@th That's neat. With colored face rendering and removal of lines on smooth surfaces (where they are not between a front- and a backface), this could give great renderings for Lego MPD files in a way that doesn't look like (because it is) rendered and edge-detected.
@chrysn smooth surfaces are a challenge - relaxing the "close enough" parameter for coplanar detection might work, although it would need some experimentation. If you're doing one-off art pieces, you can edit and color it in inkscape as much as you like...
coplanar_constant from 0.001 to 0.5 does merge the curved surfaces, although there needs to be some way to detect "is this the last visible egde" so that it doesn't get dropped.