This example is for Processing 3+. If you have a previous version, use the examples included with your software. If you see any errors or have suggestions, please let us know.

Circle Collision with Swapping Velocities by Ira Greenberg.

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

```
Ball[] balls =  {
new Ball(100, 400, 20),
new Ball(700, 400, 80)
};

void setup() {
size(640, 360);
}

void draw() {
background(51);

for (Ball b : balls) {
b.update();
b.display();
b.checkBoundaryCollision();
}

balls[0].checkCollision(balls[1]);
}

class Ball {
PVector position;
PVector velocity;

Ball(float x, float y, float r_) {
position = new PVector(x, y);
velocity = PVector.random2D();
velocity.mult(3);
}

void update() {
}

void checkBoundaryCollision() {
velocity.x *= -1;
} else if (position.x < radius) {
velocity.x *= -1;
} else if (position.y > height-radius) {
velocity.y *= -1;
} else if (position.y < radius) {
velocity.y *= -1;
}
}

void checkCollision(Ball other) {

// Get distances between the balls components
PVector distanceVect = PVector.sub(other.position, position);

// Calculate magnitude of the vector separating the balls
float distanceVectMag = distanceVect.mag();

// Minimum distance before they are touching

if (distanceVectMag < minDistance) {
float distanceCorrection = (minDistance-distanceVectMag)/2.0;
PVector d = distanceVect.copy();
PVector correctionVector = d.normalize().mult(distanceCorrection);
position.sub(correctionVector);

// get angle of distanceVect
// 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*/
PVector[] bTemp = {
new PVector(), new PVector()
};

/* this ball's position is relative to the other
so you can use the vector between them (bVect) as the
reference point in the rotation expressions.
bTemp[0].position.x and bTemp[0].position.y will initialize
automatically to 0.0, which is what you want
since b[1] will rotate around b[0] */
bTemp[1].x  = cosine * distanceVect.x + sine * distanceVect.y;
bTemp[1].y  = cosine * distanceVect.y - sine * distanceVect.x;

// rotate Temporary velocities
PVector[] vTemp = {
new PVector(), new PVector()
};

vTemp[0].x  = cosine * velocity.x + sine * velocity.y;
vTemp[0].y  = cosine * velocity.y - sine * velocity.x;
vTemp[1].x  = cosine * other.velocity.x + sine * other.velocity.y;
vTemp[1].y  = cosine * other.velocity.y - sine * other.velocity.x;

/* Now that velocities are rotated, you can use 1D
conservation of momentum equations to calculate
the final velocity along the x-axis. */
PVector[] vFinal = {
new PVector(), new PVector()
};

// final rotated velocity for b[0]
vFinal[0].x = ((m - other.m) * vTemp[0].x + 2 * other.m * vTemp[1].x) / (m + other.m);
vFinal[0].y = vTemp[0].y;

// final rotated velocity for b[0]
vFinal[1].x = ((other.m - m) * vTemp[1].x + 2 * m * vTemp[0].x) / (m + other.m);
vFinal[1].y = vTemp[1].y;

// hack to avoid clumping
bTemp[0].x += vFinal[0].x;
bTemp[1].x += vFinal[1].x;

/* Rotate ball positions and velocities back
Reverse signs in trig expressions to rotate
in the opposite direction */
// rotate balls
PVector[] bFinal = {
new PVector(), new PVector()
};

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
other.position.x = position.x + bFinal[1].x;
other.position.y = position.y + bFinal[1].y;

// update velocities
velocity.x = cosine * vFinal[0].x - sine * vFinal[0].y;
velocity.y = cosine * vFinal[0].y + sine * vFinal[0].x;
other.velocity.x = cosine * vFinal[1].x - sine * vFinal[1].y;
other.velocity.y = cosine * vFinal[1].y + sine * vFinal[1].x;
}
}

void display() {
noStroke();
fill(204);