Add example to test the smoothness/accuracy of the timestep

nfagerlund · nfagerlund · commit 80580ebd6429 · 2022-06-26T17:30:20.000-07:00
Bevy's timestep is sometimes inaccurate, and sometimes it misses its frame timings. Both
of these can manifest as hiccups and stutters in what should be smooth motion. This is
most clearly visible when using pixel-perfect movement with large visible pixels.

This new example (in the stress tests directory) scrolls a background behind a character at 120
pixels per second; by default, it makes sure to always move in whole-pixel increments. You can modify
some behavior while it's running by pressing keys:

- P: cycle between different `window::PresentMode`s.
- W: cycle between different `window::WindowMode`s.
- T: cycle between updating scroll position based on `Time.delta_seconds()`, and updating it by
  just assuming a 16.667 millisecond timestep (only useful in `PresentMode::Fifo` on a 60fps display).
- M: cycle between whole-pixel movement and simple sub-pixel movement.
diff --git a/Cargo.toml b/Cargo.toml
@@ -1246,6 +1246,10 @@ description = "Displays many sprites in a grid arragement! Used for performance
 category = "Stress Tests"
 wasm = true
 
+[[example]]
+name = "time_smoothness"
+path = "examples/stress_tests/time_smoothness.rs"
+
 [[example]]
 name = "transform_hierarchy"
 path = "examples/stress_tests/transform_hierarchy.rs"
diff --git a/examples/README.md b/examples/README.md
@@ -279,6 +279,7 @@ Example | Description
 [Many Foxes](../examples/stress_tests/many_foxes.rs) | Loads an animated fox model and spawns lots of them. Good for testing skinned mesh performance. Takes an unsigned integer argument for the number of foxes to spawn. Defaults to 1000
 [Many Lights](../examples/stress_tests/many_lights.rs) | Simple benchmark to test rendering many point lights. Run with `WGPU_SETTINGS_PRIO=webgl2` to restrict to uniform buffers and max 256 lights
 [Many Sprites](../examples/stress_tests/many_sprites.rs) | Displays many sprites in a grid arragement! Used for performance testing
+[Time Smoothness](./stress_tests/time_smoothness.rs) | Scrolls a simple background behind an animated sprite. Used for performance consistency testing.
 [Transform Hierarchy](../examples/stress_tests/transform_hierarchy.rs) | Various test cases for hierarchy and transform propagation performance
 
 ## Tools
diff --git a/examples/stress_tests/time_smoothness.rs b/examples/stress_tests/time_smoothness.rs
@@ -0,0 +1,356 @@
+//! Renders a scrolling background behind an animated sprite at a high zoom
+//! level, to test consistency and smoothness of performance.
+//!
+//! To measure performance realistically, be sure to run this in release mode.
+//! `cargo run --example time_smoothness --release`
+//!
+//! By default, this example scrolls the background at 120 pixels per second,
+//! and always moves in whole-pixel increments (since limiting movement to whole
+//! pixels at high zoom seems to make it easier to perceive any problems with
+//! frame consistency). There are several keyboard controls for changing the
+//! example's behavior at runtime:
+//!
+//! - P: Cycle the `PresentMode` between `Fifo`, `Mailbox`, and `Immediate`.
+//! - W: Cycle the `WindowMode` between `Windowed`, `BorderlessFullscreen`, and
+//!   `Fullscreen`.
+//! - T: Cycle the delta time between normal (measured from the `Time` resource)
+//!   and fixed-interval. This can be useful when trying to distinguish slow
+//!   frametimes from inaccurate time measurement. Fixed-interval delta time is
+//!   hardcoded to 1/60th of a second per frame, which will look highly wacky
+//!   unless you're using Fifo mode on a 60hz display.
+//! - M: Cycle the scrolling motion style between whole-pixel and sub-pixel
+//!   Transform increments.
+//!
+//! A number of factors contribute to scrolling without perceptible
+//! hiccups/stutters/jank, including the accuracy of the delta time measurement,
+//! the ability to consistently present frames to the GPU at the expected pace,
+//! etc. This example doesn't isolate all of those factors, but it can help
+//! identify when a problem exists and provide a starting point for further
+//! investigation.
