* Get ImageMagick subsampling factors for the target JPEG pixel format.
*
* @param string $pixelFormat one of 'yuv444', 'yuv422', 'yuv420'
* Get ImageMagick subsampling factors for the target JPEG pixel format.
*
* @param string $pixelFormat one of 'yuv444', 'yuv422', 'yuv420'