IK Leg Tutorial

Adding the Skeleton

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The leg is positioned and we're ready to add the bones.

Adding the Bones

Select Bones as the Edit item and press the '+' key (Shift and =). This is the keyboard shortcut to add a bone to the object.
Edit item

The leg with the first bone.
This bone is the thigh bone and will be the root of the IK chain.

Check that Move is the Mouse function.
Move the bone to the top of the leg and create a keyframe for it.
Mouse function
New bone position

Bone moved to top of leg.

Now we want to rotate the bone so it points down the inside of the thigh.

Change the Mouse function to Rotate and deselect the Heading constraint. This makes it easier to keep the bone inside the leg.
Rotate the bone to a Pitch of about 97 degrees and create a keyframe.
Mouse function
New bone rotation

Bone rotated into the thigh.

Next we need to increase the rest length of the bone so it runs down to the center of the knee.

Change the Mouse function to Rest Length.
Drag the mouse to change the rest length to a factor of about 5.2 and create a keyframe.
Mouse function
New bone rest length

Bone lengthened to the knee.

The thigh bone is now sized and positioned.

Next we'll put the shin bone in place.

To add the shin child bone to the thigh bone, press the '=' key. This is the keyboard shortcut to add a child to the current bone.
Notice that the child bone has the same rest length as its parent and is oriented straight to its tip. Child bones are always created this way.

The shin bone created.

The shin bone needs to be rotated back into the leg.

Change the Mouse function back to Rotate.
Rotate the bone to a Pitch of about -8 degrees and create a keyframe.
Mouse function
New bone rotation

Bone rotated into the ankle.
Next we need to decrease the rest length of the bone so it ends in the middle of the foot.

Change the Mouse function to Rest Length.
Drag the mouse to change the rest length to a factor of about 4.4 and create a keyframe.
Mouse function
New bone rest length

Bone shortened to the foot.

It may seem like we are done positioning the shin bone, but we're not. Look at the leg from the Front view. The bone needs to be centered in the ankle slightly.

Change the Mouse function back to Rotate. Re-select the Heading constraint and deselect the Pitch constraint.
Rotate the bone to change the Heading to about 5 degrees and create a keyframe.
Mouse function
New bone rotation

Bone rotated into the ankle.

The shin bone is now sized and positioned correctly.

Now for the foot bone.

Go back to the Side view. Press the '=' key again to add the foot child bone to the shin bone.
View mode

The foot bone created.

We will reposition this bone in the foot the same way we did the thigh and shin bones: rotate it into place, then resize it.

Change the Mouse function to Rotate, deselect the Heading constraint, and re-select the Pitch constraint.

Rotate the bone to a Pitch of about 82 degrees and create a keyframe.

Mouse function

New bone rotation

Bone rotated into the foot.

Change the Mouse function to Rest Length.
Resize the bone to a scale of about 3.0 and create a keyframe.
Mouse function
New rest length

Bone shortened to the fit foot.

The foot bone is now sized and positioned correctly.
With the foot bone still selected, press the '+' key one last time. This bone is not for a toe, it is a place holder at the end of the foot for the IK chain. It will help us control the foot rotation when we move the leg. Since this bone is as small as possible and does not deform any geometry, we sometimes call this a "Null Bone"

To avoid confusing this with a toe, change the bone's rest length to 0.002.
New rest length

Null bone sized down.

Now we're ready to make the bones active.

Select Bone (1) from the Selected Item pop-up menu. Press the r key. This is the keyboard shortcut to set the bone rest length.
Selected Item Pop-up

Notice the bone changes from a dotted to a solid line when it's activated.

Thigh bone activated.

Do this for all the bones in the leg.

Tip: The up and down arrows on the keyboard cycle through the selected item list. To quickly turn all of the bones on, press the down arrow key and then the r key until you have cycled through all of the bones.

The bones are now active and can deform the object. If you want to try the bones out, select Rotate as the Mouse function, re-select the Pitch constraint, and play around with bending the leg and ankle. When you've had enough of this, press Shift + Right Arrow (Next keyframe) to reset the bones back to their first keyframe.
This is why we turned off AutoKeyAdjust and AutoKeyCreate. If both of these were on, anytime you rotated one of the bones, it would stay in that position. Both of these functions are usually kept off during the set-up phase and then turned on for the animation phase.
Note: If you got yourself into trouble by having Auto Key Adjust and Auto Key Create turned on , you can fix this by turning off both items and then for each bone select reset and create a keyframe at frame 0.
You might have noticed that when the lower leg was bent backwards, the knee joint compressed down. To fix this, select Bone (2) and press the P key to bring up the Skeleton Panel. On the lower part of the panel select Joint Compensation and Joint Compensation for Parent. Leave the values at 100%, this will work fine for our leg. These functions will help keep a "volume" in the area of the knee and keep it from crimping like a bent water hose.

While we're in this panel select Bone (3) in the Current Bone pop-up menu and turn on Joint Compensation and Joint Compensation for Parent for the foot bone. This will help retain volume in the ankle area. Close the Skeleton Panel and rotate the lower leg and foot bones again. You should notice an improvement over the previous settings.

Without Joint Compensation With Joint Compensation

The skeleton is now in place; it's time to set up the Inverse Kinematics.

Set up the Scene

Setting Up the IK

To add the shin child bone to the thigh bone, press the '=' key. This is the keyboard shortcut to add a child to the current bone. Notice that the child bone has the same rest length as its parent and is oriented straight to its tip. Child bones are always created this way.
	The shin bone created.


Without Joint Compensation	With Joint Compensation