OpenCTM (MeshInspector fork)

OpenCTM is a file format and a C library for compressing 3D triangle meshes, written by Marcus Geelnard and released under the zlib/libpng license. This repository is the fork that MeshLib uses for reading and writing .ctm files.

It started from upstream v1.0.3 (2010-01-15), the last release on sourceforge.net/projects/openctm. The original README with the upstream change log and credits is kept verbatim as README.txt; the file format specification is doc/FormatSpecification.pdf.

What differs from upstream 1.0.3

Scope and build

• Library only. Only lib/ (the OpenCTM sources plus the bundled LZMA SDK) is kept. The ctmconv and ctmviewer tools, the language bindings, the Maya/Blender plugins and the upstream Makefiles are removed.
• CMake build. CMakeLists.txt builds a shared OpenCTM target and installs headers, the library and an OpenCTMConfig.cmake package, so consumers use find_package(OpenCTM CONFIG) and link OpenCTM::OpenCTM. Installation rules can be disabled with -DOPENCTM_INSTALL=OFF.
• Export macro. Symbol export is driven by CMake's OpenCTM_EXPORTS define instead of upstream's OPENCTM_BUILD; OPENCTM_STATIC still selects static linkage. On non-Windows builds every API function carries default visibility.

API additions and changes

• ctmRearrangeTriangles(context, on) (new). Upstream always sorts triangles before MG1/MG2 compression to improve the ratio, which changes their order in the file. The fork keeps that as the default, but turning it off preserves the caller's triangle order. In both modes each triangle is still rotated so that its smallest vertex index comes first, which does not change the mesh.
• Compression progress and cancellation. ctmSaveCustom gained a third parameter, a CTMcompressProgress callback (size_t pos, size_t total, void * userData) that is called during LZMA compression with the number of bytes processed so far. Returning a non-zero value aborts the save, which then fails with CTM_LZMA_ERROR. Pass NULL for the upstream behaviour. ctmSave is unchanged. Existing ctmSaveCustom call sites need the extra NULL argument.
• Empty meshes. A mesh with zero vertices and/or zero triangles can be defined, saved and loaded. Upstream rejects such data with CTM_INVALID_MESH on save and CTM_BAD_FORMAT on load, so files of empty meshes written by this fork are not readable by the original library.
• C++ wrapper. openctmpp.h uses noexcept instead of the removed throw() specification.

Loader hardening

Fixes for two heap-buffer overflows reachable from ctmLoad on a crafted file, reported by Kamal Sentassi (S9S Security Research) under coordinated disclosure in 2026:

• String lengths (#2). A file-declared string length of 0xFFFFFFFF made malloc(len + 1) wrap to a zero-size buffer that was then overrun. Such lengths and short reads are rejected, and the loader stops parsing after the first failed read instead of continuing with an error already set. ctmLoadCustom also clears a stale error from a previous load on the same context.
• Vertex and triangle counts (#3). File-declared counts drove malloc(count * stride) directly, which wraps on a 32-bit size_t (wasm32) and in the 32-bit intermediates of the temporary buffers. Counts above UINT_MAX / 16 (268,435,455) are rejected with CTM_BAD_FORMAT.

Other small fixes: compiler warnings, line endings, typos, and a Windows install rule for the DLL.

File format

The format itself is unchanged (format version 5). Files written by this fork load in upstream OpenCTM and vice versa, with the two exceptions noted above: empty meshes, and meshes above the count limit.

License

zlib/libpng, unchanged from upstream. See LICENSE.txt.