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With Linked/Appended bones with Child-Of/IK constraints, changing the Influence doesn't always work.
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--- Operating System, Graphics card ---
Windows 7 Home Premium 64-bit, Nvidia GeForce GT 545

--- Blender version with error, and version that worked ---
Blender 2.68a & Blender 2.69 RC2, This bug seems to be present in all the versions I've tested it on, which is 2.63a and up.

--- Short description of error ---
Whenever I link or append a bone, and put a child-of or IK Constraint on it, and have the target be a bone from a rig that originated in another .blend, and set keyframes to change the influence, the influence only seems to change if I'm focusing on the bone the constraint is on.

--- Steps for others to reproduce the error (preferably based on attached .blend file) ---
-Open a new .blend.
-Make a new Meta rig (or make your own armature).
-Link or append an armature from another .blend.
-Give that bone, or a bone from the meta/created rig, a Child-Of or IK constraint (other constraints may have the problem as well), and have the target the other bone/rig.
-Make 2 or more keyframes, the starting one with the influence set to 0, and the final frame with the influence set to 1, it's a good idea to have the positions change as well, so you can tell if it's worked or not. (For example, have the linked bone fall, and have the armature catch it.)
-Play the animation one focusing on the bone with the constraint, so you know how it's supposed to work. Then play it again, but this time, focusing on nothing or another bone/object.

Event Timeline

I've uploaded a video showing what I'm getting, just in case you aren't able to recreate the problem, for some reason, the bug only occurs sometimes.
Sometimes, I open up the .blend where the bug occured on another computer, and sometimes, the animation works perfectly, but other times, it doesn't.

@Bastien Montagne (mont29), is it the same issue with constraint influence you've been looking in recently?

No, don’t think so… here it’s more like missing updates, would eye at depsgraph (since behavior is OK when constrained bone is active, but animated constraint influence seems ignored when bone is not selected…).

Finally found the issue… You are using a single action for two different objects (armatures). Just putting each armature's animations into its own action fixes things.

@Joshua Leung (aligorith) not sure this is expected to work at all? I guess what’s happening is some kind of mismatch in action evaluation when checking which paths are valid?

@Bastien Montagne (mont29): That makes sense, but what I'm making these models for Unity, so how am I going to select/activate both actions if Unity only allows me to pick one?

??? Unity should allow you to select one action per object at least, I’d say?

Yeah but I export them into unity as one model, for example, if a Blender project has a character and all of his/her weapons, I export them all at once, because when I attempt to export them separately, I can't get the animations to work.

Then use a single armature in Blender too. ;)

Any news here? Otherwise I'll just close.

Well, I still have a question concerning this issue. What if I have an animation that involves 2 characters and object being passed from one to another? If I have the held object part of one armature, It'd be a lot of trouble making the other armature hold it, wouldn't it be limiting if I had all the armatures as the one? Am I making any sense?

I kind of understand what you want, but isn't this more a feature request/todo? Is that something that is supposed to work or is that just something you'd like to see? If so, it would be better to get in touch with @Joshua Leung (aligorith) and we can close the report (I'd really like to see it closed ;D).

I'm not too sure, I originally set it up so that I was able to do that, however, I encountered this error. I don't see much point of an option to select the skeleton to have an influence from if it had to be part of the same skeleton as the bone it's influencing from?

Joshua Leung (aligorith) changed the task status from Unknown Status to Unknown Status.Nov 12 2014, 1:31 AM
Joshua Leung (aligorith) edited a custom field.

Closing as a todo. I've been investigating the feasibility of allowing animation to be rooted on groups for use cases like this.