Blender Compound Parent Rigid Body Explained

The Compound Parent setting in Blender's rigid body physics allows you to combine multiple individual collision shapes into a single, unified rigid body object. By parenting child objects with simple collision shapes—such as boxes, spheres, or convex hulls—to a main parent object, you can simulate complex or concave physics objects with high accuracy and significantly better simulation performance than standard mesh collisions.

What is the Compound Parent Setting?

In Blender, physics simulations typically calculate collisions using predetermined collision shapes. While primitive shapes (Box, Sphere, Capsule) calculate quickly, they cannot represent complex or hollow objects. Conversely, the "Mesh" shape allows complex geometry but is computationally expensive and prone to physics glitches.

The Compound Parent shape bridges this gap. It tells Blender's physics engine to treat a parent object and all of its physically active child objects as a single rigid structure, summing their collision shapes together into one compound collision shape.

Key Benefits of Using Compound Shapes

  • Optimized Performance: Combining primitive shapes (like three boxes arranged in a "U" shape) runs drastically faster in simulations than using a raw triangular mesh collision.
  • Concave Collision Accuracy: It allows objects with holes, hollows, or complex protrusions to interact accurately without relying on unstable mesh-to-mesh physics.
  • Stability: Physics calculations are far less likely to cause objects to pass through one another or jitter violently.

How to Set Up a Compound Parent Rigid Body

  1. Create the Objects: Model your main object (the parent) and the smaller primitive meshes that will define its collision boundaries (the children).
  2. Configure the Parent:
    • Select the main object and navigate to the Physics Properties tab.
    • Click Rigid Body.
    • Set the Type to Active (or Passive, depending on your scene).
    • Change the Collision Shape to Compound Parent.
  3. Configure the Children:
    • Select each child collision mesh and add a Rigid Body to it.
    • Set their collision shapes to primitive types, such as Box, Sphere, or Convex Hull.
  4. Parent the Objects:
    • Select the child objects, then Shift-select the parent object.
    • Press Ctrl + P and choose Object to establish the parent-child hierarchy.

Important Considerations

  • Origins and Transforms: Always apply scale (Ctrl + A > Scale) to both parent and child objects before running the simulation to prevent collision offset errors.
  • Visibility: If child objects are only meant to serve as hitboxes, you can disable their render visibility in the Outliner while keeping their physics active.