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){
    
      // ---- CALCULATE DISTANCE ----
      dx = mouseX - x;
      dy = mouseY - y;
      distance = sqrt(dx*dx + dy*dy);
      
      
      // ---- DRAW ----
      fill(distance*4,255,0);
      rect(x,y,distance/20,distance/20);
    } 
      
      
  }  
}