+use bevy::{
+    diagnostic::{Diagnostic, DiagnosticId, Diagnostics},
+    prelude::*,
+    render::texture::ImageSettings,
+    window::{PresentMode, WindowMode},
+};
+
+// BG_WIDTH is smaller than the image's actual pixel width, because we want the
+// empty space on the background tiles to overlap a bit. That way there's always
+// a "landmark" on screen at the default window size.
+const BG_WIDTH: f32 = 755.0;
+const BG_HEIGHT: f32 = 363.0;
+const BG_TILES: usize = 3;
+const BG_SPEED: f32 = 120.0;
+
+const CUSTOM_FRAME_TIME: DiagnosticId =
+    DiagnosticId::from_u128(76860576947891895965111337840552081898);
+const MAX_FRAME_HISTORY: usize = 800;
+const FRAME_ANALYSIS_INTERVAL_SECONDS: f32 = 10.0;
+
+fn main() {
+    App::new()
+        .insert_resource(WindowDescriptor {
+            present_mode: PresentMode::Fifo,
+            mode: WindowMode::Windowed,
+            ..Default::default()
+        })
+        // Prevents blurry sprites
+        .insert_resource(ImageSettings::default_nearest())
+        .add_plugins(DefaultPlugins)
+        // Adds frame time diagnostics
+        .add_startup_system(setup_diagnostics)
+        .add_system(update_diagnostics)
+        .add_system(log_diagnostics)
+        .insert_resource(FrameAnalysisTimer(Timer::from_seconds(
+            FRAME_ANALYSIS_INTERVAL_SECONDS,
+            true,
+        )))
+        // Main app setup
+        .add_startup_system(setup)
+        .insert_resource(MoveRemainder(Vec2::ZERO))
+        .insert_resource(TimeStyle::Normal)
+        .insert_resource(MoveStyle::WholePixel)
+        .add_system(change_settings)
+        .add_system(animate_runner)
+        .add_system(scroll_background)
+        .run();
+}
+
+// Create a custom frame time diagnostic. We need this because
+// FrameTimeDiagnosticsPlugin only keeps 20 frames, which is too narrow a view
+// to be useful when hunting irregular blips.
+fn setup_diagnostics(mut diagnostics: ResMut<Diagnostics>) {
+    diagnostics
+        .add(Diagnostic::new(CUSTOM_FRAME_TIME, "frame_time", MAX_FRAME_HISTORY).with_suffix("s"));
+}
+
+// Update our custom frame time diagnostic with the delta time in milliseconds.
+fn update_diagnostics(mut diagnostics: ResMut<Diagnostics>, time: Res<Time>) {
+    diagnostics.add_measurement(CUSTOM_FRAME_TIME, || time.delta_seconds_f64() * 1000.0);
+}
+
+// Periodically analyze recent frame times and print a summary.
+fn log_diagnostics(
+    mut timer: ResMut<FrameAnalysisTimer>,
+    diagnostics: Res<Diagnostics>,
+    time: Res<Time>,
+) {
+    timer.0.tick(time.delta());
+    if timer.0.finished() {
+        let frame_times = diagnostics.get(CUSTOM_FRAME_TIME).unwrap();
+        if let Some(average) = frame_times.average() {
+            if let Some(std_dev) = std_deviation(frame_times) {
+                let mut sorted_times: Vec<f64> = frame_times.values().copied().collect();
+                sorted_times.sort_unstable_by(|a, b| a.partial_cmp(b).unwrap());
+
+                let count = sorted_times.len();
+
+                // Indexes corresponding to percentile ranks:
+                let p95 = (0.95 * count as f32).round() as usize - 1;
+                let p99 = (0.99 * count as f32).round() as usize - 1;
+                let p99_5 = (0.995 * count as f32).round() as usize - 1;
+
+                let min = sorted_times.first().unwrap();
+                let max = sorted_times.last().unwrap();
+
+                info!("-------------------------");
+                info!("Average frame time: {:.6} ms", average);
+                info!("Standard deviation: {:.6} ms", std_dev);
+                info!("Shortest frame:     {:.6} ms", min);
+                info!("95th percentile:    {:.6} ms", sorted_times[p95]);
+                info!("99th percentile:    {:.6} ms", sorted_times[p99]);
+                info!("99.5th percentile:  {:.6} ms", sorted_times[p99_5]);
+                info!("Longest frame:      {:.6} ms", max);
+                info!("-------------------------");
+            }
+        }
+    }
+}
+
+fn std_deviation(diagnostic: &Diagnostic) -> Option<f64> {
+    if let Some(average) = diagnostic.average() {
+        let variance = diagnostic
+            .values()
+            .map(|val| {
+                let diff = average - *val;
+                diff * diff
+            })
+            .sum::<f64>()
+            / diagnostic.history_len() as f64;
+        Some(variance.sqrt())
+    } else {
+        None
+    }
+}
+
+// Set up entities and assets.
