boolean n[];
int maxn;
int k;
float diam;

void setup()
{
  diam = 20;
  size(600, 600);
  frameRate(1);

  maxn = ceil(width/diam)*ceil(height/diam);        // make this instead based on width, height, diam
  n = new boolean [maxn];

  for (int i=0; i<maxn; i=i+1) n[i] = true;
  n[0] = n[1] = false;

  k = 2;
}

void draw()
{
  int i;

  // seive the primes for k
  for (int j=2*k; j<maxn; j=j+k) n[j] = false;

  // display
  background(0);

  i = 0;
  for (float x=diam/2.0; x<width; x+=diam) {
    for (float y=diam/2.0; y<height; y+=diam) {
      circle(x, y, diam, 
              color(200, 255, 255), 
              color(100, 255, 255), 
              str(i), n[i]);
      i++;
    }
  }

println(k);
textSize(200);
fill(50, 255, 255, 100);
text(k, width/2, height/2);

  // next k
  k++;
  if (k>=maxn) stop();
}

void circle(float x, float y, float diam, 
  color back, color text, String msg, boolean bright)
{
  noStroke();
  ellipseMode(CENTER);
  colorMode(HSB);
  if (bright) fill(back);
  else fill(0);       // make this a different color add arg to func?
  ellipse(x, y, diam, diam);
  textSize(diam*0.4);    // make text size scale with diam
  textAlign(CENTER, CENTER);
  fill(text);
  text(msg, x, y);
}