P: Change crop aspect ratio retaining width (or height) of existing crop like PS change canvas size

Hi, 

I will describe the situation. Every year I have a lot of portrait photos of students. For one application I need square photos. For another application I need rectangular photos. I use the crop overlay for that. I am looking for a way to change the aspect ratio (in this case from 1x1 to custom 3.4x4.5) without having to manually resize and reposition the crop area.

I imagine that once having a square crop, I can make virtual copies and change the aspect ratio in the copies. While choosing the new aspect ratio, I should be able to select also which direction (horizontal or vertical) should be kept. When selecting horizontal, only the vertical size of the crop area should be changed to meet the new aspect ratio. The left and right border remain in place.

Again while choosing the new aspect ratio I should also be able to select the direction in which the change should be applied. In my example, I should be able to select top, bottom or both. A bit like resize canvas in Photoshop.

In my example I want the change in the vertical direction, but of course there can be situations where one needs this functionality in the horizontal direction.

Writing this I realize that it is in fact very much the same as the already mentioned Photoshop functionality resize canvas. Only it is applied to the crop area.