+fn setup(
+    mut commands: Commands,
+    asset_server: Res<AssetServer>,
+    mut texture_atlases: ResMut<Assets<TextureAtlas>>,
+) {
+    // Locate the center(ish) of the background conveyer belt, so we can
+    // position the player and camera there.
+    let bg_center = Vec2::new(BG_WIDTH * BG_TILES as f32 / 2.0, BG_HEIGHT / 2.0).round();
+
+    // Set up camera.
+    let mut camera_bundle = Camera2dBundle::default();
+    camera_bundle.projection.scale = 1.0 / 4.0;
+    camera_bundle.transform.translation += bg_center.extend(0.0);
+    commands.spawn_bundle(camera_bundle);
+
+    // Set up animated player sprite.
+    let runner_texture: Handle<Image> =
+        asset_server.load("textures/rpg/chars/mani/mani-idle-run.png");
+    let mut runner_atlas = TextureAtlas::from_grid(runner_texture, Vec2::new(24.0, 24.0), 7, 1);
+    // Drop the first (idle) frame so we just have the run frames.
+    runner_atlas.textures = runner_atlas.textures[1..].into();
+    let runner_handle = texture_atlases.add(runner_atlas);
+    // Offset by half our size to find where we should place our bottom left corner.
+    let runner_location = bg_center - Vec2::new(12.0, 12.0);
+    commands
+        .spawn_bundle(SpriteSheetBundle {
+            texture_atlas: runner_handle,
+            sprite: TextureAtlasSprite {
+                anchor: bevy::sprite::Anchor::BottomLeft,
+                ..Default::default()
+            },
+            transform: Transform::from_translation(runner_location.extend(3.0)),
+            ..Default::default()
+        })
+        .insert(Player)
+        .insert(AnimationTimer {
+            timer: Timer::from_seconds(0.1, true),
+        });
+
+    // Set up scrolling background, using a conveyor belt of three long sprites.
+    let background_texture: Handle<Image> = asset_server.load("branding/banner.png");
+    for i in 0..BG_TILES {
+        commands
+            .spawn_bundle(SpriteBundle {
+                sprite: Sprite {
+                    anchor: bevy::sprite::Anchor::BottomLeft,
+                    ..Default::default()
+                },
+                transform: Transform::from_translation(Vec3::new(
+                    i as f32 * BG_WIDTH + 1.0,
+                    0.0,
+                    0.0,
+                )),
+                texture: background_texture.clone(),
+                ..Default::default()
+            })
+            .insert(Background);
+    }
+}
+
+// Change settings on demand, to display different behaviors without recompiling.
+fn change_settings(
+    input: Res<Input<KeyCode>>,
+    mut windows: ResMut<Windows>,
+    mut time_style: ResMut<TimeStyle>,
+    mut move_style: ResMut<MoveStyle>,
+    mut background_query: Query<&mut Transform, With<Background>>,
+) {
+    let window = windows.primary_mut();
+    if input.just_pressed(KeyCode::P) {
+        // P: cycle PresentMode.
+        let next_present_mode = match window.present_mode() {
+            PresentMode::Fifo => PresentMode::Mailbox,
+            PresentMode::Mailbox => PresentMode::Immediate,
+            PresentMode::Immediate => PresentMode::Fifo,
+        };
+        info!("Switching present mode to {:?}", next_present_mode);
+        window.set_present_mode(next_present_mode);
+    } else if input.just_pressed(KeyCode::W) {
+        // W: cycle WindowMode.
