p5-circles.js

var scanx = 0;
var amp;
var cy;
var cx;

function setup() {
  createCanvas(800, 800);
  background(100);
  reset();
}

function reset() {
  amp = random(15, 4             0);
  cy = random(height);
  cx = scanx + amp;
}

function draw() {
  
  if (scanx > cx + amp) {
   reset();
  }
  
  stroke(color(200, 0, 0));
  line(scanx, 0, scanx, height);
  circlePart();
  
  scanx++;
  if (scanx == width) {
    scanx = 0;
    background(100);
    reset();
  }
}

function circlePart() {
  stroke(255);
  var x = scanx - cx;
  var dy = sqrt(amp*amp - x*x);
  
  point(scanx, cy - dy);
  point(scanx, cy + dy);
}