color white=color(255,255,255);
color black=color(20,20,20);
color back=color(150,150,150);

void setup(){
  size(640,640);
  fill(back);
  rect(0,0,width,height);
  loadPixels();
  pixels[width/2+width*height/2]=white;
//  pixels[2+width*2]=white;
  updatePixels();
  frameRate(60);
}

int a=1,b=0;
void draw(){
  a=(a%3)+1;
  b=(b%5)+1;
  loadPixels();
  for(int i=a;i<width;i+=2)for(int j=b;j<height;j+=2)if (pixels[i+j*width]==white){
    for(int k=0;k<4;k++){
    int s=floor(random(4));
    switch(s){
      case 0:
        if (clear(i,j,0,-1)) {
          pix(i,j,0,-1,white);
          pix(i,j,0,0,black);
        }
      break;
      case 1:
        if (clear(i,j,1,0)) {
          pix(i,j,1,0,white);
          pix(i,j,0,0,black);
        }
      break;
      case 2:
        if (clear(i,j,0,1)) {
          pix(i,j,0,1,white);
          pix(i,j,0,0,black);
        }
      break;
      case 3:
        if (clear(i,j,-1,0)) {
          pix(i,j,-1,0,white);
          pix(i,j,0,0,black);
        }
      break;
    }}
    if (!clear(i,j, 0,-1)&&
        !clear(i,j, 1, 0)&&
        !clear(i,j, 0, 1)&&
        !clear(i,j,-1, 0))pix(i,j,0,0,black);
  }
  updatePixels();
}
void pix(int x,int y,int xx,int yy,color c){
  int p=x+(y+yy)*width+xx;
  if (p<width*2||p>=height*width-width*2) return;
  pixels[p]=c;
}
boolean clear(int x,int y,int xx,int yy){
  int p=xx+x+(y+yy)*width;
  if (p<width*2||p>=height*width-width*3) return false;
  if (yy!=0){
    if (pixels[p]==back && pixels[p+1]==back && pixels[p-1]==back&&
      pixels[p+yy*width]==back && pixels[p+1+yy*width]==back && pixels[p-1+yy*width]==back&&
      pixels[p+yy*2*width]==back && pixels[p+1+yy*2*width]==back && pixels[p-1+yy*2*width]==back
      //pixels[p+yy*3*width]==back && pixels[p+1+yy*3*width]==back && pixels[p-1+yy*3*width]==back
      )return true;
  }
  if (xx!=0){
    if (pixels[p]==back && pixels[p+width]==back && pixels[p-width]==back&&
      pixels[p+xx]==back && pixels[p+width+xx]==back && pixels[p-width+xx]==back && 
      pixels[p+width+xx*2]==back && pixels[p-width+xx*2]==back&&
      //pixels[p+width+xx*3]==back && pixels[p-width+xx*3]==back&&
      pixels[p+xx*2]==back) return true;  
  }
  return false;
  //return true;
}