = "wasm32", not(feature = "webgpu")))]
            _webgl2_padding: Default::default(),
        })
    }
}

#[doc(hidden)]
#[derive(Component, ShaderType, Clone)]
pub struct MotionBlurUniform {
    shutter_angle: f32,
    samples: u32,
    #[cfg(all(feature = "webgl", target_arch = "wasm32", not(feature = "webgpu")))]
    // WebGL2 structs must be 16 byte aligned.
    _webgl2_padding: bevy_math::Vec2,
}

/// Adds support for per-object motion blur to the app. See [`MotionBlur`] for details.
#[derive(Default)]
pub struct MotionBlurPlugin;
impl Plugin for MotionBlurPlugin {
    fn build(&self, app: &mut App) {
        embedded_asset!(app, "motion_blur.wgsl");

        app.add_plugins((
            ExtractComponentPlugin::<MotionBlur>::default(),
            UniformComponentPlugin::<MotionBlurUniform>::default(),
        ));

        let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
            return;
        };

        render_app
            .init_resource::<SpecializedRenderPipelines<MotionBlurPipeline>>()
            .add_systems(RenderStartup, pipeline::init_motion_blur_pipeline)
            .add_systems(
                Render,
                pipeline::prepare_motion_blur_pipelines.in_set(RenderSystems::Prepare),
            );

        render_app.add_systems(
            Core3d,
            motion_blur.before(bloom).in_set(Core3dSystems::PostProcess),
        );
    }
}

pub fn motion_blur(
    view: ViewQuery<(
        &ViewTarget,
        &MotionBlurPipelineId,
        &ViewPrepassTextures,
        &MotionBlurUniform,
        &Msaa,
    )>,
    motion_blur_pipeline: Res<MotionBlurPipeline>,
    pipeline_cache: Res<PipelineCache>,
    settings_uniforms: Res<ComponentUniforms<MotionBlurUniform>>,
    globals_buffer: Res<GlobalsBuffer>,
    mut ctx: RenderContext,
) {
    let (view_target, pipeline_id, prepass_textures, motion_blur_uniform, msaa) = view.into_inner();

    if motion_blur_uniform.samples == 0 || motion_blur_uniform.shutter_angle <= 0.0 {
        return; // We can skip running motion blur in these cases.
    }

    let Some(pipeline) = pipeline_cache.get_render_pipeline(pipeline_id.0) else {
        return;
    };

    let Some(settings_binding) = settings_uniforms.uniforms().binding() else {
        return;
    };
    let (Some(prepass_motion_vectors_texture), Some(prepass_depth_texture)) =
        (&prepass_textures.motion_vectors, &prepass_textures.depth)
    else {
        return;
    };
    let Some(globals_uniforms) = globals_buffer.buffer.binding() else {
        return;
    };

    let post_process = view_target.post_process_write();

    let layout = if msaa.samples() == 1 {
        &motion_blur_pipeline.layout
    } else {
        &motion_blur_pipeline.layout_msaa
    };

    let bind_group = ctx.render_device().create_bind_group(
        Some("motion_blur_bind_group"),
        &pipeline_cache.get_bind_group_layout(layout),
        &BindGroupEntries::sequential((
            post_process.source,
            &prepass_motion_vectors_texture.texture.default_view,
            &prepass_depth_texture.texture.default_view,
            &motion_blur_pipeline.sampler,
            settings_binding.clone(),
            globals_uniforms.clone(),
        )),
    );

    let diagnostics = ctx.diagnostic_recorder();
    let diagnostics = diagnostics.as_deref();

    let mut render_pass = ctx.begin_tracked_render_pass(RenderPassDescriptor {
        label: Some("motion_blur"),
        color_attachments: &[Some(RenderPassColorAttachment {
            view: post_process.destination,
            depth_slice: None,
            resolve_target: None,
            ops: Operations::default(),
        })],
        depth_stencil_attachment: None,
        timestamp_writes: None,
        occlusion_query_set: None,
        multiview_mask: None,
    });
    let pass_span = diagnostics.pass_span(&mut render_pass, "motion_blur");

    render_pass.set_render_pipeline(pipeline);
    render_pass.set_bind_group(0, &bind_group, &[]);
    render_pass.draw(0..3, 0..1);

    pass_span.end(&mut render_pass);
}