int x,y,dx,dy;
float distance;

void setup(){

  size(300,300);
  noStroke();
  
}
  
void loop(){

  for(y=0; y<300; y+=20){
    for(x=0; x<300; x+=10){
    
    dx = mouseX -x;
    dy = mouseY -y;
    distance =sqrt(dx*dx+dy*dy);
    
    fill(distance*4,0,0);
    rect(x,y,8,distance);
}
  }
}