int squareX;
int squareY;
int dx;
int dy;
float distance;
int x;
int y;

void setup(){
 size(300,300);
 rectMode(CENTER_DIAMETER);

 squareX=width/2;
 squareY=height/2;

}

void loop(){
  background(255,255,255);
  
  for(y=0;y<300;y+=10){
     for(x=0;x<300;x+=10){  

// ------CALCULATE DISTANCE------    
   dx=mouseX-squareX;
   dy=mouseY-squareY;
   distance=sqrt(dx*dx+dy*dy);
 
// ------DRAW-------     
   fill(distance*2,0,0);
   rect(x,y,10,10);
   }
  }
 }