bg_color_lerp.rs

use std::time::Duration;
use bevy::color::palettes::css::RED;
use bevy::math::VectorSpace;
use bevy::prelude::*;
use bevy::time::common_conditions::once_after_delay;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, |mut commands: Commands| {
            commands.spawn(Camera2dBundle::default());
        })
        .add_systems(Update, start_interpolation.run_if(once_after_delay(Duration::from_secs(2))))
        .add_systems(Update, interpolate.run_if(resource_exists::<TargetClearColor>))
        .run();
}

#[derive(Resource)]
pub struct TargetClearColor {
    start: Duration,
    duration: Duration,
    // LinearRgba is generally good, but you can achieve different effects with others like HSL
    start_color: LinearRgba,
    end_color: LinearRgba,
}

fn start_interpolation(mut commands: Commands, time: Res<Time>, clear: Res<ClearColor>) {
    commands.insert_resource(TargetClearColor {
        start: time.elapsed(),
        duration: Duration::from_secs(2),
        start_color: LinearRgba::from(*clear.clone()),
        end_color: RED.into(),
    });
}

fn interpolate(mut commands: Commands, time: Res<Time>, target: ResMut<TargetClearColor>, mut clear: ResMut<ClearColor>) {
    // lerp factor starts at 0 & ends at 1
    let elapsed: Duration = time.elapsed() - target.start;
    let mut ratio = elapsed.div_duration_f32(target.duration);
    if ratio >= 1. {
        ratio = 1.;
        commands.remove_resource::<TargetClearColor>();
    }
    let color_lerp = target.start_color.lerp(target.end_color, ratio);
    clear.0 = Color::from(color_lerp);
}