Looking over the library quickly, I really like it, that being said, Pillow is a much smaller and more self contained library than OpenCV, some projects try and avoid openCV if not necessary and this would extend to this project as well.
Having OpenCV as a dependency means that if a project isn't already using OpenCV, this fairly minor (in terms of scale of functionality) utility library has a lot of baggage.
In terms of convolution of implementing it, there may be a little more effort in terms of figuring some offsets (e.g., path widths may extend bounds), but it shouldn't be anything excessive I don't think.