View Full Version : Need help with "-knot" in command "curve"


bsdzhu
09 September 2008, 01:27 AM
Hi, everyone,
I am working on the command "curve" recently. Using "-p" and "-d", i have been able to create beelines and curves.
But i find it difficulte to comprehend 'flag "-k"'.
I wonder if anyone can help me. A simple example will be much useful.
Thanks!

sj_bee
09 September 2008, 10:34 AM
knots are needed in the construction of NURBS curves to define how the shape of the curve is calculated between the control points. A curve needs degree+cvs-1 knots when it is defined.

you can read about them here

http://devworld.apple.com/dev/techsupport/develop/issue25/schneider.html


these curves have the same control points but the shapes are different cos they have different knot values.

curve -d 3 -p 0 0 0
-p 0.666667 0 1.151515
-p 2 0 3.454545
-p 4 0 -1.818182
-p 6 0 3.818182
-p 8 0 -1.454545
-p 9.333333 0 0.848485
-p 10 0 2
-k 0 -k 0 -k 0 -k 1 -k 2 -k 3 -k 4 -k 5 -k 5 -k 5 ;

curve -d 3 -p 0 0 0
-p 0.666667 0 1.151515
-p 2 0 3.454545
-p 4 0 -1.818182
-p 6 0 3.818182
-p 8 0 -1.454545
-p 9.333333 0 0.848485
-p 10 0 2
-k 0 -k 0 -k 0 -k 0 -k 0 -k 0 -k 0 -k 0 -k 0 -k 0 ;

Notice that it doesnt matter what the not values are, it is their spacing which affects the shape of the curve. The knot values on these curves are the different, but their spacing is the same so the curves are the same.

curve -d 3 -p 0 0 0
-p 0.666667 0 1.151515
-p 2 0 3.454545
-p 4 0 -1.818182
-p 6 0 3.818182
-p 8 0 -1.454545
-p 9.333333 0 0.848485
-p 10 0 2
-k 0 -k 1 -k 2 -k 3 -k 4 -k 5 -k 6 -k 7 -k 8 -k 9 ;

curve -d 3 -p 0 0 0
-p 0.666667 0 1.151515
-p 2 0 3.454545
-p 4 0 -1.818182
-p 6 0 3.818182
-p 8 0 -1.454545
-p 9.333333 0 0.848485
-p 10 0 2
-k 0 -k 2 -k 4 -k 6 -k 8 -k 10 -k 12 -k 14 -k 16 -k 18 ;

If in doubt, just space your knots equally

NaughtyNathan
09 September 2008, 10:57 AM
Thanks Sam, that's really good and informative reference. Maya's docs have an awful explanation of what knots are and I've never understood them, but now I think I get it.. :)
cheers!
:nathaN

bsdzhu
09 September 2008, 05:12 AM
Good example and Useful LINK.:arteest:

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09 September 2008, 05:12 AM
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