Polysurface Split Failure


I’m trying to split this polysurface in half using the _Split command and a cutting plane. The split succeeds in Rhino 6, but it fails in Rhino 7 WIP.

Attached is a sample part (baked from a Grasshopper script) that shows the failure.

I’ve tried a bunch of different things to try and narrow down the problem and I think it has to do with four of the surfaces having a boundary edge along the splitting plane. If I explode the part and only split the three crotch surfaces, then the split succeeds. Joining the three crotch surfaces into a single polysurface and splitting the resulting polysurface also succeeds. The split only fails if all surfaces are joined into a single polysurface then split using the splitting plane.

This happened in both the August 25th build and the September 1st build of Rhino 7 WIP.

SplitTest.3dm (704.7 KB)

the upper surface is already split and the seam lies right on the split plane. Run the intersect command and you will see where the seam has a micro variance from the split plane…it’s likely under file tolerance and is what’s causing the failure.

if you extract the top surface, untrim it, run merge surface then srfseam to move the seam off the split plane it will split fine.

Or simply extract those already split bits, and then split the bottom bits and join.

I agree that fixing this in Rhino is fairly easy, but this is a small part of a model generated by a Grasshopper script. The models are currently generated in Rhino 6 from an automated process without human interaction and I don’t have this problem.

We’re already looking at rewriting the script to only split the three surfaces in the crotch, but I though it was worth mentioning this considering the same model and process produces a different result between Rhino 6 and 7.