Rigid Body Simulation¶

Overview¶

NovaPhy's rigid body pipeline handles free-floating bodies with collision detection and constraint resolution.

Creating Bodies¶

import numpy as np
import novaphy

builder = novaphy.ModelBuilder()

# Ground plane
builder.add_ground_plane(y=0.0)

# Box body (mass, half-extents)
body = novaphy.RigidBody.from_box(1.0, np.array([0.5, 0.5, 0.5]))
idx = builder.add_body(body, novaphy.Transform.from_translation(np.array([0, 5, 0])))
builder.add_shape(novaphy.CollisionShape.make_box(np.array([0.5, 0.5, 0.5]), idx))

Collision Detection¶

Broadphase¶

Filters candidate collision pairs using axis-aligned bounding boxes:

Sweep and Prune (SAP) — Default, efficient for mostly-static scenes
BVH — Better for dynamic scenes with many moving objects

Narrowphase¶

Generates exact contact points for each candidate pair:

Pair	Algorithm
Sphere-Sphere	Analytic distance check
Sphere-Plane	Signed distance to plane
Box-Sphere	Closest point on box
Box-Plane	Vertex projection
Box-Box	Separating Axis Theorem (SAT)
Convex-Convex	GJK + EPA

Solvers¶

SolverSemiImplicit (Default)¶

Sequential Impulse solver with:

Warm starting from previous frame
Accumulated impulse clamping
Coulomb friction (tangential impulse <= mu * normal impulse)
Baumgarte stabilization for penetration correction

SolverXPBD¶

Extended Position Based Dynamics with compliance-based constraints. Operates in maximal coordinates.

python demos/demo_pyramid_ball.py --solver xpbd
python demos/xpbd/demo_xpbd_stack.py

Modifying State¶

Python binding limitation

Python bindings return copies of std::vector. Use setter methods to modify C++ state:

# Wrong — modifies a copy
world.state.body_positions[0] = new_pos

# Correct — use setter methods
body.set_linear_velocity(np.array([1, 0, 0]))
body.set_angular_velocity(np.array([0, 0, 1]))

Demos¶

Demo	Description
`demo_pyramid_ball.py`	Pyramid stack with sphere projectile
`demo_friction_ramp.py`	Boxes on a 30-degree ramp
`demo_wall_break.py`	Wall hit by a sphere
`demo_dominoes.py`	Chain reaction dominos
`demo_unified_collision.py`	Gallery of all collision pairs
`demo_pyramids_numerous.py`	Stress test with many pyramids