prime_spiral.java

int width = 600;
int height = 600;
int total_points = 100000;
int point_size = 1;
float point_spacing = 1.0;

boolean isPrime(int n) { 
  if (n < 2) return false;
  else if (n == 2) return true;
  else if (n % 2 == 0) return false; 
  for(int i=3; i<=sqrt(n); i+=2) {
    if (n % i == 0) return false;
  }  
  return true;
}

void setup() {
  size(width, height);
  background(0);
  smooth();   
  noStroke();  
  spiral();
}
     
void spiral() {
  for (int n=0; n<=total_points; n++) {
           
      float theta = -sqrt(n) * TWO_PI;
      float r = sqrt(n) * point_spacing;
      float x = width/2 + cos(theta) * r;
      float y = height/2 + sin(theta) * r;
      
      if(isPrime(n)) {
        fill(100,100,255);
        ellipse(x,y,point_size*2,point_size*2);
      } else {        
        fill(50);
        ellipse(x,y,point_size,point_size);
      }
  }
}