119 lines
4.0 KiB
TypeScript
119 lines
4.0 KiB
TypeScript
import Vec2d from "./vec.js";
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import { drawCircle, resizeCanvas } from "./common.js";
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const SEGMENT_SPACING = 100;
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function initTrail(pos: Vec2d, len: number) {
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return Array.from({ length: len }, (_, i) => pos.add(new Vec2d(-1, 0).scale(i * SEGMENT_SPACING)));
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}
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// state
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let target: Vec2d | undefined = undefined;
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let trail = initTrail(new Vec2d(500, 500), 5);
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let velocity = new Vec2d(500, 500);
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let pause = false;
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let mode: "follow" | "bounce" | "circle" = "follow";
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let lastFrameTime: number | undefined = undefined;
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function update(ctx: CanvasRenderingContext2D, timestamp: number) {
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if (!lastFrameTime) lastFrameTime = timestamp;
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const dt = (timestamp - lastFrameTime) / 1000; // seconds
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lastFrameTime = timestamp;
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if (!pause) {
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switch (mode) {
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case "bounce": updateBounce(ctx, dt); break;
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case "follow": updateFollow(ctx, dt); break;
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case "circle": updateCircle(ctx, dt); break;
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default: throw new Error(`Unknown mode: ${mode}`);
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}
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}
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ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
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for (const pos of trail) {
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drawCircle(ctx, pos, 20, 0xFF00FF);
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}
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window.requestAnimationFrame(t => update(ctx, t));
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}
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function updateFollow(ctx: CanvasRenderingContext2D, dt: number) {
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if (target) {
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// update head position towards the mouse cursor
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// using lerp with a factor of 0.1 moves 10% towards the target at each
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// frame, so this is framerate dependent:
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// trail[0] = trail[0].lerp(target, 0.1);
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// use exponential decay instead, this is framerate independent:
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// a = a + (b - a) * t lerp formula
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// a = a + (b - a) * e^(-k * dt) substitute t with e^(-k * dt)
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const alpha = Math.exp(-10 * dt)
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trail[0] = trail[0].lerp(target, alpha);
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updateTrail();
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}
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}
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function updateBounce(ctx: CanvasRenderingContext2D, dt: number) {
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// update head pos
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// P_t+1 = P_t + V * t
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const newPos = trail[0].add(velocity.scale(dt));
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if (newPos.x > ctx.canvas.width - 100) { velocity.x *= -1; newPos.x = ctx.canvas.width - 100; }
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if (newPos.y > ctx.canvas.height - 100) { velocity.y *= -1; newPos.y = ctx.canvas.height - 100; }
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if (newPos.x < 100) { velocity.x *= -1; newPos.x = 100; }
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if (newPos.y < 100) { velocity.y *= -1; newPos.y = 100; }
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trail[0] = newPos;
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updateTrail();
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}
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function updateCircle(ctx: CanvasRenderingContext2D, dt: number) {
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const center = new Vec2d(ctx.canvas.width / 2, ctx.canvas.height / 2);
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const rel = trail[0].sub(center); // distance from center
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const { r, theta } = rel.toPolar();
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trail[0] = center.add(Vec2d.fromPolar(r, theta + dt));
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updateTrail();
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}
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function updateTrail() {
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// "follow the leader" algorithm: keep segments at a fixed distance
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// from each other by moving each segment towards the segment in front of it.
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for (let i = 1; i < trail.length; i++) {
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const leader = trail[i - 1];
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const delta = leader.sub(trail[i]);
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const dist = delta.length();
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if (dist > SEGMENT_SPACING) {
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const remainder = dist - SEGMENT_SPACING; // excess distance to be covered
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const p = remainder / dist; // [0, 1]
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trail[i] = trail[i].lerp(leader, p);
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}
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}
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}
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function init() {
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const canvas = document.getElementById("canvas") as HTMLCanvasElement | null;
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if (!canvas) throw new Error("unable to get canvas HTML element");
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const ctx = canvas.getContext("2d") as CanvasRenderingContext2D | null;
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if (!ctx) throw new Error("unable to get canvas 2D context");
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ctx.canvas.width = ctx.canvas.clientWidth;
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ctx.canvas.height = ctx.canvas.clientHeight;
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canvas.onmousemove = (evt) => {
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const {clientX, clientY} = evt;
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target = new Vec2d(clientX, clientY);
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}
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canvas.onclick = () => {
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pause = !pause;
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}
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window.onkeydown = (evt) => {
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if (mode === "bounce") mode = "follow";
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else if (mode === "follow") mode = "circle";
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else if (mode === "circle") mode = "bounce";
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console.log(mode);
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}
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window.addEventListener('resize', () => resizeCanvas(ctx));
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window.requestAnimationFrame(t => update(ctx, t));
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}
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init();
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