int i, j, n, m;

void setup()
{
  size(500,500);
  n = width/20;
  m = height/20;

  for(i=0; i<n; i++)
  {
    for(j=0; j<m; j++)
    {
      rect(i*20,j*20, 25,25);
    }
  }
}

void draw()
{
  if(mousePressed)
  {
    for(i=0; i<24; i++)
    {
      for(j=0; j<24; j++)
      {
        if(mouseInBox(i,j) == true)
        {
          drawPattern(mouseX,mouseY);
        }
      }
    }
  }
  //println("mouseX = " + mouseX +", mouseY = "+mouseY);
}

void keyPressed()
{
  for(i=0; i<n; i++)
  {
    for(j=0; j<m; j++)
    {
      fill(255);
      rect(i*20,j*20, 25,25);
    }
  }
}

boolean mouseInBox(int i, int j)
{
  if((mouseX > i*20) && (mouseX < (i+1)*20) && (mouseY > j*20) && (mouseY < (j+1)*20))
  {
    return true;
  }
  else
  {
    return false;
  }
}

void drawPattern(int i, int j)
{
  fill(0);
  rect(i, j, 20, 20);
  rect(i+20, j+20, 20,20);
  rect(i+20, j-20, 20,20);
  rect(i-20, j+20, 20,20);
  rect(i-20, j-20, 20,20);
}