a port of the Processing Visualization Language

CircleCollision

by Ira Greenberg. Based on Keith Peter's Solution in Foundation Actionscript Animation: Making Things Move! http://www.friendsofed.com/book.html?isbn=1590597915

Original Processing.org Example: CircleCollision

// All Examples Written by Casey Reas and Ben Fry

// unless otherwise stated.

Ball[] balls =  { 

  new Ball(100, 400, 10), 

  new Ball(700, 400, 40) 

  };



Vect2D[] vels = { 

  new Vect2D(2.15, -1.35), 

  new Vect2D(-1.65, .42) 

  };



void setup(){

  size(200, 200);

  smooth();

  noStroke();

}



void draw(){

  background(51);

  fill(204);

  for (int i=0; i< 2; i++){

    balls[i].x += vels[i].vx;

    balls[i].y += vels[i].vy;

    ellipse(balls[i].x, balls[i].y, balls[i].r*2, balls[i].r*2);

    checkBoundaryCollision(balls[i], vels[i]);

  }

  checkObjectCollision(balls, vels);

}



void checkObjectCollision(Ball[] b, Vect2D[] v){



  // get distances between the balls components

  Vect2D bVect = new Vect2D();

  bVect.vx = b[1].x - b[0].x;

  bVect.vy = b[1].y - b[0].y;



  // calculate magnitude of the vector separating the balls

  float bVectMag = sqrt(bVect.vx * bVect.vx + bVect.vy * bVect.vy);

  if (bVectMag < b[0].r + b[1].r){

    // get angle of bVect

    float theta  = atan2(bVect.vy, bVect.vx);

    // precalculate trig values

    float sine = sin(theta);

    float cosine = cos(theta);



    /* bTemp will hold rotated ball positions. You 

     just need to worry about bTemp[1] position*/

    Ball[] bTemp = {  

      new Ball(), new Ball()      };

    /* b[1]'s position is relative to b[0]'s

     so you can use the vector between them (bVect) as the 

     reference point in the rotation expressions.

     bTemp[0].x and bTemp[0].y will initialize

     automatically to 0.0, which is what you want

     since b[1] will rotate around b[0] */

    bTemp[1].x  = cosine * bVect.vx + sine * bVect.vy;

    bTemp[1].y  = cosine * bVect.vy - sine * bVect.vx;



    // rotate Temporary velocities

    Vect2D[] vTemp = { 

      new Vect2D(), new Vect2D()     };

    vTemp[0].vx  = cosine * v[0].vx + sine * v[0].vy;

    vTemp[0].vy  = cosine * v[0].vy - sine * v[0].vx;

    vTemp[1].vx  = cosine * v[1].vx + sine * v[1].vy;

    vTemp[1].vy  = cosine * v[1].vy - sine * v[1].vx;



    /* Now that velocities are rotated, you can use 1D

     conservation of momentum equations to calculate 

     the final velocity along the x-axis. */

    Vect2D[] vFinal = {  

      new Vect2D(), new Vect2D()      };

    // final rotated velocity for b[0]

    vFinal[0].vx = ((b[0].m - b[1].m) * vTemp[0].vx + 2 * b[1].m * 

      vTemp[1].vx) / (b[0].m + b[1].m);

    vFinal[0].vy = vTemp[0].vy;

    // final rotated velocity for b[0]

    vFinal[1].vx = ((b[1].m - b[0].m) * vTemp[1].vx + 2 * b[0].m * 

      vTemp[0].vx) / (b[0].m + b[1].m);

    vFinal[1].vy = vTemp[1].vy;



    // hack to avoid clumping

    bTemp[0].x += vFinal[0].vx;

    bTemp[1].x += vFinal[1].vx;



    /* Rotate ball positions and velocities back

     Reverse signs in trig expressions to rotate 

     in the opposite direction */

    // rotate balls

    Ball[] bFinal = { 

      new Ball(), new Ball()     };

    bFinal[0].x = cosine * bTemp[0].x - sine * bTemp[0].y;

    bFinal[0].y = cosine * bTemp[0].y + sine * bTemp[0].x;

    bFinal[1].x = cosine * bTemp[1].x - sine * bTemp[1].y;

    bFinal[1].y = cosine * bTemp[1].y + sine * bTemp[1].x;



    // update balls to screen position

    b[1].x = b[0].x + bFinal[1].x;

    b[1].y = b[0].y + bFinal[1].y;

    b[0].x = b[0].x + bFinal[0].x;

    b[0].y = b[0].y + bFinal[0].y;



    // update velocities

    v[0].vx = cosine * vFinal[0].vx - sine * vFinal[0].vy;

    v[0].vy = cosine * vFinal[0].vy + sine * vFinal[0].vx;

    v[1].vx = cosine * vFinal[1].vx - sine * vFinal[1].vy;

    v[1].vy = cosine * vFinal[1].vy + sine * vFinal[1].vx;

  }

}



class Ball{

  float x, y, r, m;



  // default constructor

  Ball() {

  }



  Ball(float x, float y, float r) {

    this.x = x;

    this.y = y;

    this.r = r;

    m = r*.1;

  }

}



class Vect2D{

  float vx, vy;



  // default constructor

  Vect2D() {

  }



  Vect2D(float vx, float vy) {

    this.vx = vx;

    this.vy = vy;

  }

}



// checkBoundaryCollision() function:

void checkBoundaryCollision(Ball ball, Vect2D vel){

  if (ball.x > width-ball.r){

    ball.x = width-ball.r;

    vel.vx *= -1;

  } 

  else if (ball.x < ball.r){

    ball.x = ball.r;

    vel.vx *= -1;

  } 

  else if (ball.y > height-ball.r){

    ball.y = height-ball.r;

    vel.vy *= -1;

  } 

  else if (ball.y < ball.r){

    ball.y = ball.r;

    vel.vy *= -1;

  }

}
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