public function crop($top_x, $top_y, $bottom_x, $bottom_y)
{
$image_old = $this->image;
$this->image = imagecreatetruecolor($bottom_x - $top_x, $bottom_y - $top_y);
imagecopy($this->image, $image_old, 0, 0, $top_x, $top_y, $this->width, $this->height);
imagedestroy($image_old);
$this->width = $bottom_x - $top_x;
$this->height = $bottom_y - $top_y;
}