@matematrolii/sketchbook/src/ts/physics/spring_simulation/RelativeSpringSimulator.ts

3D matematrolii playground built on three.js and cannon.js

import * as THREE from 'three';
import { SimulatorBase } from './SimulatorBase';
import { SimulationFrame } from './SimulationFrame';
import { spring } from '../../core/FunctionLibrary';

export class RelativeSpringSimulator extends SimulatorBase
{
	public position: number;
	public velocity: number;
	public target: number;
	public lastLerp: number;
	public cache: SimulationFrame[];

	constructor(fps: number, mass: number, damping: number, startPosition: number = 0, startVelocity: number = 0)
	{
		// Construct base
		super(fps, mass, damping);

		// Simulated values
		this.position = startPosition;
		this.velocity = startVelocity;

		// Simulation parameters
		this.target = 0;

		// Last lerped position for relative output
		this.lastLerp = 0;

		// Initialize cache by pushing two frames
		this.cache = []; // At least two frames
		for (let i = 0; i < 2; i++)
		{
			this.cache.push({
				position: startPosition,
				velocity: startVelocity,
			});
		}
	}

	/**
	 * Advances the simulation by given time step
	 * @param {number} timeStep 
	 */
	public simulate(timeStep: number): void
	{
		this.generateFrames(timeStep);

		// SpringR lerping
		// Lerp from 0 to next frame
		let lerp = THREE.MathUtils.lerp(0, this.cache[1].position, this.offset / this.frameTime);

		// Substract last lerp from current to make output relative
		this.position = (lerp - this.lastLerp);
		this.lastLerp = lerp;

		this.velocity = THREE.MathUtils.lerp(this.cache[0].velocity, this.cache[1].velocity, this.offset / this.frameTime);
	}

	/**
	 * Gets another simulation frame
	 */
	public getFrame(isLastFrame: boolean): SimulationFrame
	{
		let newFrame = Object.assign({}, this.lastFrame());

		if (isLastFrame)
		{
			// Reset position
			newFrame.position = 0;
			// Transition to next frame
			this.lastLerp = this.lastLerp - this.lastFrame().position;
		}

		return spring(newFrame.position, this.target, newFrame.velocity, this.mass, this.damping);
	}
}