Units and Tolerances#

Units: your choice, one choice#

The kernel is unit-agnostic — a number is a number. Brep.box(100, 100, 100) is a 100-unit cube whether you mean millimeters or inches. The convention only has to be consistent within a document and across booleans: mixing a millimeter part with a meter tool gives a valid, useless result.

Practical conventions used across the ecosystem:

• Model documents and in-repo callers assume millimeters.
• The glTF scene writer is the one place units convert: it maps Z-up millimeters → Y-up meters, per the glTF spec.

Orientation: Z-up#

World up is +Z, everywhere: cylinder axes, extrusions, gravity. Renderers that default to Y-up (three.js) need one line — camera.up.set(0, 0, 1) — or the glTF writer's built-in conversion when exporting.

Tolerances: eps is relative#

Measurement doors take an epsilon, and Brep.measurementDefaultEps() returns the kernel default: 1e-6, a relative stopping band for the volume/area quadrature — not an absolute distance.

import { Brep } from '@huukhanhnguyen/geometry'

const eps = Brep.measurementDefaultEps()
Brep.volume(part, eps) // stop refining when the remaining band < eps × magnitude

Pass it rather than hardcoding a number; the default tracks the kernel.

What accuracy to expect#

• Planar geometry (boxes, prisms): closed-form exact. Tests assert volumes to 1e-9.
• Curved geometry after booleans: the tessellator's sag shows up in measured volume. Compare with a loose band — the kernel's own parity tests use ~1e-2 relative for curved boolean volumes, not 1e-9.
• Tessellation deflection is a different tolerance: max surface-to-triangle distance in model units, controlled per call (Brep.tessellate(doc, deflection)), defaulting to the face-measured Brep.measurementDeflection(doc).
• Mesh weld has its own vertex-merge tolerance, default 1e-4 (TriangleMesh.weld).

Rule of thumb#

Exact input → exact output. The moment a curve is approximated (tessellation, faceted import, curved booleans), pick a comparison band appropriate to the approximation, not machine epsilon.

Where next#

• Guide: Tessellate for display — deflection in depth.
• Guide: Create and measure solids.