Solidly

A modular Shapely-style 3D geometry library for Python

Overview

Solidly is a 3D computational geometry library designed in the spirit of Shapely (2D). It provides robust primitives, predicates, boolean operations, and geometric transformations in a clean, NumPy-first API.

Use cases:

• CAD / modeling workflows
• Robotics & path planning
• GIS & spatial analysis in 3D
• Scientific computing

Features

Core Primitives

• Point3D, Line3D, Plane, Polygon3D, Polyhedron, Mesh, Vector3

Transformations

• translate, rotate (Rodrigues’ formula), scale

Spatial Predicates

• contains, intersects, within, touches
• Robust for convex shapes and simple edge cases

Boolean Operations

• Points, lines, polygons: union, intersection, difference
• Convex polyhedra: union (via convex hull), difference, intersection partially implemented (fails on vertex-only intersection)
• Coplanar convex polygons: intersection/difference via Sutherland–Hodgman

Robustness & Validation

• is_simple, is_convex, is_closed, almost_equals
• Area, surface area, volume, convex hull, simplify

Spatial Index & Queries

• AABB tree for fast spatial queries
• Nearest neighbor search
• Ray casting with Möller–Trumbore for Polyhedron.contains

Visualization

• Lightweight 3D plotting via matplotlib (plot() methods on all primitives)
• PyVista / vedo hooks planned

I/O & Interop

• Mesh import/export: OBJ, STL, PLY
• WKT 3D for points, lines, polygons
• Conversion from NumPy arrays
• Stubs for Trimesh/Open3D interop

Example:

from solidly import Point3D, Polygon3D, Polyhedron

# Create a polygon
p, q, r = Point3D(0,0,0), Point3D(1,0,0), Point3D(0,1,0)
tri = Polygon3D([p, q, r])
print(tri.surface_area())  # 0.5

# Build a tetrahedron
tetra = Polyhedron.from_tetrahedron(p, q, r, Point3D(0,0,1))
print(tetra.volume())      # 0.1667
assert tetra.is_closed()

# Spatial query
assert tri.contains(Point3D(0.25,0.25,0))

Roadmap

• Next sprint
  • Polygon–Plane clipping (Polygon3D ∩ Plane)
  • Convex polyhedron intersection & difference (half-space clipping)
  • More robust degeneracy handling
• Future
  • Full mesh boolean ops (non-convex, general CSG)
  • Topology utilities (edge manifold checks, mesh repair)
  • More formats (glTF, OFF)
  • Accelerated backend (C++/Rust/numba)
  • Visualization: interactive (PyVista, vedo)

Contributing

• The living design doc is in solidly.md.
• Every change must update it with summary, rationale, and limitations.
• Add/expand unit tests for new methods.
• Keep the API Shapely-style and composable.