Animating objects with WebGL

Making the square rotate

In this example, we'll actually rotate our camera. By doing so, it will look as if we are rotating the square. First we'll need some variables in which to track the current rotation of the camera.

Note: Add this code at the start of your "webgl-demo.js" script:

js

let squareRotation = 0.0;
let deltaTime = 0;

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

Note: In your "draw-scene.js" module, update the declaration of your drawScene() function so it can be passed the rotation to use:

js

function drawScene(gl, programInfo, buffers, squareRotation) {

Note: In your drawScene() function, right after the line mat4.translate() call, add this code:

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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.

Note: Add this code at the end of your main() function, replacing the existing drawScene() call:

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let then = 0;

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

  drawScene(gl, programInfo, buffers, squareRotation);
  squareRotation += 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.

Finally, we update squareRotation.

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