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Inverse Kinematics

In a previous page, we saw how to pose an armature, bone after bone – using the so called “forward kinematics” process. But Blender knows another posing method, called “inverse kinematics”.

Basically, the IK idea is to rotate (and optionally scale) automatically the bones of a chain, to follow as tightly as possible a target object’s position (and optionally rotation). In other words, it’s a way to pose an armature using just a few control elements (a bit like using hooks to deform meshes…).

Remember that inverse kinematics is only relevant with chains of bones.

There are two main types of IK:

Targetless IK

As the name implies, this is an IK-constraint which has no target. This is applied to a child bone, and influences its parents' rotations. Bear in mind that to retain a desired pose, you must keyframe every rotation value, of every bone. This kind of IK will also ignore constraints on the bone itself, only use it for constraint-less free chains.

Targeted IK

This is the original and standard type of IK, where an object or bone (uninvolved in the IK chain) is used as target. Here, you never have to key the IK chain bones, as they are permanently adjusted to follow the target. However, you must key the IK target's location.

Targetless IK

This is the “simplest” IK tool. You enable it through the Auto IK button of the Armature panel (Editing context, F9).

This is a targetless-only IK, without any of the control options you have with the constraint described below. In fact, it just dynamically creates a “virtual” targetless IK constraint each time you start to grab (G) a bone, using the whole length of the parent part of the chain this bone belongs to.

This implies that hitting G with a selected connected bone switches no more to the rotation tool, when the Auto IK option is enabled… !Be warned that automatic IK also means that you can not set up constraints for it.

If you have several selected bones, a temporary IK constraint is created for all of the most-parent bones of each involved chain.

Auto IK tool examples.

One bone initially selected.	G just hit – note the orange bone color (as with a targetless IK constraint), and the dashed line to the root of the temporary IK chain.	The tip bone if the IK chain grabbed…
Three bones initially selected, two of them belonging to the same chain. during grabbing – note that only the two “most parent” bones became IK chain tips.

IK Solver Constraint

In Blender, using inverse kinematics mainly goes through the IK Solver constraint, which is only available for bones, in Pose mode.

You can of course add an IK constraint using the standard way (i.e. via the Add Constraint button of the Constraints panel, see the constraints chapter), but you also have a specific shortcut in the 3D view. When you hit ⇧ ShiftI (or Pose » Inverse Kinematics » Add IK to Bone…), an Add IK Constraint menu pops-up, with a few options to add an IK constraint to the active bone:

• If you only have one selected bone:
  • To New Empty Object – This will create a new Empty, located at the tip of the active bone, and add to this bone an IK constraint using the empty as target.
  • Without Target – This will add to the active bone a targetless IK constraint.
• If you have another bone or object selected:
  • To Active Bone – This will add to the active bone an IK constraint, using the secondly selected bone as target.
  • To Active Object – This will add to the active bone an IK constraint, using the secondly selected object as target.

Error message If you select two bones in the same chain, Blender will refuse to create the IK constraint, with the error message “IK root cannot be linked to IK tip”… However, in an existing constraint, you can set a bone of the same chain as target, don’t know why…

Note that you can only add one IK constraint this way, else Blender complains with the error message “Pose Channel already has IK”. Anyway, it seems only the first IK constraint in a bone’s stack is effective…

If you want to remove the IK constraint of a bone, you can select it, hit CtrlAltI (or Pose » Inverse Kinematics » Clear IK…), and confirm the Remove IK constraint(s) menu that pops-up. Note that this will delete all IK constraints a bone may have.

We saw in the visualization page that an IK constrained bone turns either Orange (for a targetless constraint) or Yellow (for a targeted constraint). In addition, a dashed line of the same color is drawn, between the tip of the constrained bone, and the root of the root bone of the IK chain – all bones between those two included are affected by the IK constraint.

Targetless

The IK Solver constraint is an exception, inasmuch it is designed to work even without any target specified (i.e. when it’s in “red state”).

In this case, it behaves exactly as the Auto IK editing option – but with all the advanced settings of the constraint (see below).

Just remember that with a targetless IK constraint, the bones of the IK chain are not permanently controlled by the constraint – you have to key them if you want to animate them…

Targeted

This is the classical version of IK. When a target is specified (as any constraint, either an object – typically an empty – or a bone), the IK constraint will transform the bones of its chain so that, as much as possible, the tip of its owner bone follows, tracks the center of the target.

When targeted, the constraint permanently affects its chain, so this is an easy way to pose and animate an armature, just by controlling a few external objects. You have no more to pose nor key the bones belonging to the IK chain (even though it is still possible, for very specific needs…).

Targeted IK example.

A bone IK constrained to an empty target.

The target selected…

…And grabbed.

Tip In general, you’ll parent the targets of the IK constraints to their armature, so you can move the armature without deforming its pose…

Options

The IK constraint has quite a few settings to control its behavior.

Target

The object or bone to use as IK target (if void, the targetless IK behavior takes place). As with any other constraint, if you specifies the name of an armature in the first OB field, you can specify the name of one of its bones in the BO field that appears below, to use this bone as target. Similarly, if you specifies the name of a mesh or lattice object in the OB field, you can specify the name of one of its vertex groups in the VG field that appears below, to use this vertex group’s center as target.

Pole Target

This second target allows you to define the “pole” of the IK chain, which is the direction in which a knee, or middle joint in the chain, should point track to. It behaves as any constraint target option (i.e. you can use an object, bone or vertex group as pole target).

