int drawCount = 0;

void setup() 
{ 
  size(500,500);
  framerate(15);
  ellipseMode(CENTER);
  background(0);
  noStroke();
} 

void draw()
{
  if (drawCount % 15 == 0)
  {
    background(0);
  }
  drawCrazyBackground(18);
}

void drawCrazyBackground(int bsize)
{  
  int x = 0;
  int y = 0;
  
  while (x < width)
  {
    while(y < height)
    {
      if (dist(mouseX,mouseY,x,y) > 50)
      {
        fill(255,255,255,25);
        ellipse(x,y,bsize,bsize);
      }
      else
      {
        fill(255,0,0,75);
        ellipse(x,y,bsize,bsize);
      }
      y = y + bsize;
    }
    y = 0;
    x = x + bsize;
  }
}