Multiple Extrude Pilaster

[willofmaine]

Is there any way to get rid of the "X" twist in this pilaster? It's a multiple extrude, 6 segments/7 polygons. The bottom three polygons are all the same. It is between the uppermost of these bottom three and the first slightly smaller polygon that the "X" is occurring.

Any thoughts greatly appreciated!

[michaelk]

I think if you mirror the top 4 polygons (non duplicating mode) the twist will go away.

(or the bottom 3....)

hth

mk

[willofmaine]

If I do that, I get a double "X"...

[Benson Shaw]

This, from VW help:

Multiple Extrude

A pyramid, sphere, or other 3D object can be created from a series of 2D objects using the Multiple Extrude command. Using locus points can provide a point of reference with this command.

The process matches segments on each object to create the extrude. The segment order may be reversed for one or more of the objects, producing unexpected results (such as a twisted extrusion). If this occurs, convert the objects to open polygons prior to selecting the Multiple Extrude command.

-B

[bcd]

Surprisingly 2d polygons remember how they were drawn. If the drawing direction is different for multiple extrude generation objects the result will be twisted.

You can untwist them by rotating flipping or I imagine mirroring symmetrical objects.

In this case it looks like all your cross-sections are behaving except the belt which needs a toss.

[willofmaine]

Thanks for the responses.

Using multiple extrude seems like an odd way to create a sphere. Anyway...

I'd read the bit about using locus points as a point of reference, but I'm not sure what that means. If the locus point is included in the extrude, it simply adds another segment and everything converges at the point of the locus.

I tried open polygons, and even aligned the open segments from one polygon to the next, all to no avail.

I've de-composed and re-composed each polygon in an effort to 'wipe their memories;' still the same twist.

I've tried alternately sending the lower polygons to the back, behind each other, but the twist persists.

The Loft Tool (no rail) seems to do the trick. At the cost of more memory and management and reduced flexibility, but at least it works!

Thanks again, Will

[Benson Shaw]

I played around with a shape like yours, created by making an original poly as a rectangle with flutes removed with clip surface command. Then I duplicated the orig poly with the Dupe in Place pref. #1-4 were all same, #5,6,7 were each scaled to 95% of the predecessor. I aligned them all to back edge and ran the Multiple extrude command. It made the extrude with no twist, but really whacked the system. I got the beachball of doom for every action on the extrude - select, deselect, move, change fill color, etc.

I'm using VW2010 and snow leopard, so this was a bit of a surprise. It's supposed to be so good at 3d. I don't have VS2008 on my system any more. Maybe this should be done with fewer extrude polys. Two for the bottom, two for any taper, and one more if the taper goes straight.

Good luck!

-B

[Kevin McAllister]

Can you post an example file? I'm not sure I entirely understand the shape if it requires so many polygons.

When I've had trouble with multiple extrudes, I've made smaller segments and then used the Add Solids to combine the pieces. It allows you to retain the history and tends to cause less problems.

Kevin

[willofmaine]

Hi Benson,

Interestingly enough, I had ended up doing what you've described.

While originally my flutes were tapered, they did not get closer to each other as they approached the top; so, the polygons were not simply scaled-down versions of their predecessors. They were developed from a round column which involved some amount of additions, subtractions and flipping. (Clearly a good recipe for twisted extrudes... )

Then I realized that duplicating and scaling the same polygon might make VectorWorks happier and also that a pilaster with its flutes getting closer to each other towards the top might actually be a good thing. So, except for using different percentages, I did pretty much exactly what you did. And, like you, there were no twists.

Are your flutes arcs? If so, that may be why it 'whacked' your system. I found I had to use the Regular Polygon tool and subtract "circles" with something like 22 flat faces to make the flutes. Otherwise, I think VW tries to make zillions of tapered slivers to describe the tapered, curved flutes...

Thanks, Will

[willofmaine]

Hi Kevin,

The shape is a pilaster with the gradual convex curve ("taper" or entasis) of a classical column. I'll try and attach a file...

Will

[Benson Shaw]

I like that - Twisted Extrudes - could be a really great garage band we've not yet heard of.

And, good reminder - My source polys were probably the cause of beach balling. After my clip surface, I was so intent on making the extrude that I never looked at the vertex count.

-B

[Kevin McAllister]

Hi Will,

Thanks for the file. Is this the one that shows a twist? Or is this the fixed version? It shows up correctly in VW2010.

It makes more sense now that there are so many slices, as its not really curved so much as facetted. I think I would have built it as two pieces, the straight bottom half and the facetted top half, allowing me to reduce the the number of slices in the bottom half to two.

Benson, I had the same result as you when I started making a version using circles and clip surface, spinning beach ball. I think its because my 3D conversion for curves is set to very high. Will's version has a fixed number of points in the flutes, whereas the number of points in ours changes as the 3D conversion factor is changed.

If you can make a version with the loft rail command it probably actually is more efficient. My experience has been the NURBs can help the efficiency of a file.

Kevin

[willofmaine]

Hi Kevin,

Here is the original Twisted Extrude file, not to be confused with any garage band of the same name...

Right, it's faceted to create the illusion of a curve, successful because the curve is so slight.

If you did it as two components, wouldn't the bottom just be a simple extrude?

Roughly speaking, doesn't the file's having to track two components offset any gain in reducing the number of polygons in a single multiple extrude? And from a user's point of view, it seems fewer components are easier to manage...

I did compare a file with the twisted extrude to a file with a loft surface, and the file with the loft surface was a 1/2 MB bigger. Hardly a conclusive experiment, I know... But also NURBS seem much more difficult to edit than the polygons of a multiple extrude...

Will