spirograph.pde

int t = 0;
float r = 50.0;
float r2 = 400.0;
float k1 = 10 * exp(1);
float k2 = 0.1 * PI;
float x;
float y;
float offsetX = 500;
float offsetY = 500;
color from;
color to;
float fromOpacity = 60.0;
float toOpacity = 0;
float opacityCycleOffset = 0.05;
float colourCycleOffset = 0.0;

void setup() {
   size(1000, 1000);
   colorMode(HSB);
   from = color(120, 255, 255);
   to = color(180, 255, 255);
   background(0, 0, 255);
   stroke(
     lerpColor(from, to, ((k1*t)/(2*PI+colourCycleOffset))%1),
     lerp(fromOpacity, toOpacity, ((k1*t)/(2*PI+opacityCycleOffset))%1)
   );
   x = offsetX + r * cos(t * k1) + r2 * cos(t * k2);
   y = offsetY + r * sin(t * k1) + r2 * sin(t * k2);
}

void draw() {
  stroke(
     lerpColor(from, to, ((k1*t)/(2*PI+colourCycleOffset))%1),
     lerp(fromOpacity, toOpacity, ((k1*t)/(2*PI+opacityCycleOffset))%1)
   );
   float x_ = offsetX + r * cos(t * k1) + r2 * cos(t * k2);
   float y_ = offsetY + r * sin(t * k1) + r2 * sin(t * k2); 
   line(x, y, x_, y_);
   x = x_;
   y = y_;
   t++;
}