Simple circle in circle illusion.

circleInCircle.pde

int c = 255;
float angle = 0;

void setup() {
    size(500, 500);
    noStroke();
}

void draw() {
    background(0);
    translate(width / 2, height / 2);
    c = 255;
    cic(width / 2, 10, angle, PI / 16.0); 
    angle += PI / 48.0;
}

void cic(float radius, float rSub, float angle, float aAdd) {
    fill(c);
    c = 255 - c;
    ellipse(0, 0, radius * 2, radius * 2);
    radius -= rSub;
    angle += aAdd;

    PVector p = PVector.fromAngle(angle + aAdd);
    p.mult(rSub * 0.6);
    translate(p.x, p.y);
    if (radius >= 1) {
        cic(radius, rSub, angle, aAdd); 
    }
}

circleInCircle2pde

int c = 255;
float angle = 0;

void setup() {
    size(500, 500);
    noStroke();
}

void draw() {
    background(0);
    translate(width / 2, height / 2);
    c = 255;
    float rSub = map(mouseX, 0, width, 5, 20);
    float aAdd = map(mouseY, 0, height, PI / 2.0, PI / 48.0);
    cic(width / 2, rSub, angle, aAdd); 
    angle += PI / 23.0;
}

void cic(float radius, float rSub, float angle, float aAdd) {
    fill(c);
    c = 255 - c;
    ellipse(0, 0, radius * 2, radius * 2);
    radius -= rSub;
    angle += aAdd;

    PVector p = PVector.fromAngle(angle + aAdd);
    p.mult(rSub * 0.6);
    translate(p.x, p.y);
    if (radius >= 1) {
        cic(radius, rSub, angle, aAdd); 
    }
}