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.

Sine.

Smoothly scaling size with the sin() function.

 
float diameter; 
float angle = 0;

void setup() {
  size(640, 360);
  diameter = height - 10;
  noStroke();
  noStroke();
  fill(255, 204, 0);
}

void draw() {
  
  background(0);

  float d1 = 10 + (sin(angle) * diameter/2) + diameter/2;
  float d2 = 10 + (sin(angle + PI/2) * diameter/2) + diameter/2;
  float d3 = 10 + (sin(angle + PI) * diameter/2) + diameter/2;
  
  ellipse(0, height/2, d1, d1);
  ellipse(width/2, height/2, d2, d2);
  ellipse(width, height/2, d3, d3);
  
  angle += 0.02;
}