# Why trimmed surface and untrimmed surface are introduced in Rhino

Hello all,

I understand the difference between trimmed surface and untrimmed surface, and some commands must consider surface property, such as Match surface. I also read the Rhino level 2 tutorial and our forums.

In 2D plan, circle with Planar Surface command - the circle is trimmed surface. Similarity, square with Planar Surface command - the square is untrimmed surface.

My question is that why trimmed surface and untrimmed surface ( two concepts) are introduced in Rhino. Why one trimmed surface is not a new untrimmed surface. The goal or reason we must distinguish trimmed surface and untrimmed surface when Mcneel writes the program ?

Maybe some basic concepts I missed.

Thank you very much.

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Most of the time PlanarSrf will give you a trimmed surface, except I guess for the special case of a rectangle that it recognizes and makes untrimmed.

The ability to trim surfaces is the âkillerâ feature of NURBS, the reason ALL CAD software uses them. In mesh-based workflows trying to just drill a hole in something is an ordeal that canât produce an actual circle and changes the structure of the surrounding area.

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A surface has two domains. U and V.

If you revolve a line in a plane, the result can be a circular untrimmed surface. (two objetcs on the left).

A simple circular surface is a trimmed surface when it has been created with a circle (closed curve).

After splitting the circle into 4 pieces however, you can use the command _EdgeSrf which lets you choose up to 4 edges and the result is an untrimmed surface.

circles.3dm (114.7 KB)

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Yes. All NURBS surfaces are essentially like a rectangular rubber sheet with 4 sides. The sheet can be stretched and twisted in any way but the basic ârectangularâ structure remains - the âUâ and âVâ directions that Martin mentions above. There is one possibility to collapse one or two of the edges to zero length to create âpolesâ - this is how one can get a sphere or a cone, or even the flat circle above (which is actually just a âflatâ cone). But aside from the collapsed outer edges, the rectangular UV structure always remains.

There are however an infinite number of surface configurations that cannot be represented with just the ânaturalâ untrimmed edges. A simple example would be a planar surface that has more than 4 edges. This type of surface cannot be defined as a single untrimmed surface defined by its natural edges. It can only be described by a trimmed surface - one where the natural edges have been cut away by the outer border, or, it must be broken up into several joined untrimmed surfaces that have four natural edges or less.

It is for this reason that it is very important to understand the difference between untrimmed and trimmed surfaces and their respective properties and roles in Rhino.

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Thank you Helveyosaur, Martin and Jim. The explanation helps me a lot, although I have to spend more time to think, comparation and more model practice.

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Yes of course. I just corrected it above.