Stuck on Simple IK

I feel a bit silly posting this, but I’m evaluating Bongo right now, and not having much luck getting simple results:/

I’m trying to duplicate the door hinge tutorial video, but hitting the wall, right off the bat.

Animations that I’d be using Bongo for would be so much more complex than this that it seems daunting…Anyway, here’s a few screen grabs that show this teeny stumbling block:


It’s a bit hard to see, based on your screengrabs, what is going wrong. Can you give some more info or better still, post the model you are working on.

Hi Luc,

Pardon the delay - been analog the past little while. Thanks for even trying to decipher my query. I realize I should have included the file.

I’ve tried again, and still not getting results as I’d expect. What I’m finding unclear is the role grouping plays in IK (if at all). I mean grouping objects with points that represent pivots or constraints of said objects.

I’ve attached the file I’ve been playing with. Any help is most welcome.


door hinge_EM.3dm (225.4 KB)

Grouping definitively is NOT playing a role in IK. Parent/child relationship is too. On you find a little PDF tutorial that deals with these matters. I feel you might benefit from reading this while examining the accompanying models.

A child acts (moves, rotates, scales etc…) just like his parent thereby using the pivot of the parent. That’s what is going on with Object 2 in your model. By making it a child of the door (Object 2) it will rotate around the pivot of the door. Object 2 is needed to locate the point where the door pump arm is to grasp. In fact the door is hardly playing any role in this IK structure; it is the spindle on the door that will activate the door-pump arm. This is a “pulled” chain in terms of my tutorial.

You don’t need points to represent pivots. The advantage of a point is that his pivot by default coincides with itself. Other objects (surfaces or solids) get their pivot by default on their center of gravity. I see you already figured out the technique of relocating an objects pivot (as you did with the door).

The pomp’s arm needs a simple hierarchy structure which I installed for you in this model: door hinge_EM 001.3dm (262.6 KB)
The point ‘point on end of chain’ is needed to mark the end of ‘link 2’, being the element which will grab the door.
I replaced your point Object 2 by a line because the point point wasn’t exactly on the same altitude of ‘point on end of chain’ which would create a problem for the IK solver. By making Object 2 a line the constraint can locate itself at the desired altitude.

I hope this gives you a start. Any more questions? Just post!

Thanks for taking the time in providing the samples and explanations Luc. Much appreciated.
I read through your Explications .pdf and accompanying .3dms. Cool stuff. Brought up lots of questions for me. For instance: why does a point require a tracker point that occupies the same location?
Anyway, back to the door hinge animation:

  • the idea of using a curve instead of a point to alleviate altitude (in)tolerance is interesting.
  • I’m still not able to make the point on end of chain constrain to object 2 (curve on door) in a way that forces Link2/Link1 to ‘follow’ the door’s movement. You can see what I figured would work, but does not in the attached file.

Again, your help getting this into my little brain is appreciated.


door hinge_EM 002.3dm (257.0 KB)

HA! got it! Finally:)

Thanks again for your help Luc.


Congrats. In your 002 model the pivot of the ‘point on end of chain’ was moved away from the point.

I’m not sure I understand your question correctly.
I assume it is the matter I tried to explain by means of figure H in the tutorial. Also you may be confused by the fact that I only use point objects in the example models. They all work the same way when solids (or surfaces or curves) are used.
In this model snakes.3dm (257.4 KB) you see 5 identical chains. When the head is animated (moved, rotated, scaled) the attached links (children, grandchildren etc… ) perform the same action. When on the other hand the tail is animated directly, it acts by itself relative to his parent(s), hence kind of disturbing the initial setup of the chain. The only way to get the chain work as a hole is to constrain the tail (or another link in the chain) to an object. The constrained object tries to keep his pivot somewhere on the target. When you want the constraint to follow the target exactly it is necessary to use a point.

Keep on rocking. Luc

Interesting. Thanks for taking the time (again) to explain this. Much appreciated!

It seems that there’s much more to Bongo than meets the eye (par for the RMA course, of course:).