Understanding trimmed surfaces

Is there a tutorial or good concise explanation about the differences between trimmed and untrimmed surfaces and when/if you need to shrink trimmed surfaces?


Hi Dennis - a trimmed surface is one that has one or more curves added to the surface, so to speak, that tell Rhino to hide some parts of the surface and show you others. In other words trimming a surface does not remove any of the surface, it just hides it.
The underlying surface is always arranged as a grid - albeit often a distorted grid. That is, control points, for instance are in rows an columns even of these are wiggly - in short the arrangement, topology of a surface has a very particular organization and this is completely separate from the 3d shape of the trimmed face that you see in Rhino - that can appear as an arbitrary shape - like a circle for instance. When you shrink a surface, Rhino reduces, if possible (it’s not always) the underlying surface to a grid which just surrounds or brackets the trims, plus a small margin. The trimmed face does not change, but if a surface is shrunk, UntrimAll will result in a different surface compared to the pre-shrunk version, if that’s a word.

Try it on a sphere - trim with a small circle, leaving a little disc surface. Turn on points - see how all the points for the full sphere are there? Now ShrinkTrimmedSrf - the points suck down to just surround the trim. UntrimAll - depending on where the trim falls in the sphere, you’ll get a little rectangular or possibly a small circular surface (if the trim was across the pole of the sphere).

Any of that make sense?


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The Level 2 Training guide has a section called: NURBS Topology
It is a good introduction to these important details.
It includes trimmed surfaces.


Yes. Very helpful. Thanks!