index.js

  // right and left wrists are the objects given by the model
  const rx = rightWrist.position.x;
  const ry = rightWrist.position.y;
  const lx = leftWrist.position.x;
  const ly = leftWrist.position.y;
  
  // vertical and horizontal distance
  var dy = ly - ry;
  var dx = lx - rx;
  
  var theta = Math.atan2(dy, dx); // range (-PI, PI]
  alpha *= -180 / Math.PI; // rads to degs, range (-180, 180]