Splines, and their direction...

[Kelley]

Can someone explain the 'direction' of splines? I have never understood that.

[Rodney]

Can someone explain the 'direction' of splines? I have never understood that.

 

There are a few topics that discuss this but we'll have to ferret them out.

 

As a starting place I'll say that every spline has an origin (initial Control Point) and all other Control Points extend from that point.

It's not always easy to know which is the origin and we can trick A:M into assigning new points.

 

The example of this is to create a two CP Path in a Choreography and then alternatively extend that path in one direction and then delete that extra spline length and extend the spline from the other direction. An object with a Path Constraint will assume that the newly added Control Point is an indicator of the direction of the spline.

 

1 ------- 2

X ------- 1 -------- 2

1 ------- 2 -------- X

 

Here the X indicates a new Control Point added to the spline.

In the first case the new CP between 0 and 1 has the spline moving to the left.

In the second case the spline is moving to the right.

 

I suppose the downside of this is that the direction of a spline can change based on additions to the spline if we expect them to maintain the same direction.

[robcat2075]

"spline direction" in modeling is usually a question of how 2 splines intersect or how the ends of splines are attached.

 

If you consider what appears to be an

 

X

 

it could be either two straight splines crossing each other or two L-shaped splines that meet, then turn in opposite directions.

 

>

 

Two straight splines crossing is the right way. Two Ls sharing a corner is wrong.

 

The video in this post will introduce you to some related problems of connecting splines...

 

http://www.hash.com/forums/index.php?s=&am...st&p=365435

[Rodney]

I hope this isn't too esoteric, but I found this written by Howard Trickey (author of the AI and Font Wizards). He had this to say regarding some of the difficulty he experienced in determining the direction of splines to ensure proper continuity:

 

...construct the splines in a specific order (do the closed outlines first, THEN attach the splines that connect them, so that they were always

connecting to CPs in the "middle" of splines) in order to avoid unintentional spline unifications.

 

Perhaps you should ask Dan for a flag option to AttachCPs that would prevent this behavior when it is not desired. In the absence of that, you might need to put in a test to see if the CP you are attaching is at the end of its spline, and if so, add a temporary extra CP to extend the spline, do the attach, then delete that extra CP (mimicing a trick that one sometimes has to use when modeling by hand).

 

I believe this and other early plugin programming information still resides in the yahoo group where they discussed the development information.

 

The point of posting this is to emphasize that it isn't always clear which way a spline is directed. The test that can be performed to ensure a particular direction is to add and then immediately delete a new CP at the end of to the desired direction of the spline. By doing this (and at least in theory) we can always guarantee the splines direction.

[Kelley]

Thanks, guys for the come-back. That cleared things up a bit. I had always seen the matter of 'direction' as being one where Spline A had a left-right direction, while Spline B might have a right-left. And no way tto tell which is which, or what the import of that diection might be. Once rid of that basic misconception, things look much better.

 

Nice bit of sleuthing, Robcat, on seeing that table top as one whole spline that ran all the way around.