In this lesson, you will learn how to replace expensive per-polygon collision meshes with lightweight primitive shapes to ensure your physics simulations remain performant.
When you import a high-fidelity 3D model into Unreal Engine, the engine defaults to "Use Complex Collision as Simple" if no collision is provided. While this accurately matches the visual geometry, it forces the CPU to perform complex intersection tests against every single polygon. In a scene with multiple moving objects, this calculation cost skyrockets, causing frame drops.
The Power of Primitives
To keep your simulation smooth, we replace these complex meshes with Custom Collision Shapes. These are simple mathematical volumes—specifically Spheres, Capsules, and Boxes—that the Chaos physics solver can process almost instantly.
Instead of the engine checking if a bullet hit one of 5,000 tiny triangles on a barrel, it performs a simple distance check against a single Capsule or Box primitive.
Implementing Custom Collision
You can configure this directly in the Static Mesh Editor. Follow these steps:
- Open your Static Mesh.
- Navigate to the Collision settings in the Details panel.
- Set Collision Complexity to
Use Simple Collision as Complex. - In the top toolbar, use the Collision menu to add a primitive shape (e.g.,
Add Box Simplified Collision). - Scale and rotate the primitive to fit your mesh silhouette.
If you are generating these via code or through data-driven workflows, you can influence the generation by adjusting the CollisionSettings within the mesh asset to prioritize simplified hulls.
Why This Matters
By using primitives, you reduce the "Triangle Count" of the physics scene. Even if you have fifty barrels in a room, the physics solver only calculates collisions for fifty simple boxes rather than tens of thousands of complex polygons. This optimization is critical for maintaining consistent performance during intense physics-based interactions, such as explosions or debris-filled environments.
Key Takeaway: Replace complex, high-polygon collision meshes with simplified primitive shapes to drastically reduce CPU overhead and ensure stable frame rates during physics-heavy gameplay.