Gd::filter

public function filter(Mask $mask)
{
	if ($this->resource === null)
	{
		return false;
	}

	$transparentColor = -1;
	if ($this->format == FileImage::FORMAT_GIF)
	{
		// Process transparency for GIFs
		$transparentColor = $this->getTransparentColor();
	}

	// Fix left top corner
	$newPixel = $this->calculatePixel($mask, 0, 0);

	$result = imageconvolution($this->resource, $mask->getValue(), 1, 0);

	if ($result)
	{
		// Fix left top corner
		imagealphablending($this->resource, false);
		imagesetpixel($this->resource, 0, 0, $newPixel);

		// restore transparency
		if ($transparentColor >= 0)
		{
			$this->restoreTransparency($transparentColor);
		}
	}
	return $result;
}