About

Submitting a 3D model with non-manifold edges or degenerate faces to a slicer, game engine, or CNC pipeline causes silent failures. Print jobs abort mid-layer. Boolean operations produce garbage geometry. Physics engines crash on zero-area triangles. This tool parses your OBJ or STL file and computes the Euler characteristic χ = V − E + F, identifies every non-manifold edge (shared by ≠ 2 faces), flags degenerate triangles with area below 1×10⁻⁸ units², detects isolated vertices, and verifies normal orientation consistency across the entire mesh. All analysis runs locally in your browser. No file is uploaded to any server.

The watertight test confirms whether your mesh forms a closed 2-manifold surface, a prerequisite for 3D printing (per ISO/ASTM 52915) and volumetric calculations. Limitations: this tool processes triangulated meshes. Quad or n-gon faces in OBJ files are automatically triangulated via fan decomposition, which may not match your modeler's triangulation for concave polygons. STL binary format is supported alongside ASCII. Models exceeding 500K faces may take several seconds to analyze.

Formulas

The fundamental topological invariant for mesh validation is the Euler-Poincaré formula:

χ = V − E + F

For a closed, genus-g surface: χ = 2 − 2g. A watertight sphere has χ = 2. A torus has χ = 0. For a valid closed triangular mesh, the edge-face relationship holds: E = 3F2.

Triangle area via cross product:

A = 12 | (v₁ − v₀) × (v₂ − v₀) |

Where v₀, v₁, v₂ are triangle vertex positions. A face is degenerate when A < ε (typically 1×10⁻⁸). An edge is non-manifold when its face-adjacency count n ≠ 2. Boundary edges have n = 1.

Reference Data

Check	What It Detects	Impact If Ignored	Standard / Reference
Non-Manifold Edges	Edges shared by ≠ 2 faces	Boolean ops fail, slicer crashes	ISO 52915, general manifold topology
Non-Manifold Vertices	Vertices where face fans don't form a disk	Mesh splitting errors in subdivision	Euler-Poincaré formula
Degenerate Faces	Triangles with area < 1e-8	NaN normals, rendering artifacts	Floating-point geometry best practices
Isolated Vertices	Vertices referenced by zero faces	Wasted memory, export bloat	OBJ/STL spec compliance
Flipped Normals	Inconsistent face winding order	Inverted shading, inside-out prints	Right-hand rule (CCW winding)
Watertight / Closed	All edges shared by exactly 2 faces	Cannot compute volume, print fails	ISO/ASTM 52915 for AM
Euler Characteristic	χ = V − E + F	Genus > 0 means holes or handles	Euler-Poincaré theorem
Bounding Box	AABB min/max in all 3 axes	Scale verification for manufacturing	Coordinate system validation
Surface Area	Sum of all triangle areas	Material estimation, coating calc	Cross-product area formula
Duplicate Faces	Two or more faces with identical vertex indices	Z-fighting, doubled thickness	Mesh cleanup standard practice
Self-Intersection	Triangles penetrating each other (basic check)	Invalid solid, slicer errors	Computational geometry
Edge Count Ratio	E ≈ 1.5F for closed triangular mesh	Deviation indicates mesh issues	Combinatorial topology
Component Count	Number of disconnected mesh islands	Unintended separate shells	Graph connectivity (BFS/DFS)
Vertex Valence	Min/Max/Avg edges per vertex	Extreme valence causes shading issues	Mesh quality metrics
Aspect Ratio	Worst triangle aspect ratio	Poor FEM results, bad UV mapping	Mesh quality for simulation

Frequently Asked Questions

A non-manifold edge is shared by more than 2 faces (like a T-junction where 3 surfaces meet at one edge) or by only 1 face (a boundary). Slicer software cannot determine inside vs. outside at these edges, causing layer generation to fail or produce unpredictable infill. Fix by splitting the shared edge into separate edges for each surface pair.

For a closed mesh with no holes, χ = V − E + F should equal 2 (for genus 0, like a sphere). Each topological handle (like a torus hole) subtracts 2, so a torus has χ = 0. If your mesh is supposed to be a simple closed solid but χ ≠ 2, it has topological defects: boundary edges, non-manifold geometry, or unintended handles.

Degenerate triangles produce undefined or NaN surface normals because the cross product of two collinear edge vectors is a zero vector. This cascades into lighting calculations (black spots), physics engines (collision detection returns NaN), and UV mapping (infinite distortion). Most renderers silently skip them, masking the problem until export.

The tool performs a basic self-intersection check using bounding-box overlap tests between face groups. For meshes under 10K faces, it can detect obvious penetrations. Full exact self-intersection testing (Möller - Trumbore on all face pairs) is O(n²) and impractical for large meshes in a browser without spatial indexing. Use dedicated tools like MeshLab for exhaustive self-intersection analysis on complex models.

Watertight means every edge is shared by exactly 2 faces - no boundary edges, no holes. Manifold is stricter: watertight plus no edge shared by more than 2 faces and no vertex where the face fan doesn't form a topological disk. A mesh can be watertight but non-manifold if, for example, two cones share a single vertex (bowtie configuration). Both conditions are required for valid 3D printing.

Quad faces (lines starting with "f" followed by 4 vertex indices) are split into 2 triangles using fan triangulation: vertices (0,1,2) and (0,2,3). This is correct for convex quads but may produce overlapping triangles for concave quads. The checker flags faces with more than 4 vertices and applies the same fan decomposition, noting the count in the report.