Annoying Curve Normals

[paperfish]

Hey all!

...so i've created a curve and am looking at it's normals at certain points along it. (MFnNurbsCurve.normal)

I've found that it actually returns normals that are on the concave side of a point. That being the case, the normals flip from one side to the other along the curve. How can i get the normals of a curve across one side??

I'll try and attach a picture to show the problem in more detail.

Thanks!

-Paperfish

[joojaa]

You cant its a limitation of the mathematical part of the curve (whan the curve points straight up in its local space k becomes unlimitted wich forma a mtahematical poit of view i can asume you understand is death for a computer calcs integrity), So its underlying mathematical problem porblem.

IF you want to iterate along the curve then its possible to discern a flip but that doesnotmake for a very goood utility node as youd need to sample prhibitivly much.

So you do what i do when i use path animation for instance. You create a ribbon thats linear the one way and cubic the other and make a curve smack middle on this. Now if you manipulete this ribbon you get a nice nonflipping solution because you can read the underlying normal. As a bonus you get free controll over twisting!

[rob-bateman]

Game programming gems 2 details a parrallel transport frame to handle this, the outcome is similar to the ribbon method above...

[paperfish]

Thanks guys!

I had NO idea what you where talking about when you mentioned "parallel transport frames"! : D But i did some reading and found this pretty good PDF on the technique:
http://www.cs.indiana.edu/pub/techreports/TR425.pdf

It's really starting to make sense! Thanks a lot!

-Paperfish

[joojaa]

The ribbon method offcourse has its benefits,

a) its allready defined
its easily manipulateble for controll
c) its essentialy the same solution as paralell transport frames
d) the api commands automativcaly take the underling surface normal into accound if you wish
e) it works on all current solutions and nodes that are already in maya.

And this what could be made for maximal development effor benefit is a command that automatess the making of the nonflipping ribbon form a curve in the firstplace. In wich case you get the benefit of both worlds. thisway you wouldnt need to recreate thing such as path anim curve flow etc etc. But could use them as is. And use the same technology for your custom nodes.

[sagun-manandhar]

Hi Joojaa,

didn't qutie get by your lines :

"So you do what i do when i use path animation for instance. You create a ribbon thats linear the one way and cubic the other and make a curve smack middle on this. Now if you manipulete this ribbon you get a nice nonflipping solution because you can read the underlying normal. As a bonus you get free controll over twisting! "

Could you please expalain a bit more on this ?

/users/rob-bateman