[joshvm]

I would guess the library uses OpenCV primitives to do things like figure out the size of the label before drawing the background fill underneath. That's normally where the complexity comes in for drawing labels. I'm not sure if you can do that easily with ImageDraw?

For users of this library, OpenCV is pretty likely to already be installed anyway.

Another couple of reasons - OpenCV in my experience handles nonstandard images much better than Pillow (eg many channels, >8 bit). It also has a uniform Python/C++ interface so this could easily be ported if desired. I maintain a C++/Qt desktop annotation app and this would definitely be something to crib from for visualisation. Although in this case image loading could be done somewhere else and the library could just take a Numpy array.

Edit: turns out it's possible with a few extra steps, but it's not trivial. You need to handle ascender/descenders, etc: https://stackoverflow.com/questions/43060479/how-to-get-the-....

OpenCV will tell you the bbox of what it just drew.