Rhodonea Curve

This is some exploration of Rhodonea curves (a.k.a rose curves). Also see what Jason Davies did here.

<!DOCTYPE html>
<title>Rhodonea Curve</title>
<style>
  body {
    font-family: Monospace;
    margin:0px;
  }
  canvas {
    position: absolute; 
    top: 0px; 
    left: 0px; 
    z-index: -2;
  }
  .controls {
    background-color: rgba(255, 255, 255, .75);
    box-shadow: 0px 0px 5px rgb(200, 200, 200);
    margin: 5px;
    padding: 10px;
    z-index: 9;
    display: inline-block;
  }
</style>

const canvasCtx = canvas.getContext("2d");
canvasCtx.fillStyle = 'rgb(255, 255, 255)';
canvasCtx.strokeStyle = 'rgb(255, 0, 0)';
canvasCtx.lineWidth = 1;

const length = 7200;
let t = 1;

const state = {
  a: Math.ceil(Math.random() * 20),
  b: Math.ceil(Math.random() * 20),
  oldA: 0,
  oldB: 0,
}

// make sure a and b are different
state.b = state.b === state.a ? Math.ceil(Math.random() * 20) : state.b;

document.querySelector("input[name=a]").value = state.a;
document.querySelector("input[name=b]").value = state.b;

const aText = document.querySelector(".aText");
const bText = document.querySelector(".bText");
aText.innerHTML = "a: " + state.a;
bText.innerHTML = "b: " + state.b;

update();

function updateRange(input) {
  state[input.name] = Number(input.value);

if (input.name === "a") aText.innerHTML = "a: " + state.a;
  else bText.innerHTML = "b: " + state.b;

if (t >= 1) {
    t = 0;
    update();
  } else {
    [state.oldA, state.oldB] = [state.a, state.b];
  }
}

function update() {
  canvasCtx.fillRect(0, 0, innerWidth, innerHeight);

t = t >= 1 ? 1 : t + .05;

if (t === 1) [state.oldA, state.oldB] = [state.a, state.b];
  else requestAnimationFrame(update);

canvasCtx.beginPath();

let i = length;
  while (i--) {
    const oldX = getVal(state.oldA, i, "sin") + getVal(state.oldB, i, "sin");
    const oldY = getVal(state.oldA, i, "cos") + getVal(state.oldB, i, "cos");
    const x = getVal(state.a, i, "sin") + getVal(state.b, i, "sin");
    const y = getVal(state.a, i, "cos") + getVal(state.b, i, "cos");
    canvasCtx.lineTo(tween(oldX, x) + innerWidth/2, tween(oldY, y) + innerHeight/2);
  }
  canvasCtx.stroke();  
}

function getVal(val, n, func) {
  return  Math[func](2 * Math.PI * val * n / length) * Math.min(innerWidth, innerHeight) / 5;
}

function tween(oldVal, newVal) {
  return oldVal * (1 - t) + newVal * t;
}
</script>