OpenCGM

An open-source CGM (Computer Graphics Metafile) parser and CGM→SVG converter.

CGM is the vector format that aerospace, defence, and automotive technical publications have standardised on for three decades — it is what S1000D and ATA iSpec 2200 illustrations are authored in. The open-source tooling for it has not kept up: RALCGM is unmaintained, and ImageMagick and Inkscape have effectively no usable CGM support.

OpenCGM is a modern, actively developed implementation: a complete ISO/IEC 8632 parser, a Skia-backed renderer, and an SVG emitter with profile support for WebCGM 2.1 and S1000D.

• Parses CGM binary and clear-text encodings per ISO/IEC 8632-3:1999 and ISO/IEC 8632-4:1999
• Converts CGM to SVG with WebCGM 2.1 and S1000D output profiles
• Preserves application structure (APS) — layers, names, screentips, link URIs, and region hotspots survive the conversion
• Handles the parts that usually break: NURBS approximation, cell arrays, tile rasters (JPEG / PNG / bitmap / CCITT G4), pattern and hatch geometry, and font substitution with ACI font normalisation
• Ships a C API for embedding, a CLI, and a validation tool
• Round-trips CGM → CGM as well as converting

Building

Requires CMake 3.15+, a C++17 compiler, and optionally Skia for the native renderer (consumed via vcpkg; see vcpkg.json, and configure with -DENABLE_SKIA_RENDERER=ON).

cmake -B build
cmake --build build

Binaries land in build/bin: opencgm_cli, cgm_validate, and the opencgm shared library.

Usage

Command line

opencgm_cli --profile s1000d input.cgm output.svg

Common options:

• --profile <s1000d|webcgm|compat> — output profile (s1000d is the default)
• --font-map <file> — JSON font map describing CGM font substitutions and optional embedded WOFF2 resources
• --text-as-path / --text-as-path-threshold <value> — outline text for pixel-exact verification
• --validate-geometry — enforce viewBox/viewcontext consistency; tune with --geometry-tolerance <value> (default: 0.01 user units)
• --geometry-log / --raster-log — emit diagnostics to stderr

opencgm_cli --help lists every flag.

Library

#include <opencgm/cgm_file.h>

auto cgmFile = cgm::BinaryCGMFile("input.cgm");

for (const auto& command : cgmFile.getCommands()) {
    // inspect or transform
}

cgmFile.writeFile("output.cgm");

A stable C API is available in include/opencgm/c_api.h for binding from other languages.

Layout

Path	Contents
src/core/, src/classes/	CGM file handling and data structures
src/commands/	CGM command implementations
src/import/, src/export/	Binary and clear-text parsing and writing
src/svg/	SVG emission and profile handling
src/skia/	Skia renderer
src/nurbs/, src/xcf/	NURBS approximation, XCF support
src/validation/	Profile and geometry validation
include/opencgm/	Public headers and C API
tests/	Unit tests and corpus-driven conversion tests

Tests

cmake -B build -DBUILD_TESTS=ON
cmake --build build
ctest --test-dir build

That runs green on a fresh clone. Sample corpora are not vendored — the WebCGM 2.1 Conformance Test Suite and the S1000D Bike Data Set are third-party data under their own terms — so tests needing a corpus skip until you fetch one:

python scripts/fetch-testdata.py
export OPENCGM_SAMPLES_DIR=$PWD/testdata

The WebCGM suite downloads automatically from OASIS with a pinned checksum and activates 78 further tests. The S1000D Bike Data Set is distributed through the S1000D users' portal and must be installed by hand; the script prints instructions.

Conversion coverage

Every file in the WebCGM 2.1 Conformance Test Suite (353 CGMs) converts successfully under all three output profiles, producing well-formed SVG:

Profile	Converted	Failures
webcgm	353/353 (100%)	0
s1000d	353/353 (100%)	0
compat	353/353 (100%)	0

Full results, including how much the profiles actually diverge from one another, are in docs/corpus-report.md. The report is generated by scripts/corpus-report.py and is reproducible from a clean checkout.

This measures conversion coverage and SVG well-formedness, not visual fidelity, and is not a WebCGM conformance certification.

Conformance checkpoints

The WebCGM static test suite ships its methodology in machine-readable form: static10/webCGMsuite.xml lists the official operator checkpoints for each of its 232 rendering test cases. scripts/conformance.py runs them:

	Checkpoints
total	912
decided mechanically	438 (48%)
passed	438
failed	0
require an operator	474 (52%)

Full results by CGM category are in docs/conformance-report.md. Checkpoints that describe what a person should see are written to an operator worksheet (--worksheet) pairing each reference image with this converter's rendering.

This is not a conformance certification, and OpenCGM does not claim WebCGM conformance: that is defined for viewers, and the suite's 20tests, 21tests and dynamic10 modules almost entirely exercise a viewer's DOM and XCF APIs and link navigation, which a converter has no part in. The static10 module is the part that tests rendering, and it is what is measured.

scripts/visual-compare.py additionally rasterises converted output and ranks cases by divergence from the reference images (docs/visual-comparison.md) as a regression screen; it has already surfaced real gaps.

Contributing

See CONTRIBUTING.md. The most useful contributions are usually narrow: a CGM file that renders wrong, plus a test pinning the correct behaviour.

License

Licensed under the Apache License 2.0.

OpenCGM is a C++ port of the codessentials.CGM C# library by Toni Wenzel, used under the MIT License. See NOTICE and THIRD-PARTY-NOTICES.md for full attribution.

Commercial

Reforge, a commercial desktop application built on this engine — batch conversion, folder monitoring, a validation UI, and hotspot export — is available separately. OpenCGM itself is and remains Apache-2.0.