This is most useful in knees and elbows, but would also work to allow you to control which way a tail “crinks” when compressing. Parent an empty to the armature, and then use it as a pole target. Animate the location of that empty, which would guide the bone chain’s orientation as it bends. For knees, float the empty out in front of the leg (study mocap to see how the leg/hip rotation results in knee orientation during the walk cycle). For elbows, float the empty behind the arm, and study mocap to see how the elbows fly out or stay tucked in for different movements.

Pole target example.

The selected empty is now the pole target.	Moving it on the other side of the “root-tip” line of the chain “mirrors” the chain.
The same set, but side view – the pole target is in the same plane as the armature.	Now, the pole target is “going down”, and the IK chain “tracks” it.

Use Tail

When enabled, this button indicates to Blender to use the tip of the constrained bone as the last element of the IK chain (remember that currently, in Blender UI bones’ roots are called “heads”, and bones’ tip, “tails”…). This is now the default behavior (and the most intuitive one), but in older versions of Blender, it used to be the owner’s root, and not its tip, that was the last IK element – which implied that the constrained bone didn’t participate to the IK solution…

ChainLen

This setting allows you to control the length of the IK chain, in number of bones. The default 0 value means “use as much bones as possible”. A value of 1 limits the chain only to its owner bone (having more or less the same effect as Track To constraint…), etc.

ChainLen example.

An IK chain of 0 length (as there are four bones in the chain, you’ll get the same result with a ChainLen of 4 or more).

An IK chain of 2 bones length.

PosW

The “position weight” setting is only relevant in case of tree IK (see below). It specifies the influence, the importance this IK constraint should have in the global tree IK solution.

Rot

When enabled, this button forces the last element of the IK chain (i.e. by default, the constrained bone) to follow the rotation of its target (both in global space).

W

The “rotation weight” setting is only relevant in case of tree IK (see below), when Rot is enabled. It specifies the influence, the importance this IK constraint rotation should have in the global tree IK solution.

Follow target rotation example.

With Rot disabled.	With Rot enabled.
	With Rot enabled, and the empty target rotated.

Stretch

When enabled, this button allows the IK constraint to stretch (i.e. scale) the bones in its IK chain, to adapt there length to the target’s position.

Note that:

• Enabling this option won’t be enough, you’ll also have to adjust the bones’ Stretch property to non-null values (see below).
• With tree IK, its behavior is quite strange – it seems that it only works (more or less…) when it is enabled in only one IK constraint.
• The bones are stretched to avoid as much as possible their rotation (i.e. to maintain as much as possible their original rotation) – in general, this implies that bones “aligned” with the target translation are scaled more than bones “perpendicular” to it…

Bone stretching example.

The IK chain in “rest position”.	The target moved, with the three first bones stretchable (note how they scale differently).
	The same target position, with Stretch disabled…

Iterations

The maximum number of iterations allowed during the IK solving calculations. The higher this value, the more precise the IK result, but the heavier it gets to compute… The 500 default value should be enough in most situations, only very complex ones (like multi-level tree IKs) might require you to increase this setting.

Controlling Bone Rotation and Scale

You can further control an IK chain with each of its bones’ settings. Indeed, the sub-panels of bones implied in one or more IK chains (found in the Armature Bones panel) have an extra set of options, which allow you to block/limit there own rotation and scale (there “degree of freedom”, or DoF), during the IK solving process.

Do you remember the elbow example of the constraint page (you can’t rotate a human elbow backward, etc.)? Well, you can “constrain” IK bones the same way with these controls – with further advanced options, like rotation stiffness…

Rotation locks and limits are defined with respect to the rest position of the bone.

Lock X Rot, Lock Y Rot, Lock Z Rot

When enabled, these buttons prevent any rotation of the bone around its relevant axis(axes).

Stiff X, Stiff Y, Stiff Z

Only available when the relevant “lock” button is disabled. These numeric fields controls how “hard” it is for the IK to rotate the bone around its axes, from 0.0 (no stiffness) to 0.990 (nearly blocked – however, it does not prevent you to manually rotate the bone, as do the “lock” buttons…).

Limit X, Limit Y, Limit Z

Only available when the relevant “lock” button is disabled. These options allow you to limit the rotation of the bone within a given range. When enabled for a given bone axis, two numeric fields appear, where you can set the minimum (from -180° to 0°) and maximum (from 0° to 180°) allowed rotations.

There is a goody for the Limit X and Limit Z options: when enabled, these limits are materialized in the 3D views, as a red arc of a circle for the X rotation range, and a blue arc of a circle for the Z rotation range – and when both limits are enabled, you also get a semi-transparent gray section of a sphere, materializing the whole area the bone’s tip is allowed to reach. Obviously, the Limit Y is not concerned by this feature, as it limits the bone’s roll…

Stretch

This value is the “reversed stiffness” of the bone IK scaling. It only has an effect when the Stretch option of the IK constraint is enabled. It ranges from 0.0 (no scaling allowed) to 1.0 (no stiffness in stretching).

Tree IK

Blender supports “tree IK”, i.e. multiple IK chains sharing a common part. The bones belonging to this common part are simultaneously affected by all the IKs – there respective influence determined by their PosW (and optionally RotW) value(s).

Tree IK example.

A tree IK: both IK chains share the same root bone, as shown by their “IK line”.

The “cross” empty has been translated, which affects both IK chains.

Same situation, but the PosW setting of the selected bone IK constraint has been set to 0: it affects no more the other IK chain.