Store the selected sizes?

Hi! First of all, great idea on this plugin.

It reminds me of how Drupal manages different sizes. Drupal checks of the size exists, of not, then generates it.
Though I have one question. When a size is requested, does this plugin generate and store the image? Or does it regenerate the image every time?

Thanks, Jory

https://wordpress.org/plugins/intermediate-image-sizes/