In this lesson, you will learn how to reduce the computational cost of articulated mechanical bodies by simplifying collision geometry and tuning the constraint solver’s precision.
The Cost of Complexity
When you build complex mechanical assemblies—like a multi-jointed robot arm or a hanging chandelier—Unreal Engine has to solve mathematical equations every frame to ensure the parts move together without intersecting. If you have too many constraints or overly detailed collision shapes, the CPU overhead spikes, leading to "physics jitter" or frame rate drops.
Simplify Collision Hulls
The most common performance drain is using "Complex as Simple" collision on physics bodies. Every polygon in a mesh is a point the constraint solver must calculate. Instead, open your Physics Asset (PhAT) and replace high-poly meshes with primitive shapes like Spheres, Capsules, or Boxes.
- Why? Primitive math is significantly faster for the engine to calculate than checking intersections against hundreds of individual triangles. If a joint requires detail, prefer a Convex Hull over a complex mesh.
Adjusting Constraint Solver Iterations
Even with simple collisions, a long chain of constraints (like a crane cable) can become unstable. By default, the engine uses a global iteration count to solve these physics "equations." If your simulation is wobbling or acting "stretchy," you might be tempted to increase these iterations, but that kills performance.
Instead, check the Physics Asset Tool settings for your constraints. You can modify the Projection Settings to snap parts together more aggressively rather than relying on high-iteration counts to force them into place.
If you are using Physics Constraints in a Blueprint or C++, avoid excessive stiffness. If the physics don’t need to be perfect, lower the Linear/Angular Tolerance to allow the solver to "give up" on a perfect result slightly earlier, saving CPU cycles.
// Example: Modifying constraint stability instead of cranking up iterations
ConstraintInstance.ConstraintBone1 = "Arm_Joint";
ConstraintInstance.LinearLimit.bSoftConstraint = true;
ConstraintInstance.LinearLimit.Stiffness = 500.0f; // Lower stiffness reduces solver stress
Key Takeaway: Prioritize using primitive collision shapes over mesh-based geometry and tune constraint tolerance to reduce solver overhead without sacrificing visual stability.