Spline control twists bones


Jack of many trades

Simple animation, 6 sheets of paper coming out of a printer. A chain of bones down one edge to let the paper bend a bit as it comes out and feed up the paper support (inkjet style printer). The bones are linked to a set of nulls as spline control. Funny thing is how it feeds the first 3 pages perfectly but the last 3 twist as they come out of the printer and intersect with the pages already at rest. Tried to lock the axis and tried to keyframe the angles to 0 degrees.

I'm cloning the nulls and bumping each master null for the spline control up a few millimeters. How can I lock movement on certain axises? I even did a limit on movement but the twisting ignores that.

I did notice that when I drop a keyframe to animate on the Z axis it flings the paper to a new location on the spine and I have to move it back to the right location. Been going at this for two days between other work and it's looking like a lost cause for me. The splines are nice for organic movement but may not have as much use for mechanical things (like sifi doors for example that could close then lock in place on a spine path)

Any suggestions welcome. Thanks.

View attachment Printer scene and object.zip


Goes bump in the night
You need to spin the nulls to "direct" the spline direction. Also you can choose which axes are or arent controlled by the spline via the motion options as with other constraints. The Z thing is down to the fact that the Z value (input) represents distance along the sline (from the spline start point), but outputs the position relative to parent.

All spline control detail here...



Jack of many trades
Thanks RebelHill, I'll take a look at that video when I get home with a faster connection.

The thing is that I did double check the nulls now that you mentioned it, but none of them are twisted. Why then is the object twisting slightly as it moves along the path? I'm smelling a bug here some place. I'll try setting up some new objects and a new scene and see if I can replicate the issue. There is some interesting angles being added to keep the path in reach I assume, I quickly tried on this scene to delete the nulls and bones. I then recreated them and as soon as the controlling bone has a second keyframe for me to animate the Z position the angle went from 180 to 180.9 degrees.

Odd, but I'm certainly not near good enough to imagine I'm doing it all right.


Eat your peas.
I don't know if this will fix the issue you described but here's a trick Celshader taught me a while back when I was having issues with the bank getting all screwy on some of my nodes:

1. Go to frame zero and select all your spline path nodes.

2. Open the motion panel.

3. Under Rotation, set the Heading Controller to Spline Control.

4. Press Enter twice to keyframe the nodes.

5. Set the Heading Controller back to Keyframe.

This procedure will align the bank for all the nodes to follow the path and allow you to continue editing the path in Keyframe mode. This usually fixes the occasional '180 flip' issue I run into. Obviously, you can try variations of the above if this isn't exactly what you're trying to fix.

Hopes this helps.



Active member
I tend to leave the Bank Control at Keyframes but explicitly set the Bank value on all spline nodes to 0. Then, I manually adjust the Bank per node as appropriate to the path.

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