body {
  width: 100vw;
  height: 100vh;
  overflow: hidden;
  background: #00f;
}
canvas {
  cursor: pointer;
}
#canvas {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 100%;
  height: 100vh;
  z-index: 10;
}
#can {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 100%;
  height: 100vh;
  z-index: 10;
  mix-blend-mode: multiply;
}