How to check valid size of images before crop

[TINHCX-GMAIL]

I am researching nginx module "http_image_filter_module" and use the
crop function
but I wonder how to check valid size of images before crop them.

_____________________
​​

[E]: tinhcx at gmail.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.nginx.org/pipermail/nginx/attachments/20150919/b88175db/attachment.html>