varying bounce angle (#90)

Examples/Pong/Sources/Pong/Game.swift

@@ -106,12 +106,35 @@ class Ball: Sprite.Sprite {
 
  var velocity = Vector(x: 4, y: 5)
 
+ static func computeNewVelocity(collisionPoint: Float, speed: Float, direction: Bool) -> Vector {
+ // Maximum and minimum angle from vertical
+ let maxAngleDegrees = 20
+
+ let bounceDegrees = Float(180 - 2 * maxAngleDegrees) * collisionPoint + Float(maxAngleDegrees)
+
+ let bounceRadians = bounceDegrees * Float.pi / 180
+
+ let unitVectorX = sinf(bounceRadians)
+ let unitVectorY = cosf(bounceRadians)
+
+ let flip: Float = direction ? -1 : 1
+
+ let velocityX = speed * unitVectorX * flip
+ let velocityY = speed * unitVectorY
+
+ return Vector(x: velocityX, y: velocityY)
+ }
+
  func reset() {
  position = Point(x: Display.width / 2, y: 10)
  velocity.x *= Bool.random() ? 1 : -1
  velocity.y = abs(velocity.y)
  }
 
+ func speed() -> Float {
+ sqrtf(velocity.x * velocity.x + velocity.y * velocity.y)
+ }
+
  override func update() {
  let collisionInfo = moveWithCollisions(
  goal: position + velocity
@@ -128,7 +151,9 @@ class Ball: Sprite.Sprite {
  } else {
  synth.playNote(frequency: 220.0, volume: 0.7, length: 0.1)
  if collision.normal.x != 0 {
- velocity.x *= -1
+ let distance = collision.otherRect.center.y - collision.spriteRect.center.y
+ let positionWithinPaddle = ((distance + collision.spriteRect.height / 2 + collision.otherRect.height / 2) / (collision.spriteRect.height + collision.otherRect.height))
+ velocity = Ball.computeNewVelocity(collisionPoint: positionWithinPaddle, speed: speed(), direction: velocity.x > 0)
  }
  if collision.normal.y != 0 {
  velocity.y *= -1