For the time being, the GUI is usable, but not overly pretty. The color choosers look a little helpless, ROI selection is single-shot, wireframe colors are not automatically adjusted. There is exactly one menu, adding an unnecessary click to get to the commands. There are no icons for them. Image sizes have gone wild.

It's not fully clear whether shell mode should be left in. It's not clear either whether it should be mandatory (likely shouldn't), and what consequences that has for the OpenCV requirement.

Shell mode doesn't draw helper lines. Shell mode also doesn't provide a means of configuring them. Shell mode doesn't allow a way to modify colors. Anything the UI is enhanced with in the meantime that is technically doable from shell will also be missing from shell mode.

Potentially it makes sense to refactor the code such that there is a common middle layer for options and distribution of the same data according to the same logic from different user interfaces to different technology-centered implementations.