Odd geometry effects in Sketchup

Eric The Viking · 16 Mar 2015

I must be doing something daft - often the case!

Draw a circle. Extrude it to a cylinder and shrink one end about the centreline, so you have a truncated cone. Now rotate the end circle by, say 60 degrees. I'm not sure what this object is properly called, but it's similar in shape to a power station cooling tower.

Have a look at the hidden geometry - it's weird!

Sketchup makes the initial cyilnder 'faced' with identical rectangles all round the sides. When you shrink one end, those rectangles become trapeziums ("trapezia"?), but when you rotate it, SU has to break them into two triangles each, to accommodate the twist.

The trouble seems to be (and I've tried this several times), the triangles aren't applied identically. On the ones I've tried, most of the circumference is correct, but there are a couple of places where the wrong points are joined together (end to end of the cone-thingy). Actually that's unfair: about 1/3 of the circumference has the wrong points joined together, but it's not obvious - you see the transition between correct and incorrect sections in two places, so it looks like there are two errors when in fact there are many.

I've dismantled one of these objects by erasing connecting lines and redrawing them in the proper place: you can make it work, but the SU engine doesn't seem to want to.

I've obviously bumped into another SU oddity and would be grateful for any light shed on this.

E.

PS: I haven't tried yet with an odd number of facets - I'm nervous in case it causes space-time to warp!

Brentingby · 17 Mar 2015

I think you are bored.

I see what you mean and I don't know why it does it. On the other hand, I wonder what you are trying to represent with that drawing.

Eric The Viking · 17 Mar 2015

It's genuinely something I'm working on, but not woodwork (nor a power station, truthfully). It's an experimental shape on the way to something else.

Thanks for the reply, B.

Now I know it does that, I can watch out for it. I'll try an odd number of facets (in part out of curiosity), but I've only today left on my Pro evaluation period and I ought to be trying other bits really. Haven't played with LayOut and really should.

I have to say that, so far, it's been really stable under Wine. I can crash it, but usually only if I ask it to do something really stupid involving the GUI, and it's consistent rather than random. Oh, and it is successfully spawning (Linux) Firefox now, usually with the right help pages. I think that's a Linux bugfix actually, but helpful nonetheless. Warehouse doesn't work from the SU GUI, but that's OK.

Brentingby · 17 Mar 2015

There's probably a better way to construct the shape you're after that would give you the same triangles all the way around. One would be to make the top and bottom circles and then connect vertices offset to the desired angle. You should only need to draw two triangles. Then copy/rotate them around the center to file in the rest of the faces.

I'm happy to hear you're getting good results on Wine. Of course the SU GUI for the warehouses won't work but we know why, right.

xy mosian · 17 Mar 2015

This is probably totally irrelevant but I believe the profile of a cooling tower is a parabola. Now how to draw a parabola in Sketchup is way beyond me, but that and the Follow Me tool might be another way of doing it.
xy