FOSSIL
May 14, 2026 · View on GitHub
FOrtran Stereo Litography parser
A pure Fortran 2003+ OOP library for reading, writing, and manipulating STL mesh files.
| 📂 STL I/O Load and save ASCII or binary STL files. Format is auto-detected; bad input (NaN/Inf coordinates) is rejected via a status code | 🔧 Surface manipulation Translate, rotate, mirror, resize, clip, and merge STL surfaces with a clean type-bound API | 🩺 Surface analysis & repair Sanitize pipeline detects and fixes degenerate slivers, literal duplicates, disconnected edges, non-manifold edges, and inward-pointing normals. is_watertight / is_manifold / is_volume predicates summarise the result | 📏 Signed distance Bit-exact closest-facet lookup via best-first traversal with d² pruning. Sign via Bærentzen–Aanæs pseudo-normal (default), ray intersection, or solid angle |
|---|---|---|---|
| ⚡ SAH BVH acceleration Binary BVH built with the surface-area heuristic — adapts to triangle density. ~100× faster distance queries than the legacy octree on dragon-scale meshes. Octree remains selectable | 🧪 OOP / TDD designed Two public types — surface_stl_object and facet_object — every public operation exercised by the regression suite | 🚀 Advanced geometry processing Booleans, winding number, alpha wrap, isotropic remesh, decimation, marching cubes, SDF segmentation, ray queries, cotangent Laplacian, per-vertex curvature, Taubin smoothing — twelve libigl/CGAL-class primitives, every one a single TBP | 🆓 Free & open source Multi-licensed — GPLv3 for FOSS projects, BSD 2/3-Clause or MIT for commercial use. Fortran 2003+ standard compliant |
Documentation
For full documentation (guide, API reference, examples, etc...) see the FOSSIL website.
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|---|---|
the dragon STL test (src/tests/dragon.stl) has 6588 triangular facets. With the default SAH BVH and the pseudo-normal sign algorithm, signed-distance queries on a 16³ grid run in ~0.022 s vs ~0.987 s on the legacy octree — about a 45× speedup at this size, growing past 100× on larger meshes. Bit-exact correctness vs brute force is asserted by the regression suite. | automatic repair of disconnected edges. |
Advanced features
FOSSIL ships a focused subset of the libigl / CGAL geometry-processing toolkit, drawn directly from the standard references in the field. Each is a single type-bound procedure on surface_stl_object; every one has a dedicated page with a CFD-relevant motivation, a worked Fortran example, and an honest list of known limitations.
| Feature | Reference |
|---|---|
| Boolean operations — union, intersection, difference, symmetric difference | Zhou et al. 2016 |
| Self-intersection detection / resolution — Möller tri-tri broad-and-narrow phase | Möller 1997 |
| Mesh decimation — quadric edge collapse with normal-flip / non-manifold safety | Garland & Heckbert 1997 |
| Generalized winding number — robust inside/outside on dirty STL, hierarchical Barnes-Hut traversal | Jacobson 2013 + Barill 2018 |
| Marching cubes — isosurface extraction with the SDF→STL roundtrip | Lorensen-Cline 1987 |
| Alpha wrapping — guaranteed watertight 2-manifold surrogate from any triangle soup | Portaneri et al. 2022 |
| Isotropic remeshing — uniform-edge re-tessellation with optional sharp-feature preservation | Botsch & Kobbelt 2004 |
| SDF segmentation — per-facet semantic labels via Shape Diameter Function + Gaussian-mixture clustering | Shapira, Shamir & Cohen-Or 2008 |
| Ray-mesh intersection queries — closest hit, all hits, any-hit early-exit | Möller-Trumbore |
| Cotangent Laplacian + barycentric mass — sparse SPD operator; foundation for curvature and smoothing | Pinkall & Polthier 1993; Meyer et al. 2003 |
Per-vertex Gaussian + mean curvature — angle-defect Gaussian and signed mean curvature from H n = (1/2) M⁻¹ L V | Meyer et al. 2003 |
| Mesh smoothing (explicit + Taubin λ|μ) — production-grade denoiser for CFD-grade STL | Taubin 1995 |
Authors
Stefano Zaghi · stefano.zaghi@cnr.it
Chief Yak Shaver, Accidental Research Scientist, and HPC Farmer — CFD researcher who decided that one more day debugging Fortran build systems was one day too many, opened a Python REPL "just to prototype," and now finds himself maintaining a meshing library, a chimera assembler, an MPI load balancer, and the seven blog tabs he keeps meaning to read.
Claude (Anthropic)
Omniscient Code Oracle & Tireless Rubber Duck — AI pair programmer, responsible for writing the boring parts so humans don't have to.
Contributions are welcome — see the Contributing page.
Copyrights
This project is distributed under a multi-licensing system:
- FOSS projects: GPL v3
- Closed source / commercial: BSD 2-Clause, BSD 3-Clause, or MIT
Anyone interested in using, developing, or contributing to this project is welcome — pick the license that best fits your needs.
Quick start
use fossil
use penf, only: R8P
use vecfor, only: ex_R8P
implicit none
type(surface_stl_object) :: surface
real(R8P) :: d
! Load (ASCII or binary, auto-detected) and run the full repair pipeline.
call surface%load_from_file(file_name='cube.stl', guess_format=.true.)
call surface%sanitize ! degenerate / duplicate / non-manifold / winding
print '(A)', surface%statistics()
! Signed distance from a point — uses SAH BVH + pseudo-normal sign by default.
d = surface%distance(point=2.0_R8P * ex_R8P, is_signed=.true., is_square_root=.true.)
print '(A,ES12.5)', 'signed distance = ', d
call surface%translate(x=1.0_R8P, y=2.0_R8P, z=0.5_R8P)
call surface%save_into_file(file_name='cube-moved.stl')
See src/tests/ for more examples including clipping, distance queries, and validity predicates (is_watertight, is_manifold, is_volume).
For interactive use without writing Fortran, the companion fossilizer CLI (src/app/fossilizer.f90) wraps the library for command-line STL analysis and manipulation.
Install
FoBiS
Standalone — clone with submodules and build:
git clone https://github.com/szaghi/FOSSIL --recursive && cd FOSSIL
fobis build --mode static-gnu # build static library
fobis build --mode tests-gnu && ./scripts/run_tests.sh # build and run tests
As a project dependency — declare FOSSIL in your fobos and run fetch:
[dependencies]
deps_dir = src/third_party
FOSSIL = https://github.com/szaghi/FOSSIL
fobis fetch # fetch and build
fobis fetch --update # re-fetch and rebuild
CMake
git clone https://github.com/szaghi/FOSSIL --recursive && cd FOSSIL
cmake -B build && cmake --build build && ctest --test-dir build
As a CMake subdirectory:
add_subdirectory(FOSSIL)
target_link_libraries(your_target FOSSIL::FOSSIL)