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Action Constraint doesn't evaluate B-Bone properties
Closed, ArchivedPublicTO DO


System Information
Windows 7 x64
NVIDIA GeForce GTX 580

Blender Version
Broken: 2.79 5bd8ac9

Short description of error
Action Constraint doesn't evaluate B-Bone properties

Exact steps for others to reproduce the error
Create an Armature, and then an Action with keyframes on a bone's B-Bone properties channels. Assign an Action Constraint to said bone using this Action. When the Target Object is transformed, the constrained bone's Location/Rotation/Scale will change as specified in the Action, but any B-Bone properties animated in the Action stay the same.

In the attached .blend, the Action has keyframes for both Location and B-Bone Curve In Y. The Action Constraint on Bone.001 uses Bone as a Target Object. Move Bone +Y and Bone.001's Location changes as expected, but the B-Bone Curve In Y stays the same.

Event Timeline

Joshua Leung (aligorith) lowered the priority of this task from 90 to Normal.Sep 25 2017, 6:02 AM
Joshua Leung (aligorith) edited a custom field.

As per our tracker policy, technically this isn't considered a bug.

This is because the constraint system is only supposed to operate on the transforms of the object/bone it operates on (i.e. strictly, what can be handled by affecting the 4x4 transform matrices of those entities; as you might've noticed, things like custom properties are also unaffected by the Action Constraint). As the Bendy Bone controls are not included in these matrices, by definition, this use case is not supposed to work.

That said, I can understand why this might be useful. I've noted this as one of the things to consider for future changes in this area.

Oh, I see! I thought the B-Bone curve properties were considered transforms. Sorry about that.


Closing this as invalid, as the constraint system is made to only manipulate the transforms. You can use drivers on the BBone properties instead, to have the values be computed from other properties.