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Non-orthogonal Collision with Multiple Ground Segments

by Ira Greenberg

Based on Keith Peter's Solution in Foundation Actionscript Animation: Making Things Move!

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/**
 * Non-orthogonal Collision with Multiple Ground Segments 
 * by Ira Greenberg. 
 * 
 * Based on Keith Peter's Solution in
 * Foundation Actionscript Animation: Making Things Move!
 */

Orb orb;

PVector gravity = new PVector(0,0.05);
// The ground is an array of "Ground" objects
int segments = 40;
Ground[] ground = new Ground[segments];

void setup(){
  size(640, 360);
  // An orb object that will fall and bounce around
  orb = new Orb(50, 50, 3);

  // Calculate ground peak heights 
  float[] peakHeights = new float[segments+1];
  for (int i=0; i<peakHeights.length; i++){
    peakHeights[i] = random(height-40, height-30);
  }

  /* Float value required for segment width (segs)
   calculations so the ground spans the entire 
   display window, regardless of segment number. */
  float segs = segments;
  for (int i=0; i<segments; i++){
    ground[i]  = new Ground(width/segs*i, peakHeights[i], width/segs*(i+1), peakHeights[i+1]);
  }
}


void draw(){
  // Background
  noStroke();
  fill(0, 15);
  rect(0, 0, width, height);
  
  // Move and display the orb
  orb.move();
  orb.display();
  // Check walls
  orb.checkWallCollision();

  // Check against all the ground segments
  for (int i=0; i<segments; i++){
    orb.checkGroundCollision(ground[i]);
  }

  
  // Draw ground
  fill(127);
  beginShape();
  for (int i=0; i<segments; i++){
    vertex(ground[i].x1, ground[i].y1);
    vertex(ground[i].x2, ground[i].y2);
  }
  vertex(ground[segments-1].x2, height);
  vertex(ground[0].x1, height);
  endShape(CLOSE);


}









class Ground {
  float x1, y1, x2, y2;  
  float x, y, len, rot;

  // Constructor
  Ground(float x1, float y1, float x2, float y2) {
    this.x1 = x1;
    this.y1 = y1;
    this.x2 = x2;
    this.y2 = y2;
    x = (x1+x2)/2;
    y = (y1+y2)/2;
    len = dist(x1, y1, x2, y2);
    rot = atan2((y2-y1), (x2-x1));
  }
}

class Orb {
  // Orb has positio and velocity
  PVector position;
  PVector velocity;
  float r;
  // A damping of 80% slows it down when it hits the ground
  float damping = 0.8;

  Orb(float x, float y, float r_) {
    position = new PVector(x, y);
    velocity = new PVector(.5, 0);
    r = r_;
  }

  void move() {
    // Move orb
    velocity.add(gravity);
    position.add(velocity);
  }

  void display() {
    // Draw orb
    noStroke();
    fill(200);
    ellipse(position.x, position.y, r*2, r*2);
  }
  
  // Check boundaries of window
  void checkWallCollision() {
    if (position.x > width-r) {
      position.x = width-r;
      velocity.x *= -damping;
    } 
    else if (position.x < r) {
      position.x = r;
      velocity.x *= -damping;
    }
  }

  void checkGroundCollision(Ground groundSegment) {

    // Get difference between orb and ground
    float deltaX = position.x - groundSegment.x;
    float deltaY = position.y - groundSegment.y;

    // Precalculate trig values
    float cosine = cos(groundSegment.rot);
    float sine = sin(groundSegment.rot);

    /* Rotate ground and velocity to allow 
     orthogonal collision calculations */
    float groundXTemp = cosine * deltaX + sine * deltaY;
    float groundYTemp = cosine * deltaY - sine * deltaX;
    float velocityXTemp = cosine * velocity.x + sine * velocity.y;
    float velocityYTemp = cosine * velocity.y - sine * velocity.x;

    /* Ground collision - check for surface 
     collision and also that orb is within 
     left/rights bounds of ground segment */
    if (groundYTemp > -r &&
      position.x > groundSegment.x1 &&
      position.x < groundSegment.x2 ) {
      // keep orb from going into ground
      groundYTemp = -r;
      // bounce and slow down orb
      velocityYTemp *= -1.0;
      velocityYTemp *= damping;
    }

    // Reset ground, velocity and orb
    deltaX = cosine * groundXTemp - sine * groundYTemp;
    deltaY = cosine * groundYTemp + sine * groundXTemp;
    velocity.x = cosine * velocityXTemp - sine * velocityYTemp;
    velocity.y = cosine * velocityYTemp + sine * velocityXTemp;
    position.x = groundSegment.x + deltaX;
    position.y = groundSegment.y + deltaY;
  }
}

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