createCanvas(400, 400);

}

​

function draw() {

  background(220);

  const T = millis() * 0.001;

  // https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/createRadialGradient

  const gradient = drawingContext

    .createRadialGradient(0, 0, width*0.2, 0, 0, width*1.3);

​

  gradient.addColorStop(0, `hsl(${(T*10)%360}deg 100% 50%)`);

  gradient.addColorStop(0.5, `hsl(${(T*10+100)%360}deg 100% 50%)`);

  gradient.addColorStop(1, `hsl(${(T*10+200)%360}deg 100% 50%)`);

​

  // Set the fill style and draw a rectangle

  drawingContext.fillStyle = gradient;

  rect(50, 50, width-100, height-100);

}