Help with Rhino Splines

[andrewjohn81]

once you learn to create curves in Rhino you may wonder why Illustrator doesn't offer both types.

The cv curves are probably the best way to start.

Start with creating a degree 2 curve. Notice that it will take 3 points to create an actual curve. Some other programs don't even show you a curve until you have the minimum amounts of points to create that degree curve.

Make 4 points and hit close and you'll have some sort of elipse. I actually can't stand degree 2 curves, so I'm going to move on. Feel free to play around with them and get comfortable if you wish.

Now make a degree 3 curve. This is closer to what illustrator has. The only difference is the order in which you create the second part of the curve. The first cv is the origin of course. The second would be similar to the handle that you would drag out in illustrator. The third cv would be like the handle for the second point. The fourth would be just like the the point in illustrator that you would normally drag the previous point out of. You'll figure that out pretty quick. At first just experiment with making 4 cv's and figure out how that works. After that ever 3 cv's effect the ones next to it and it becomes more confusing.

I've gotta go for the day, but that should get you started. Hope that helps a bit.

[JimCarruthers]

I guess V4 will have Illustrator-style curves, but the reason it hasn't had them is that such curves aren't as "smooth" as control-point curves, and as such not suitable for a basis for making freeform organic surfaces. Once you get used to them, you will probably like control point curves betterl

[andrewjohn81]

I was playing with that today, but I didn't see that you could edit that curve in that way again. I don't really like it as much anyway, but I still wanted to play with it a bit. If you can't edit it that way I wouldn't see the point really.
Is there a way to go back and edit a curve in that way in V4? I didn't find it, but didn't look all that far. I just get to play with it occasionally when rendering on the other computer or when I have a few minutes between something else.

[OamadeusO]

I know what you mean mate. You get used to a way of working and then have to change.
For me its the workflow. I can click the points, click drag out the handles, hold down space and move the current point. All without changeing tools.

Theres a pluggin here that allows you to copy and paste from illustrator. Not tried it myself because i'am trying to be good. :D
http://www.marcotraverso.it/artifex3d/freeplug-ins.html

[trainee]

Thank you for your replies!


@andrewjohn:
I played around with CVs trying to understand what you have written, and I realised that I was using Edit-Point-mode much too often. Control-Point-Mode seems to be the way to go.
Understanding the difference between the two is very critical, I believe.

Another question that came to my mind: Do I need anything else than degree 3 curves? What do you use?

@amadeus:
Thank you for the link, but I'd prefer to stay in one app. Copy/Paste seems to be too cumbersome.

[andrewjohn81]

Well, I usually only use degree 3 and degree surfacing. If I am trying to keep geometry light then you can rebuild something like a straight fillet to be degree 1 in one direction, and degree 3 in the other.

For very complex surfaces I occasionally use degree 5. This is only when there is a surface when I want a noncalculated bend in the middle. It is also a good cheat if you need some slight tweaking. Many times you can get away with rebuilding to degree 5 and pushing and pulling CVs around. With degree 5 it is much less noticeable than with 3. You will get a much smoother result.

But yes, for the most part, any curved surface is degree 3 if I am building it. It is much easier to get proper parameterization that way. Many times match edge doesn't close gaps and can really F things up. If the two surfaces have matching params then you can just snap the edge cvs together and you will get perfect positional tolerance. If you create a line connecting one cv to another then you can create tangency manually. Many people think this can't be done, but I just think they are dumb. I attatched a couple screen grabs of this because it's difficult to describe.

I was too lazy to upload a bunch of files or anything so I just made an animated gif

I created an obvious example. First it shows the gap, but the surfaces have the same parameters. The first step shows that I moved a cv to get positional tangency. If you want to blend in the middle you could create a curve to both cvs and use the mid snap. In most cases that I have the cvs are so close together that isn't necessary.
The second step shows two lines I created. One was created by making a line from one cv to another, skipping the edge cv. Then I moved the edge CV to that line using the near snap. If you can't change the edge, then you would want to use the other method. I created a line from an adjacent cv on the surface that was not going to change to the shared edge cv. I then used the extend command to get it beyond the next cv. The next step is to snap that cv to the line.
Both of those cases cause the surface to be tangent where the two surfaces meet.
If anyone notices that closer to the screen the surfaces aren't tangent, that's because I only did it for those cvs. You'd have to do it to all the shared edge cvs in order for it all to be tangent. It's a pain in the but, but sometimes the tools fail, and that's sometimes quicker than rebuilding over and over and over in different orders. If you do some more organic shapes you will quickly find out that these complex surfaces can be a nightmare.

edit: oops, typed the wrong hyperlink, duh!
-sorry it's so darn big.

http://andycg.com/temp/CVexample.gif

[JimCarruthers]

Degree 3 is the minimum degree you need for 'freeform' shapes, as you go up in degree each control point has a more subtle effect on the whole curve/surface, technically higher degrees are "smoother" but beyond degree 5 the difference is academic.

