CTM File

What are CTM files and how to open them

Are you having problems opening a CTM file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CTM files.

What is a CTM file?

CTM files have multiple uses, and Charpad Format is one of them. Read more about the other uses further down the page.

Charpad Format

Charpad Format files contain complete project data created by CharPad (and SpritePad) by Subchrist Software, used to design character graphics, tiles, and maps for Commodore 64 and related 8-bit platforms. They store all of a project's information: character sets (pixel images for each "char"), tilesets (logical combinations of characters; optional), screen maps (which tiles/characters go where), color attributes, display mode and palette settings, and other project-specific metadata. These files are meant to preserve a project between edit and export stages, so that later the graphics, maps or source exports (e.g. binary or assembly) can be done using the stored data without recreating it.

The format is written by CharPad (Free / Pro version) and SpritePad when saving a project. Projects in this format enable visual editing of character images, screen layouts or tile maps, palette control, and export to binary files, assembly code or image import. Multiple versions of the format exist (e.g. version 7, version 8.x), each supporting more features such as ECM display mode, higher tile/tab counts, richer color handling. Older versions of CharPad may not support features present in later versions when saving to older format versions.

Files start with the ASCII signature "CTM" at offset 0, to identify the format.

How to open CTM files

Important: Different programs may use files with the CTM file extension for different purposes, so unless you are sure which format your CTM file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested seven different CTM openers which you will find listed below.

Last updated: September 19, 2026

All known file formats using extension .CTM

While Charpad Format is a popular type of CTM-file, we know of 2 different uses of the .CTM file extension. Different software may use files with the same extension for different types of data.

OpenCTM 3D Mesh

OpenCTM 3D Mesh files store triangle-mesh geometries in a compact, compressed binary form. The format was created to reduce the size of 3D model files (those built from triangles) for storage and transmission while preserving useful mesh data such as vertex positions, surface normals, and texture coordinates. A mesh writer (usually the official OpenCTM library or tools using it) converts meshes produced by modeling, scanning, or procedural generation into this format; downstream tools or renderers load or convert from OpenCTM to use or display the mesh.

OpenCTM supports three compression modes. The "RAW" mode stores data uncompressed; "MG1" employs lossless compression using techniques like prediction plus entropy encoding (including LZMA); "MG2" is a configurable lossy mode that quantizes floating-point data before compressing, trading accuracy for smaller size. The format defines indexed triangles (three vertex indices per face) and stores arrays of vertex data. Per-vertex normals are optional and texture (UV) coordinate sets may be included; custom attribute maps per vertex are supported. Materials, animations, and embedded texture image files are not part of the format.

Internally the file begins with a four-byte ASCII signature "OCTM" located at offset zero. It uses little-endian 32-bit integers and 32-bit IEEE-754 floats. Attributes like CTM_HAS_NORMALS, CTM_VERTEX_COUNT, CTM_INDICES etc are encoded as named metadata fields in the file's header and following arrays.

Various apps that use files with this extension

These apps are known to open certain types of CTM files. Remember, different programs may use CTM files for different purposes, so you may need to try out a few of them to be able to open your specific file.

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about CTM file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.