Challenge 2: Unnatural Mesh Distortion When Posing a Shoulder — Solution Walkthrough

The answer:

Per this chapter's own warning box, automatic weight assignment often
needs manual correction, especially at joints where multiple bones
influence overlapping geometry — a shoulder is exactly this kind of
joint, where the automatic assignment can blend adjacent bones' own
influence incorrectly, causing the visible distortion the student is
seeing.

Why the shoulder specifically causes this problem:

A shoulder joint sits at the intersection of the upper-arm bone and
the torso/spine bone, with mesh geometry in that area needing a
carefully balanced blend of influence from both. Automatic weight
assignment doesn't always get that balance right on its own,
especially in complex, overlapping areas like this one.

What the student should do about it:

Per this chapter's own recommendation, manually correcting the weight
painting in that specific joint area — adjusting how strongly each
nearby bone influences the affected vertices — fixes the distortion.
Testing the rig through its own full range of motion before
considering weight painting finished is exactly the practice this
chapter recommends to catch this kind of problem early.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise checks that joint distortion after automatic weight
assignment is correctly traced back to incorrect automatic blending at
overlapping-influence areas, with manual weight-painting correction
identified as the fix.