function fill($color = '#000000')
{
$rgba = $this->normalize_color($color);
$fill_color = imagecolorallocatealpha($this->image, $rgba['r'], $rgba['g'], $rgba['b'], $rgba['a']);
imagealphablending($this->image, false);
imagesavealpha($this->image, true);
imagefilledrectangle($this->image, 0, 0, $this->width, $this->height, $fill_color);
return $this;
}