CO File

What are CO files and how to open them

Are you having problems opening a CO 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 CO files.

What is a CO file?

A .CO file is a Cult3D Object file.

Cult3D Object files are finalized 3D interactive scene files produced by Cult3D Designer. They are optimized for display and sharing, especially on the web, and include embedded mesh geometry, textures, animations, event-logic (such as Java classes or event maps), lighting, sound, and other display properties. These are the completed or "viewer" versions of a design, intended for presentation rather than further editing.

Cult3D Viewer objects are created by exporting from a full Cult3D project, which itself uses editable files: design meshes are imported from CAD tools (e.g. via 3D Studio MAX), project layout and event logic are constructed in Cult3D Designer, and finally saved as a Viewer object file. Once saved as a Viewer object the file is read-only and cannot be reopened or edited in the Designer; this preserves the published form and protects the creator's design. Viewable via the Cult3D Player plug-in or browser ActiveX/embed object, the Viewer object can be embedded in web pages.

The file begins with a specific byte signature: at offset zero it contains the ASCII string "C3DF0940". It uses a compact, compressed binary form, and omits editable design data like source meshes or layout references once exported.

How to open CO files

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

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

Last updated: August 31, 2026

Various apps that use files with this extension

These apps are known to open certain types of CO files. Remember, different programs may use CO 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 CO file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.