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

void setup(){

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

 
  
  

  for(y=0;y<300;y+=10){
    for(x=0;x<300;x+=10){

      dx=mouseX -x;
      dy=mouseY-y;
      distance=sqrt(dx*dx+ dy*dy);

      fill(0,0,255-distance*4);
      rect(x,y,x,y);
    }

  }

}