Could someone explain why/how this sample scene kit code works? - ios

Question

I am working with scene kit and am trying to make a first person game. I found this sample code for making the first person camera with a pan gesture. Everything works but I do not understand what is going on here. Could someone explain what is happening?
func lookGestureRecognized(gesture: UIPanGestureRecognizer) {
//get translation and convert to rotation
let translation = gesture.translationInView(self.view)
let hAngle = acos(Float(translation.x) / 200) - Float(M_PI_2)
let vAngle = acos(Float(translation.y) / 200) - Float(M_PI_2)
//rotate hero
heroNode.physicsBody?.applyTorque(SCNVector4(x: 0, y: 1, z: 0, w: hAngle), impulse: true)
//tilt camera
elevation = max(Float(-M_PI_4), min(Float(M_PI_4), elevation + vAngle))
camNode.rotation = SCNVector4(x: 1, y: 0, z: 0, w: elevation)
//reset translation
gesture.setTranslation(CGPointZero, inView: self.view)
}

Answer 1

Here is the same code, with a few additional comments:
func lookGestureRecognized(gesture: UIPanGestureRecognizer) {
// Create Translation variable, containing the
// "distance" traveled by the finger since the last event
let translation = gesture.translationInView(self.view)
// From this distance, calculate how much the camera should move
// 1) horizontally, 2) vertically using angles (FPS controls)
let hAngle = acos(Float(translation.x) / 200) - Float(M_PI_2)
let vAngle = acos(Float(translation.y) / 200) - Float(M_PI_2)
// Apply the horizontal angle to the Hero node as a force to
// Make it rotate accordingly (physics body use forces to move)
heroNode.physicsBody?.applyTorque(SCNVector4(x: 0, y: 1, z: 0, w: hAngle), impulse: true)
// Use the other angle to look up and down, clamped to ±pi/4
elevation = max(Float(-M_PI_4), min(Float(M_PI_4), elevation + vAngle))
// Apply the new angle to teh camera on the X axis
camNode.rotation = SCNVector4(x: 1, y: 0, z: 0, w: elevation)
// Set the translation to 0 to avoid accumulation
// the next time the event is triggered
gesture.setTranslation(CGPointZero, inView: self.view)
}
This should help understand, let me know if you need more details on how it works!
(Note: the "Distance" is actually a 2D Vector)
EDIT: here's a better explanation of the angle:
let hAngle = acos(Float(translation.x) / 200) - Float(M_PI_2)
First, the translation (so pixel distance on x) is divided by 200. This is to both slow down the movement and (unsafely) keep x between -1 and 1.
Acos gives the arc cosinus of a value. The result is between 0 to pi and (to simplify) only works for x from -1 to 1. Here's a graph for it: http://www.wolframalpha.com/input/?i=acos%28x%29-pi%2F2
Since we want to move in positive and negative directions, we remove half of the max value (M_PI_2, which is pi/2) to keep the result within -pi/2 to pi/2
In the end, if you move your finger 200 pixels in a direction, you would look pi/2=90° on your screen.