6.5 – Animation – Inverse Kinematics and Constraints

In Pose Mode, we have seen that within an armor, and in particular a chain of bones, the transformations applied to a single bone are only inherited by the subsequent bones and not by the previous ones. Armors therefore follow a direct kinematics. Now let’s select the last Bone of any chain (still in pose mode) and go to the Bone Constraints properties context, icon , here we see that the individual bones of an armor have specific constraints (identical to those seen in paragraph 6.4) due to the fact that the usual constraints in the Constraints context will act on the entire armor instead. Let’s see the operation of the interesting IK constraint (inverse kinematics).

With the default options, we just need to move (press G) the last bone in the chain (which has become amber) to see the entire chain adapt smoothly and organically to this movement. This will be useful when we need to accurately place only the last bone in the chain, while allowing the rest of the armor to adapt automatically (respecting any chosen rotation, translation, and scale limits).

The number of bones involved in this process is decided in the Chain Length field of the constraint; the default value of Zero extends the inverse kinematics to the entire chain. By inserting an object, for example a cube, in the target field, the bone’s tail will follow and point to it despite the absence of parentage links (in this case, the bone with the IK constraint modifier will turn yellow).

Subsequently, we can insert another object, for example another cube, into the scene and apply a Copy Location Bone Constraint to the first bone in the chain (with the new cube as the target), thus finding ourselves with our chain constrained between two objects and able to adapt naturally to their translations and other transformations depending on the Constraints used.

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