Lists numbers in columns and highlights primes

primesCascading.pde

//inspired by Max Cooper - Aleph 2 (Official Video by Martin Krzywinski)
//https://www.youtube.com/watch?v=tNYfqklRehM
import java.math.*;
BigInteger prime;
int n = 0;
int fontsize=12;
int x=0, y=0;
int repeat=1;
void setup() {
  size(720, 720);
  background(0);
  textSize(fontsize);
}
void draw() {
  float alpha = map(constrain(repeat, 1, 100), 1, 100, 4, 30);
  fill(0, alpha);
  square(-9, -9, 2*width);
  //why background(0,alpha); does not work?

  for (int i=0; i<repeat; i++) {
    prime = BigInteger.valueOf(n);
    if (prime.isProbablePrime(1)) fill(255, 0, 0);
    else fill(0, 255, 0);
    text(n, 5+x, fontsize+y);
    n++;
    //println(n + ", pos=(" +x +","+y + ") speed="+repeat);
    y+=fontsize;
    if (y>height) {
      y = 0;
      //faster alternative to String.valueOf(n).length()*fontsize
      int digitfontsize = (int)(Math.log10(n)+1)*fontsize;//get how many digits the number n has for horizontal spacing
      x += digitfontsize;
      if (x > width-digitfontsize) {
        x=0;
        if (repeat < 250) repeat++;
      }
    }
  }
}