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spline end tangency dumped (#1928)
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jol  
#1 Posted : Friday, December 26, 2008 5:18:19 AM(UTC)
jol

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Convert a Control Point Spline into an Interpolate Spline

Tangency on the endpoints is lost

so this seems to be a cr@p conversion !

see image

WE NEED BETTER SPLINE TOOLS
jol attached the following image(s):
tangency lost.jpg (21kb) downloaded 5 time(s).

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bobc  
#2 Posted : Friday, December 26, 2008 5:48:13 PM(UTC)
bobc

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Hi Jol -
I think what you meant by tangency on the endpoints is lost. Is that the radius of curvature at the endpoints has been change to and infinite radius of curvature.

This characteristic is one of my pet peeves about interpolated spline routine found in most every CAD system I have uncounted. The ends of the spline are relaxed and no attempt is made to carry curvature through to the ends.

If you have every set a spline (batten) to a set of points, you will know that extra spline weights are needed beyond the end points to have the spline fair properly.

I spent 43 years with Douglas, Northrop & Boeing as a loft engineer and we never used this type of spline routine, we wrote our own that emulated the true fairing process.

I need to talk to Tim about this.

Robert Chubb
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jol  
#3 Posted : Friday, December 26, 2008 6:19:01 PM(UTC)
jol

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Robert, excellent - yes you should !

You probably know more about these things than anyone

- this losing the end tangency is one issue, others include

- simply the purity with which they're shown on screen - we have lost AntiAlias here on the Mac

- You need to use nice clean interpolate splines in order to get good cross-overs for editable network surfacing, but every time you offset them, split them, join them or breathe too hard, they drop back into control point splines, which you then have to redraw (ideally)
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unique  
#4 Posted : Saturday, December 27, 2008 6:50:18 AM(UTC)
unique

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Quote:
Hi Jol -
I think what you meant by tangency on the endpoints is lost. Is that the radius of curvature at the endpoints has been change to and infinite radius of curvature.


Hi Bob - Glad you could join in and I hope we see a lot more of you in cases when nurbs expertise is needed ;)
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jol  
#5 Posted : Saturday, December 27, 2008 8:29:18 AM(UTC)
jol

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>>I think what you meant by tangency on the endpoints is lost. Is that the radius of curvature at the endpoints has been change to and infinite radius of curvature.

Robert ..

I've been watching this and .. it just happens - I can't predict when. There must be a trigger of course .. but both endpoint tangents becomes (effectively) zero without warning

With really long slow splines, it's dangerous, cos I can't see them relaxing

However, as somebody else noted (I think Steve) - it seems to happen most on splines that begin and end at the same point

Hopefully we can track this down

This loss of tangency and the lack of (ability to make) spline symmetry are my main concerns
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jol  
#6 Posted : Saturday, December 27, 2008 1:14:01 PM(UTC)
jol

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It just happened again - and is driviing me nutttZ

Look at the spline end tangents - they've all been dumped !

This was after a scale command - see pic
jol attached the following image(s):
spline endpoint tan dump.jpg (48kb) downloaded 5 time(s).

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Steve.M  
#7 Posted : Friday, June 12, 2009 10:07:27 PM(UTC)
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The changing of a CP spline to Int spline is still a problem.

Currently I have to first convert to a B-spline, which does retain Angle to work plane, then to change that to an Int spline, which does then again retain Angle to workplane and the angles are checked in the object info data.

There is a slight drift along the spline, about 0.03mm on my checks, but at least I dont have to realign the endpoint angles.

EDIT:
Unfortunately the start position of a closed CP spline moves when converted to a B-spline.

What is it with this program and splines :(

- Steve
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Tim Olson  
#8 Posted : Monday, June 22, 2009 11:03:39 AM(UTC)
Tim Olson

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>>Convert a Control Point Spline into an Interpolate Spline

>>Tangency on the endpoints is lost

Jol

Fixed in 838.

Tim
Tim Olson
IMSI Design/Encore
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Tim Olson  
#9 Posted : Monday, June 22, 2009 11:06:00 AM(UTC)
Tim Olson

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>>Unfortunately the start position of a closed CP spline moves when >>converted to a B-spline.



I could not repeat this. If anyone has an example file that would be very helpful.


Regards

Tim
Tim Olson
IMSI Design/Encore
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