Boundry Interpolation Behaviour for Blenders Sub Division Modifier.

Edited: Thanks To both Eugene and Lillya for pointing me in the right direction on this, that's awesome!

For those who find this and want a TLDR there is currently a branch of Blender 2.9 by Piotr Ostrowski (postrowski) which adds the additional functionality to the open subdiv modifier flawlessly! Which I’m sure many, many artists will be very happy about, what a hero!

And it works like this:

The standalone branch can be downloaded from his google drive for now here: drive.google.com/file/d/1Jue4yqgVZ_hccBpvKZQHUrO94EYFHH3N/view

For the forum post go here: devtalk.blender.org/t/subdivision-behaviour-is-different-than-in-any-other-software-causing-compatibility-issues/13032/17

and the official build submission page, here: developer.blender.org/D8485

lets hope this makes it to the stable build, very exciting!

-------Original post-----------

Original title: More Boundry Interpolation options for Blenders Sub Division Modifier

Hey Folks, I’m a concept artist looking to use 3d to speed up my workflow and was messing around with the sub div modifier, I would like to use it to rapidly concept thumbnails of vehicle 'skins' while gaining a good idea of how light and shadow would fall on its shape, using subdiv saves a lot of time as you are just using a small amount of edges to describe a ton of detail.

Now stay with me as I mention the Max word but in 3ds max subdivision on a plane seems to work differently. As you begin to extrude from a plane the edges are preserved like they were sharp, this means you always know what 'body' of your mesh is going to look like because the ends react a lot more how you might expect them to react in a CAD program. Allowing you to rapidly concept results. ...problem is, its max. yuck.

Where in blender the result is this:

Why not just crease edges? because you have to constantly redefine them, which is fine for production pipelines but breaks this technique for concept. see bellow. Every time you have to mess around with edges is time you are not designing. Making this technique sadly unviable in Blender. :(

But all is not lost ...maybe? I was pokeing around the Pixar documention for open sub div which from what I can gather both blender and max use and noticed that Pixar defines differnt behaviour for how to define edge behaviour on non-closed objects.

Link: graphics.pixar.com/opensubdiv/docs/subdivision_surfaces.html

To me it appars that blender is using VTX_BOUNDRY_EDGE_ONLY where Max is useing VTX_BOUNDRY_EDGE_AND_CORNER.

I was wondering if it was possible to expose this option to the subdiv modifier UI as a checkbox for Catmull-Clark.

I'd be vary thankful if someone would share the technical knowhow for how one might go about calling this function in a python script? or is it too deep in the open sub div API?

Cheers!