A sample code in Processing: a (translucent) rainbow in the sky / Processing で Perlin noise を使った雲と半透明な虹

rainbow-and-sky.pde

size(800, 400);
colorMode(HSB, 360.0, 1.0, 1.0, 1.0);

// The sky by the perlin noise
for (int i = 0; i < width; i++) {
  for (int j = 0; j < height; j++) {
    stroke(210, noise(0.005 * i, 0.005 * j), 1.0);
    point(i, j);
  }
}

//  A translucent rainbow
noFill();
for (int i = 0; i <60; i++) {
  stroke(i*i/10, 1, 1,  0.3 * exp(-(i-30)*(i-30)/300));
  ellipse(width/2, 1.2 * height, 600-2*i, 600-2*i);
}