+        let next_window_mode = match window.mode() {
+            WindowMode::Windowed => WindowMode::BorderlessFullscreen,
+            WindowMode::BorderlessFullscreen => WindowMode::Fullscreen,
+            WindowMode::Fullscreen => WindowMode::Windowed,
+            _ => WindowMode::Windowed,
+        };
+        info!("Switching window mode to {:?}", next_window_mode);
+        window.set_mode(next_window_mode);
+        if next_window_mode == WindowMode::Windowed {
+            window.set_resolution(1280.0, 720.0);
+        }
+    } else if input.just_pressed(KeyCode::T) {
+        // T: cycle delta time style
+        let next_time_style = match *time_style {
+            TimeStyle::Normal => TimeStyle::Fixed,
+            TimeStyle::Fixed => TimeStyle::Normal,
+        };
+        info!("Switching time style to {:?}", next_time_style);
+        *time_style = next_time_style;
+    } else if input.just_pressed(KeyCode::M) {
+        // M: cycle scroll motion style
+        let next_move_style = match *move_style {
+            MoveStyle::WholePixel => MoveStyle::SubPixel,
+            MoveStyle::SubPixel => {
+                // Re-lock the background positions to whole-pixel boundaries
+                for mut transform in background_query.iter_mut() {
+                    transform.translation = transform.translation.round();
+                }
+                MoveStyle::WholePixel
+            },
+        };
+        info!("Switching scroll style to {:?}", next_move_style);
+        *move_style = next_move_style;
+    }
+}
+
+// Increase+loop the player sprite's frame index, per its animation timer.
+fn animate_runner(
+    time: Res<Time>,
+    texture_atlases: Res<Assets<TextureAtlas>>,
+    mut query: Query<
+        (
+            &mut AnimationTimer,
+            &mut TextureAtlasSprite,
+            &Handle<TextureAtlas>,
+        ),
+        With<Player>,
+    >,
+) {
+    for (mut sprite_timer, mut sprite, texture_atlas_handle) in query.iter_mut() {
+        sprite_timer.timer.tick(time.delta());
+        if sprite_timer.timer.finished() {
+            let texture_atlas = texture_atlases.get(texture_atlas_handle).unwrap();
+            sprite.index = (sprite.index + 1) % texture_atlas.textures.len();
+        }
+    }
+}
+
+// Scroll the background in pixel-perfect increments, re-using the sprites as
+// they scroll off the left side.
+fn scroll_background(
+    time: Res<Time>,
+    time_style: Res<TimeStyle>,
+    move_style: Res<MoveStyle>,
+    mut move_remainder: ResMut<MoveRemainder>,
+    mut query: Query<&mut Transform, With<Background>>,
+) {
+    let delta = match *time_style {
+        TimeStyle::Normal => time.delta_seconds(),
+        TimeStyle::Fixed => 1.0 / 60.0,
+    };
+    let move_input = -Vec2::X * BG_SPEED * delta;
+    // Complain if the raw movement amount is unexpectedly big:
+    if move_input.x.abs() > 2.5 {
+        info!("Big jump: {} px", move_input.x.abs());
+    }
+    // Calculate how many pixels to scroll this frame, and save any
+    // leftover/leftunder for future frames:
+    move_remainder.0 += move_input;
+    let move_pixels = match *move_style {
+        MoveStyle::WholePixel => move_remainder.0.round(),
+        MoveStyle::SubPixel => move_remainder.0,
+    };
+    move_remainder.0 -= move_pixels;
+
+    // Move the background tiles.
+    for mut transform in query.iter_mut() {
+        // First, move this tile to the back of the line if it just scrolled past zero.
+        if transform.translation.x < 0.0 {
+            transform.translation.x =
+                BG_WIDTH * (BG_TILES - 1) as f32 + 1.0 - transform.translation.x;
+        }
+        // Next, move the amount we calculated:
+        transform.translation += move_pixels.extend(0.0);
+    }
+}
+
+// Marker struct for player sprite
+#[derive(Component)]
+struct Player;
+
+// Animation time for player sprite
+#[derive(Component)]
+struct AnimationTimer {
+    timer: Timer,
+}
+
+// Marker struct for background sprites
+#[derive(Component)]
+struct Background;
+
+// Sub-pixel movement accumulator for background sprites
+#[derive(Component)]
+struct MoveRemainder(Vec2);
+
+// Timer for printing frame time analysis
+struct FrameAnalysisTimer(Timer);
+
+// Enums for changing runtime settings
+#[derive(Debug)]
+enum TimeStyle {
+    Normal,
+    Fixed,
+}
+
+#[derive(Debug)]
+enum MoveStyle {
+    WholePixel,
+    SubPixel,
+}
