Single Point Matched Curve Tutorial

[VIRTUALENVIRONS]

The object that is completed using a set of matched curves that turns 180° and then is joined with no seam using the "close" option in the Loft NURBS dialogue box.  

 

Although this object has no engineering value its geometry is a good example of how Matched Curves can help is constructing complex objects.

 

 

[line-weight]

@VIRTUALENVIRONS I have a couple of requests.

 

- Please can you try and keep discussions relating to a particular topic in one thread where possible. I think there are now 4 or 5 separate thread where you mention this basic concept, which you call "single point matched curves" and still it's unclear what is special about it. It makes it quite annoying to engage in any discussion because it abruptly comes to a halt on one thread, and the only way to find the continuation of it is to happen to come across some other thread. Plus, the work anyone puts into making any points in response gets lost. If you are having trouble re-locating threads you previously started or posted on, use the "follow topic" button whenever you contribute.

 

- Please can you post the file itself that you show in the video, so that we can examine it for ourselves. If you are genuinely interested in us understanding what you are trying to say. The video is too short and fast for anyone to follow and replicate all the steps properly. For example, at the end where you are showing the isoparm you don't show what happens at the critical bit which is where the "seam" would be and where VW closes the curve.

[VIRTUALENVIRONS]

Hi Line-weight,

Thank you for the comments.  The curves are already posted in an earlier post as you pointed out in a separate thread.  This is just a Tutorial off my Youtube channel.

 

Unfortunately, this topic seems to be like the curve itself, it has no end, but it also seems to exact a lot of emotion.  If you notice I don't call the curve what it is.  I have to describe it in a paragraph when one word would do.

 

Single Point matched curves are basic curves, but they can have several forms.  If you notice in my tutorial I create a curve with 24 points, but if you put a circle down and use "Convert to NURBS", you would only have 6 points and they are much different.  Their controls points are off the curve.  

 

The point to take away is that build your own curves so Vectorworks is not forced to make any decision on how they will be interpreted.

 

If you have any questions on building curves, etc., just ask.

 

regards....Paul

 

[line-weight]

9 hours ago, VIRTUALENVIRONS said:

Hi Line-weight,

Thank you for the comments.  The curves are already posted in an earlier post as you pointed out in a separate thread.  This is just a Tutorial off my Youtube channel.

 

right, but how is anyone coming across this thread supposed to be able to find it?

 

Anyway, the file is here:

 

 

[line-weight]

14 hours ago, VIRTUALENVIRONS said:

The object that is completed using a set of matched curves that turns 180° and then is joined with no seam using the "close" option in the Loft NURBS dialogue box. 

 

The thing is, the "matched curves", more conventionally called the "profile curves" in the loft command, they don't really turn by 180 - because each curve is not the same. They have to be individually made (in a very laborious way) so that they each have a different start and end point. What happens is that the profile shape is distorted as it runs around the path, so that when it meets itself again at the "join", the two profile curves that are joined are not rotated relative to one another.

 

Ok, so you might say this is pedantic, but the point is that it severely limits what can be achieved with this method, because it doesn't work for any profiles with sharp edges. Only curved shapes that can be smoothly transitioned between each other. That's why it doesn't work for the mobius strip.

 

 

[line-weight]

10 hours ago, VIRTUALENVIRONS said:

Single Point matched curves are basic curves, but they can have several forms.  If you notice in my tutorial I create a curve with 24 points, but if you put a circle down and use "Convert to NURBS", you would only have 6 points and they are much different.  Their controls points are off the curve.  

 

The point to take away is that build your own curves so Vectorworks is not forced to make any decision on how they will be interpreted.

 

 

And there's another problem, which is that the above isn't actually true, as far as I can see.

 

Using the 24 locus points method does not produce an oval with the same geometry as the one produced by the oval tool. This is visible in the tutorial video. Here is a zoomed in view of the original oval (red) and the one generated with the 24 points (green). They don't match and the green one has become asymmetrical.

 

 

I believe this also means that all of the versions of the oval used to create the loft are not quite the same (because of the different start/end points).

 

This may be why the resulting solid object is ever so slightly messy.

 

There are also some issues in controlling the exact rate of "rotation" around the path/rail because when you "rotate" by one of those 3-segment steps you are not rotating by exactly 30 degrees. And, if instead of rotating by 180 degrees, you wanted to rotate by some non-regular angle, say 87 degrees and then back again, you'd have to make the ovals using a huge number of locus points (360, I think).

 

Doing a convert-to-nurbs on an oval does appear, as far as I can see, to produce an oval that is geometrically the same rather than an approximation.

[VIRTUALENVIRONS]

Hi Line-weight.  You have a lot of stuff here to discuss.

 

You have given this a lot of thought.  I did not think anyone would forensically go through as you did, well done.  Full disclosure, tutorials can be tricky to write.  You get better after a while, but people take short cuts.  See Point 1

 

"That's why it doesn't work for the mobius strip."   This is not a Mobius Strip remember, simply how to close a sequence of curves that turn 180 degrees.

 

1 - Even though I show picking off 24 points I only picked off 12 to show the method for speed sake, so the curve is a little wonky.  24 would be smoother and more accurate, 48  even better etc.  I suppose it could be considered like conversion factors in VW.

2 -  Matched curves are profile curves, but a better description is profile curves that are matched.  If you take a half circle and use Convert to NURBS and then use the method I used, they are not matched and in sequence will produce a different looking object.

3 - Matched curves are not a secret.  They are as basic as using the extract iso-parm.

4 - Profile with sharp edges will work, but perhaps not using the closed scenario, I have not looked at that.

 

NURBS curves are not the same as 2D primitives as you no doubt are aware, but doing comparisons of them is not useful either.  NURBS are a different beast.

 

In summary.  Although I have written tutorials for Vectorworks itself at there behest, there is more I don't know about Vectorwoks NURBS than I do

 

regards...Paul

[line-weight]

That's all fair enough.

 

In my opinion, NURBS modelling is not very usefully implemented in VW, and it really doesn't help that there is no proper documentation.

 

Unfortunately, examining the method you describe doesn't change my mind.

 

To generate that particular geometry, I think it would be more efficient, more easily editable, and give a cleaner result, to use the subdivision tool.

[VIRTUALENVIRONS]

As you say fair enough.  Each of us will have our own methodology.  I enjoy discussing.  

 

BTW, I used change vertex mode to alter the corners to sharp in some of the curves and it still closes.  Mysteries of Vectorworks.

regards....Paul