-dx;
    }
    if (y < 0 || y > gl.canvas.height) {
      dy = -dy;
    }

    div.style.left = Math.floor(x) + "px";
    div.style.top  = Math.floor(y) + "px";
    textNode.nodeValue = now.toFixed(2);

    // Every frame increase the rotation a little.
    rotation[1] += rotationSpeed * deltaTime;

    // Tell WebGL how to convert from clip space to pixels
    gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

    // Clear the canvas
    gl.clearColor(0, 0, 0, 0);
    gl.clear(gl.COLOR_BUFFER_BIT | gl.