Ball Corner Matching

Inspired by Sky Greenawalt’s excellent YouTube series on Primary Surfacing, I’ve been making a renewed effort to build clean surfaces and really understand what’s going on (I’ve been winging it for years).

One area that’s come up on a recent project is ball corners. In the past I would create a surface with stacked control points - I’d like to do better.

I came across an Alias tutorial by Andreas Hopf on YouTube, which is very helpful. However, I’m struggling to work out how to manipulate the three patches to achieve continuity towards the centre. It looks so easy in Alias to adjust the CVs - what would the equivalent workflow be in Rhino?Ball Corner Matching.3dm (234.5 KB)


try blendedge in v7

Thanks Kyle. I’ve just installed the latest WIP and will give it a try.

Glad your’e enjoying the vids! Here’s another way to approach this issue:

For stock Rhino, one way to change the angle of those two edges so you can “open up” the edge at the top is to create some planes that intersect the side blends, intersect and MatchSrf to move the edges back. Hopefully that makes sense, but holler if it doesn’t!


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Thanks very much, Sky. And thank-you for the YouTube videos - they really are excellent. I watched them all last week instead of Netflix - I wish I’d seen them years ago.

I’ll give the other method in the link a try. Thanks for the suggestion.

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I have also watched your videos and they have very much helped. I have read your reply regarding stock Rhino and would very much like a little more detail on… “to create some planes that intersect the side blends, intersect and MatchSrf to move the edges back”. Thanks for your time and efforts to educate us.


Ask and ye shall receive!


Crystal clear, very much appreciated


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Nice video. I’m not convinced by your pull method though, as it will move points not necessarily in the direction of the control cage. In this simple case you could use _Shear and keep the points in a straight line in one go.

I use the pull to a plane method all the time to clean up surface edges, but for more complicated geometry you generally need to run MatchSrf after to make sure things didn’t affect your continuity - with Preserve Isocurve Direction turned on.

This is where VSRs split surface comes in handy as it produces resulting surfaces that are untrimmed. Would be a great addition to Rhinos toolset too @pascal

Rhino’s way is currently ‘TestTrimRefit’ in v7 wip

Oh, didn’t know of that one.
I need to try it, thanks!

Thanks very much for the tutorial, Sky. It works beautifully.

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Shear works really well for this - thanks for the tip.

Oh Shear is SUPER DOPE! Thanks for the tip @Gijs! Took me a bit to figure it out, but you can actually use it very similar to how I move surface corners using the VSR Control Point Modeling tool in “Extrapolate” mode, grabbing the surface corners. I played around with it - you can also use it to move more complicated surfaces that have higher degrees, and select all the points except the opposite edge. Gooood stuff!

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An alternative to Shear is to use Project in the direction which the surface being modified is straight.

have you tried rail revolve?

Thanks for the suggestion, Matt. I have used revolve in the past, including with the rail option. In fact that’s how I used to create this type of corner but am now trying to avoid stacking control points to get better continuity. The method Sky suggested is the best I’ve tried so far, although I’d still like to work out how to match the three patches shown in my first post, just as another option.

this is also quite interesting: from patch to single span using XNurbs:

Ball Corner Matching - Xnurbs.3dm (1.6 MB)