Animating objects with WebGL

Making the square rotate

Let's start by making the square rotate. The first thing we'll need is a variable in which to track the current rotation of the square:

var squareRotation = 0.0;

Now we need to update the drawScene() function to apply the current rotation to the square when drawing it. After translating to the initial drawing position for the square, we apply the rotation like this:

  mat4.rotate(modelViewMatrix,  // destination matrix
              modelViewMatrix,  // matrix to rotate
              squareRotation,   // amount to rotate in radians
              [0, 0, 1]);       // axis to rotate around

This rotates the modelViewMatrix by the current value of squareRotation, around the Z axis.

To actually animate, we need to add code that changes the value of squareRotation over time. We can do that by creating a new variable to track the time at which we last animated (let's call it then), then adding the following code to the end of the main function

  var then = 0;

  // Draw the scene repeatedly
  function render(now) {
    now *= 0.001;  // convert to seconds
    const deltaTime = now - then;
    then = now;

    drawScene(gl, programInfo, buffers, deltaTime);

    requestAnimationFrame(render);
  }
  requestAnimationFrame(render);

This code uses requestAnimationFrame to ask the browser to call the function "render" on each frame. requestAnimationFrame passes us the time in milliseconds since the page loaded. We convert that to seconds and then subtract from it the last time to compute deltaTime which is the number of second since the last frame was rendered. At the end of drawscene we add the code to update squareRotation.

  squareRotation += deltaTime;

This code uses the amount of time that's passed since the last time we updated the value of squareRotation to determine how far to rotate the square.