2d Polygon vs. 3d Polygon vs. NURBS

[cberg]

Can anybody give me a primer about the differences between 2d Polygons, 3d Polygons and NURBS curves and when to use them.  Compared to other programs, VW seems to have a fairly muddled taxonomy of 2d/2.5d/3d Objects.

 

In Rhino, for example, a curve is a curve is a curve, and they all behave pretty much the same with hatches, behavior, fills, etc.

 

By contrast, in Vectorworks, why do some operations work with NURBS curves, others 2d Polygons, and still others with  3d Polygons?  For example, one can extrude a 3d Polygon along a path.  But only loft NURBS curves.  You can extrude 2d polygons, but not 3d polygons or NURBS.  However the Push Pull tool works with 2d Polygons, 3d Polygons as well as NURBS. It seems kind of random, and I always find myself working with the wrong type of objects.

 

Are there any rules of thumb,  or are we resigned to rote trial and error?

 

Edited March 7, 2017 by cberg

[Kevin McAllister]

What are you referring to as a 3d polygon?

 

Personally I only use two types in general - polylines for 2d shapes and NURBS (curves and surfaces) for 3d shapes. I often start as polylines and convert to NURBS since polylines are friendlier for editing. They can do everything you need. As you say, VW really only has 2d (screen plane), 2.5d (layer plane) and true 3d.

 

21 hours ago, cberg said:

Compared to other programs, VW seems to have a fairly muddled taxonomy of 2d/2.5d/3d Objects.

 

I think this is because VW has always been a hybrid program.

 

21 hours ago, cberg said:

By contrast, in Vectorworks, why do some operations work with NURBS curves, others 2d Polygons, and still others with  3d Polygons?  For example, one can extrude a 3d Polygon along a path. 

 

Don't do this..... 3d polygons create more geometry than you need. Use polylines (or rectangles, circles and other standard 2d shapes) and NURBS curves. The Extrude Along Path will auto-convert a 2d path to a NURBS curve or you can give it a 3d NURBS curve as input. It doesn't care if the profile stays a 2d shape which makes editing easier later.

 

21 hours ago, cberg said:

But only loft NURBS curves.

 

True. But you can start with 2d polylines and convert them to NURBS which is often easier.

 

21 hours ago, cberg said:

You can extrude 2d polygons, but not 3d polygons or NURBS. 

 

You can use the Shell tool to "extrude" a NURBS surface.

 

21 hours ago, cberg said:

However the Push Pull tool works with 2d Polygons, 3d Polygons as well as NURBS. It seems kind of random, and I always find myself working with the wrong type of objects.

 

It definitely is random and more modern tools work with more things (eg. the Push Pull tool). Older tools have never been updated to take advantage of new technology added on the back end.

 

Kevin

 

[cberg]

Very helpful, thank you. 

 

The 3d Polygon appears right next to the NURBS curve tool in my 3D Pallette, so I always think I should use it. I didn't realize that it doesn't do anything useful (and in fact, using it is not good for the model)

 

It would be great if some of the older objects could be retooled to be more consistent!

[rDesign]

1 hour ago, cberg said:

The 3d Polygon appears right next to the NURBS curve tool in my 3D Pallette, so I always think I should use it. I didn't realize that it doesn't do anything useful (and in fact, using it is not good for the model)

 

I wouldn't say that 3D Polygons don't do anything "useful". Personally, I often prefer 3D Polygons over NURBS surfaces as I have never figured out how to successfully edit NURBS surfaces in a predictable manner (this is probably more a comment on my lack of skills / experience with NURBS). 

 

One benefit that 3D Polygons and NURBS surfaces both have is that they can display Renderworks textures, whereas 2D Polygons cannot.

Edited March 8, 2017 by rDesign

[cberg]

Thank you again.  I didn't realize that 3d Polygons can accept textures. 

 

Why are NURBS so hard to edit?  They don't seem to operate by the normal rules of VW editing behavior? 

 

Can't they all just be curves (or whatever we want to call them) that behave consistently? 

[Kevin McAllister]

Its true the NURBS can be difficult to edit. That's why I often start with a polyline and convert it. I think its because there's no proper tool to edit points in 3d (see image). If we had this tool you would have the control you need in 3d to position a point.

When you double click on a NURBS curve to edit it, you do get options to control which axis you're editing in the mode bar, but its a clunky way to work and you can't select multiple axes at once. I tend to edit point co-ordinates in the OIP or create reference geometry to snap to.

 

Another piece of the NURBS puzzle is understanding their degree. If you double click on the NURBS curve tool it will allow you to set it. In its simplest form degree 1 is like making a 2d polygon (corners), degree 2 is like making a 2d polyline (curves or corners), and degree 3 (3d curve with either curves or corners).

 

Kevin