Scherenschnitt-Animation

<p>
  WebGL animation by clearing the drawing buffer with solid color and applying
  scissor test.
</p>
<button id="animation-onoff">
  Press here to <strong>[verb goes here]</strong> the animation.
</button>

<canvas>Your browser does not seem to support canvases.</canvas>

body {
  text-align: center;
}
canvas {
  display: block;
  width: 280px;
  height: 210px;
  margin: auto;
  padding: 0;
  border: none;
  background-color: black;
}
button {
  display: block;
  font-size: inherit;
  margin: auto;
  padding: 0.6em;
}

const canvas = document.querySelector("canvas");

// Variables to hold the WebGL context, and the color and
// position of animated squares.
const gl = getRenderingContext();
let color = getRandomColor();
// Unlike the browser window, vertical position in WebGL is
// measured from bottom to top. In here we set the initial
// position of the square to be at the top left corner of the
// drawing buffer.
let position = [0, gl.drawingBufferHeight];

gl.enable(gl.SCISSOR_TEST);
gl.clearColor(color[0], color[1], color[2], 1.0);

const button = document.querySelector("button");
let timer;

function getRenderingContext() {
  canvas.width = canvas.clientWidth;
  canvas.height = canvas.clientHeight;
  const gl = canvas.getContext("webgl");
  gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);
  gl.clearColor(0.0, 0.0, 0.0, 1.0);
  gl.clear(gl.COLOR_BUFFER_BIT);
  return gl;
}

function startAnimation(evt) {
  button.removeEventListener(evt.type, startAnimation, false);
  button.addEventListener("click", stopAnimation, false);
  document.querySelector("strong").textContent = "stop";
  timer = setInterval(drawAnimation, 17);
  drawAnimation();
}

function stopAnimation(evt) {
  button.removeEventListener(evt.type, stopAnimation, false);
  button.addEventListener("click", startAnimation, false);
  document.querySelector("strong").textContent = "start";
  clearInterval(timer);
}

stopAnimation({ type: "click" });

// Variables to hold the size and velocity of the square.
const size = [60, 60];
let velocity = 3.0;
function drawAnimation() {
  gl.scissor(position[0], position[1], size[0], size[1]);
  gl.clear(gl.COLOR_BUFFER_BIT);
  // Every frame the vertical position of the square is
  // decreased, to create the illusion of movement.
  position[1] -= velocity;
  // When the square hits the bottom of the drawing buffer,
  // we override it with new square of different color and
  // velocity.
  if (position[1] < 0) {
    // Horizontal position chosen randomly, and vertical
    // position at the top of the drawing buffer.
    position = [
      Math.random() * (gl.drawingBufferWidth - size[0]),
      gl.drawingBufferHeight,
    ];
    // Random velocity between 1.0 and 7.0
    velocity = 1.0 + 6.0 * Math.random();
    color = getRandomColor();
    gl.clearColor(color[0], color[1], color[2], 1.0);
  }
}

function getRandomColor() {
  return [Math.random(), Math.random(), Math.random()];
}