Null Problem

[Mike Moon]

I attached my model to a cube instead of a null this time. I moved the null on top of the model. Then parented the model to the cube. Pressed one frame forward and the cube stayed in place and moved the ladder as if the cube was in 0,0,0. Same problem. I don’t know what else to do.

[Adam Trachtenberg]

When you parent an object to another object, the child essentially has it’s own local coordinates, but those coordinates are relative to the parent’s coordinates. Say, for example, you have your cube at 0,0,100 and animate it to 0,0,200. Those coordinates are relative to world 0,0,0.

Now you create a null at 0,0,100 and drop the cube into it. Now the cube’s position is relative to the null’s position, so it thinks that 0,0,100 is actuall 0,0,0. Consequently, it will jump to 0,0,100 in the null’s space, which unfortunately is 0,0,200 in world space.

There are a number of ways around this. The most straightforward is parent the cube *before* you animate it.

Or, instead of parenting, you could use Xpresso or set-driven keys to have the null’s position drive the cube’s position. To do that, go to the null’s Coord. tab in the attributes manager (not the coordinates manager) and right-click on one of the “P.”s (for Position). From the menu, choose Animation>Set Driver. Then repeat for the cube, but choose Set Driven (relative).

A third way to do it would be use a constraint tag on the cube, with a parent or PSR constraint. The key in either case is to check the “Maintain Original” box in the Offset section.

[Mike Moon]

Thanks for your thorough answer. I guess I should just try to animate before I start adding nulls. It’s just that sometimes, I animate and then later, I use the nulls to tweak the positioning. Of the two other option, the Set Driver and Driven Xpressos seemed to work. I would just need to adjust the nulls underneath which isn’t that big of a deal. I probably should work with less nulls! The constraint tags didn’t really work for me in this case. Thanks for your help!