I was very excited that I reached a milestone in my implementation of cropping for Filmulator, so I recorded a short video even though the feature isn't done. Only one corner has the full behavior right now, and it can't even actually crop the image (just draw the guides), and the appearance will change for the final version, but I still wanted to post it.
The goal of Filmulator is to have a minimum number of controls without limiting what you can do. So, to minimize the amount of checkboxes and spinboxes and drop-down boxes needed for cropping, I designed it to let you control everything with just the mouse and two modifier keys.
You can drag around the crop itself by clicking and dragging within the boundaries, you can resize the edges by dragging on each edge, and you can freely resize two edges at once by dragging on a corner.
By holding the Ctrl key while dragging a corner, the aspect ratio will be fixed to what it was when you pressed Ctrl.
But here's the important part:
By holding the Shift key while dragging a corner, the aspect ratio will snap to one of a variety of common aspect ratios; press Ctrl to hold that one while you resize so you don't have to be as careful with mouse positioning.
It only has some aspect ratios built-in: 1:1, 5:4, 4:3, 3:2, 16:9, 2:1, 2.35:1, and 3:1. If you have any that you're burning to have included, let me know, because I'm not really planning on having there be too many built-in.
Let me know what you think!