4 bones limitation and weight painting

1. The Skeleton and the "Glue" (Weight Painting)

In a game, a 3D character's skin (the mesh) doesn't know how to move on its own. We use Bones (also called Nodes) to deform it. Weight Painting is the process of telling the game: "This specific point on the skin (vertex) must follow this bone."

To visualize this, modders use a color code called a "Heat Map":

• 🔴 Red (~100 %): The vertex follows the bone blindly. If the bone moves 1cm, the skin moves 1cm.
• 🟠 Orange (75%): The bone has a strong grip, but another bone is starting to pull slightly.
• 🟢 Green (50%): A perfect balance between two bone influences.
• 💎 Light Blue (25%): The bone barely brushes against the skin.
• 🔵 Dark Blue (<25%): The influence is almost zero.

2. The Hardware Limit: The "Rule of 4"

Skyrim’s engine has a strict physical limit: A single vertex can only be "glued" to a maximum of 4 bones. In complex areas like the Torso (chest and belly), these 4 slots are filled instantly:

• Spine Bone (The "Anchor," for breathing/bending)
• Breast Bone 1 (Main physics)
• Breast Bone 2 (Secondary physics)
• Breast Bone 3 (Detail physics)

The Problem: If you want to add a piercing or a medal with its own physics, you have no "slots" left. To add the piercing's bone, you must remove the influence of one of the other 4. If you remove the Spine (the Green or Blue weight), the skin won't stretch correctly anymore. If you remove a Breast bone, the piercing won't follow the body's jiggle.

3. Safe Zones vs. Conflict Zones

The Torso and Belly (Critical Zones): this is where the problem is most severe. The skin here is pulled in every direction. Every vertex already has its 4 "bone friends," and there is no room for a guest (an accessory).

Arms, Legs, and Pelvis (Free Zones): here, movement is simpler—mostly like a hinge. Often, a vertex only uses 2 bones (e.g., Forearm and Elbow). This leaves 2 empty slots to easily add bracelets, pouches, or daggers with their own physics.

4. The "Piercing moving 2x" Glitch

This is the direct result of the 4-bone limit. If a modder removes the Spine's influence on the skin to make room for the Piercing's bone:

The Body Skin is still being held back by the Spine (it only moves 50% of the distance).

The Piercing, however, is now glued at 100% (Red) to the Breast bone.

Result: During a jump, the piercing travels twice as far as the skin. It literally "flies" out of the chest before snapping back.

Summary for a Newbie

It’s like trying to hold a package with 5 ropes when you only have 4 fingers. You are forced to let go of one rope, and the moment you do, the whole package deforms.