Exercise 4 — Spine and Leg Chains

10 points · Week 3 · Course Home

What you're building

Two clean joint chains on the practice mesh — and, more importantly, the ability to prove they
are clean.

Time: 75 minutes
New tools: Joint Tool, Orient Joint, Local Rotation Axes display

Open the practice mesh. Display → Animation → Joint Size — set it large enough to see
inside the mesh.
Skeleton → Create Joints (Joint Tool). Reset the tool options.
In the side orthographic view, click to place the root at the world origin between the
hips, then place 4–5 spine joints following the spine's S-curve up to the base of the neck.
Press Enter to finish.
Look at the last joint you placed. It has no child, so it took its orientation from its parent.
A joint's orientation comes from the next joint in the chain — with nothing to point at,
there is nothing to orient to.
Display → Transform Display → Local Rotation Axes. Turn it on and leave it on. Look down
each bone and confirm the X axis points along it.

Select a middle spine joint and rotate it 30° in Y. Everything above it swings. Look at the
Channel Box: rotateY now reads 30. Undo.
Now select the same joint and move it forward 3 units. The chain follows. Look at the local
rotation axes on its parent: X no longer points down the bone. The bone moved; the
orientation did not follow.
Two rules come out of steps 6 and 7. Write them in your submission:
- Position by moving, never by rotating
- After moving, re-orient
Skeleton → Orient Joint (options). Primary axis X, secondary axis Y, secondary axis
world orientation +Y. Check Orient child joints. Apply to the root.
Re-check the local rotation axes. X should point down every bone.
Check the Channel Box on each joint: rotate reads 0, 0, 0, and jointOrient carries the
orientation. This is what a clean joint looks like. Anything else will not return to bind
pose.

In the side view, draw the leg chain: hip → knee → ankle → ball → toe.
Place the knee slightly forward of the straight line between hip and ankle. This is the
preferred angle — it tells the IK solver in Week 9 which direction the leg bends.
Switch to the front view. Everything you drew in the side view is sitting at X = 0. Move
the whole leg chain out to the character's left leg, then correct each joint's position
against the mesh.
Re-orient the leg chain — repeat step 9 on the hip joint.
Parent the hip joint under the root joint.

Rename to convention. See Naming Convention. The suffix declares the
joint's role: _bnd for joints that will carry skin weights, _tip for terminal joints that
won't.
```
root_jnt
spine_01_bnd … spine_05_bnd
L_hip_bnd
L_knee_bnd
L_ankle_bnd
L_ball_bnd
L_toe_tip
```
The toe tip exists to orient the ball joint and define its bone length. It will not be in the
bind.
Evaluation order test: rotate root_jnt — everything follows. Rotate spine_02_bnd —
only what is above it follows, and nothing below moves. Undo both.
Hinge test: rotate L_knee_bnd in each axis in turn. It should bend cleanly in exactly one
axis. If the knee bends on a diagonal, the orientation is wrong — go back to step 9. If it
still fails, delete the chain and redraw it. That is a normal outcome, not a failure.
Zero every rotation. Parent the root joint under the Joints group. Save.


`rotate` reads 0,0,0 on every joint; `jointOrient` carries all orientation
X points down every bone
Knee hinges on one axis only, with a preferred angle set
Root at world origin, single root, parented under `Joints`
Named to convention with correct role suffixes
Both rules stated correctly

Orientation stored in rotate, because joints were rotated into place
Leg drawn in side view and never corrected in front view — the whole chain sits at X = 0
End joint left oriented to its parent instead of to world
Root joint placed at the pelvis rather than world origin. This will fail the engine check in
Week 4.