@jeff_hammond: Your advice to use _DupEdge looks to be very useful.
when you say ‘renders’, what do you mean- exactly? do you have an example render which shows the seam problem?
(like, is there literally a line being drawn at the seam or do you mean you can see a hard(er) transition where the seam is?)
I had the opportunity to play with Penguin render, which I tried to create the same effect as Pen mode. You can see the seams.
I’m not sure what the workaround here, because Penguin does not seem to have a way to hide edges but show curves (they treat edges of surfaces and curves as the same thing). I assume that you can do a really manual workflow by creating one layer with nothing but the shaded surface, then create a layer with the outlines, which you’ve manually created using _DupeEdge, then combine the two images later.
However, with all this in mind, I think the best workflow for my purposes is none of the above. I had a quick look at the videos suggested in this post:
and I think for what I want (technical drawings with crisp lines but with a little shading), the Make2D option is the best way to go. The curves are brought into Illustrator, cleaned up, and we can use any renderer (with no edges) and overlay the vector image with the shading from the renderer.
Either way, it looks like there’s some manual work involved, but that’s probably fine given that you’d like to clean up the lines in any case, and there’s not enough control with any of the default renderers.