how to check width and height of image before resize

[eluser]Tống Kiện Phi[/eluser]
In the example Image Manipulation Class i saw this code:

Code:

$config['image_library'] = 'gd2';
$config['source_image'] = '/path/to/image/mypic.jpg';
$config['create_thumb'] = TRUE;
$config['maintain_ratio'] = TRUE;
$config['width']  = 75;
$config['height'] = 50;

$this->load->library('image_lib', $config);

$this->image_lib->resize();

Now, i want to check size image if width more than 150px it's must resize, on the contrary not resize