[OamadeusO]

Sorry if this sounds dense but what are "degree 2,3,4 etc" curves?

[trainee]

Sorry if this sounds dense but what are "degree 2,3,4 etc" curves?

Splines or curves are mathematical objects described by polynomial functions. The degree of the curve corresponds to the degree of the polynomial. I hope I got that right.

If you're interested in the underlying mathematics, this would be a good start:
http://en.wikipedia.org/wiki/Nonuniform_rational_B-spline (http://http://en.wikipedia.org/wiki/Nonuniform_rational_B-spline)

http://en.wikipedia.org/wiki/Spline_%28mathematics%29


@andrewjohn:
Thank you for your description. I will try this out.
There seems to be a lot more to learn about NURBS-modeling but I'm really enjoying it.

If you have more advice for me or a tutorial, everyone should do, please let me know!

[andrewjohn81]

I went to school for animation and only one of the teachers I had knew much of anything about nurbs, so I kinda learned stuff on my own from reading here and there and from my old boss, who was an excellent modeler in Alias.

I learned on Maya, which was nice because sometimes you could only do things the manual way to get even close.

One of those links sent me to Microsoft for some reason.

I like the graphical way of showing the degree of things a bit better. I'll try and do that when I get home. Maybe I'll bust out my personal edition of Maya to show that. I wouldn't have to save anything.

Even though you tell Rhino to create a degree 3 curve after making 2 CV's it gives you the results of a degree 1 curve. That may be part of the confusion.

Basically though, the degree tells you how many CV's will make up one span. Of course, it doesn't count CV's. If you make only one span, though, and have the minimum number of CV's then you could count the CV's and subtract one.

I'll try and make up a document soon that will explain it. Maybe I'll put it on my never updated website and just post the link straight to the page. I'm so damn lazy with that site. They made us do it in school and I haven't updated it since.

[Glyptic]

I use Rebuild quite a lot. Knock out your orignial curve as a guesstimation, then Rebuilt and pull Conrol Points. You can reset the number of Control Points with Rebuild, so you don't have to think about it when you are sketching. Good for smoothing also.
If you need a middle, use odd numbers of Control Points. And yes, they seem better than Edit Points to me.
I was used to Corel, and missed it at first. Then got used to Rhino, and Corel now seems clunky.
Still getting used to C4D with the selection to play with. But it seems a better compliment to Rhino than Corel.
Rhino is fast and easy once you get used to it. Great for sketching and modeling.

[andrewjohn81]

Ok, I knocked out the first section of this basic nurbs explaination. I'm sure it will eventually grow to an huge size eventually, but it's a start.
I just did it in about an hour, while talking to a girl on IM, so don't expect much. It's definitely a WIP. I'll post a link in the tuts as well, since it is more fitting there, but it's relative to this too.

So here it is (http://andycg.com/NURBS/)

Please let me know if there are any huge problems. You can PM me so as not to clutter up the threads needlessly.

I'll get to surfaces next. It's just curves for now. I'll get to the 3d stuff next, although if you didn't know this stuff it should be easy enough to apply if it makes sense. It's really simple, but suprisingly enough, not all that many people know the basic stuff.

[OamadeusO]

Thanks for doing the work to explain this to me. I'll have a good read of this tonight.

I can see that in the transition from 2D art (illustrator) to 3D i'am going to have to be more carefull about my lines. In my illustrations I can have thousands of points without any trouble.

In 3D nurbs it seems we do not have the luxury to be wastefull...

[andrewjohn81]

You are correct about the not being wasteful. I think it's more, being careless though. Somtimes something won't matter so you leave it. It somehow always comes back to bite you in the butt later though.

So I added some surfacing to that plain old page that I created. I think it's pretty useful.

I remember where I saw most of the similar information. I don't remember the book, but it was showing how to put a round object in a square hole. You actually end up changing the square hole to round, but it was a good tutorial. It was a Maya tutorial, but it can easily be used for Rhino.

I don't know how many people will care, but I did the page in Maya, not Rhino. Maya's a bit faster for most things like this because you can move things around faster. Hope nobody minds. I call out any differences when there are any, but there are very few since it is quite elementary.

here it is -- http://andycg.com/NURBS/ (http://andycg.com/NURBS/)

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