Solid vs hollow object


Hi Everyone,

since a few day’s I’m now practicing Rhino and one mayor question that pops up is the following. When I make a box like shape (from the box menu) and cut it with another object it appears the shape is hollow. What I want is an solid shape with the cutout only. When I use the command createsolid it doesn’t work.
How do I remake an object solid?

motus_head.3dm (45.0 KB)

(David Cockey) #2

First, an important concept. A “solid” in Rhino is one or more surfaces joined together which make up the surface of the solid.

CreateSolid trims and joins a group of intersecting surfaces to form a closed surface if possible. It does not create new surfaces. The same results can be achieved using Trim, Split, Join and Delete.

The “Boolean” commands are designed to be used with closed surfaces (aka solids) so that the appropriate parts of the surfaces are trimmed and joined for the appropriate result. Be aware that the Boolean commands do not always work so sometimes you will need to use the separate Trim or Split, Join and Delete commands.

(Chris Kuether ) #3

motus_headck.3dm (66.7 KB)
I wonder if this attachment is what you need.


Thank you, that is indeed what I need. With closed sides also in the inner shape. How did you do it?


Thanks David for the background info! it is good to know.

(Chris Kuether ) #6

What you provided was an open polysrf. The edges of the opening were naked. I used extrudecrv to deepen the opening, then cap to make it closed. Simple, straightforward.

(John Brock) #7

The concept to keep in your head when modeling is Rhino is not a solid modeler.
Rhino is a surface modeler that can make “closed, solid, polysurfaces” that are treated as your concept of a solid.

That description needs to be unpacked.

  • Closed - The polysurface has no “naked” or unjoined edges.
  • Solid - The surface normal direction for all surfaces has been changed to point out, away from the enclosed volume.
  • Polysurface - The object is consists of multiple surfaces.

Here’s a “closed, polysurface”. It’s not “solid” because it has a non-manifold edge:

Here is a “closed surface” It is made up of a single surface. Two opposite edges are Joined making a “Seam”, and the other two edges have a length of zero so they are called “singularities”.

(David Cockey) #8

However “solid” operations such as the Boolean commands and Volume work with a closed surface with a non-manifold edge such as @John_Brock 's example above, with all the surfaces on the exterior of the “solid”.
solid dc 1.3dm (634.1 KB)

(David Cockey) #9

@John_Brock Are you claiming that spheres and similar closed single surfaces are not “solids” in Rhino? If so why are the commands Sphere, Ellipsoid and Torus on the Solids tab and on the Help page list for creating solid objects. That help page says a A solid is a surface or polysurface that encloses a volume . Nothing about non-manifold edges.

(John Brock) #10

No, Spheres are solids and have their normals pointing in the correct direction.
My goal with the description was to get @Morgan_Ruben thinking about modeling in terms of how Rhino works instead of in terms of the goal of the modeling.

Users that come from a solid modeling background generally have a little trouble making the mental leap from solids to surfaces.

Non-manifold polysurfaces can cause lots of problems so Rhino generally tries to avoid them.
V6 does a better job or warning the user and marking them problems when Booleans fail.


Thanks for all the usefull info John! Much appreciated!