Why there is an 'Extrusion' geometry type exist?

Dear all,

I am a bit confused about a basic issue - what is extrusion and why we need to develop such a geometry type?
I remember at the early version of grasshopper we don’t have ‘extrusion’, we only have ‘surface’ and ‘polysurface’(brep). That is easy to understand. But now I realize that sometimes there is a type ‘extrusion’ (eventhtough it is just a simple surface with 4 edges extruded by one line. You cannot use a surface parameter to set it and you cannot use evaluate surface button… I have explode it to achieve what I want… and sometimes my old script fails now because this ‘extrusion’ type appears
I just wonder what is the benefit of bring this ‘extrusion’ object type.

Thanks for any hint.


– Dale

I see, thanks Dale

You should be able to reference extrusions using a Brep parameter.

I see, so I can pass brep to Evaluate Surface Button. I still stuck at the outdated understanding that you cannot pass brep to Evaluate Surface…

Thanks David

You can, provided the brep only contains a single face. As soon as brep has two or more faces it no longer has a single uv domain which can be evaluated.

The reason extrusions are implemented as types of surfaces rather than types of brep is because the Rhino SDK didn’t allow the latter without breaking it. Conceptually, an extrusion is clearly a polysurface, since it may have vertical creases and end caps. However we either had to add an entirely new geometry type or make it a kind of surface.

I have a better understanding now, thanks for your explicated explanation.