What is the AC file format?
.ac files belong to the category of 3D model files.
Files with the .ac extension contain three-dimensional models generated in the AC3D software. This application allows for creating 3D objects and modifying existing projects; it is used in video game production and is particularly popular for flight simulation assets such as FlightGear and X-Plane add-ons. AC3D has been in development since 1992, originally created by Andy Colebourne and now maintained by Inivis Limited.
In .ac files, stored are complete three-dimensional models as plain ASCII text. The contents include:
- positions and sizes of objects in the scene,
- polygon definitions of individual surfaces,
- material properties (colour, shininess, transparency),
- texture references and UV mapping coordinates,
- hierarchical groupings of objects (parent/child relationships).
Texture image files are stored separately and referenced by filename path inside the .ac file - they are not embedded in the model file itself.
Files with the .ac extension are most often used to save work conducted in the AC3D application. In this way, three-dimensional model projects can be transferred between computers or shared online, for instance via file exchange platforms.
.ac files can also be converted into other formats using dedicated converters. Many programs for creating and animating three-dimensional graphics - including Blender - support importing AC3D models directly without prior conversion.
Security & safety
RISK: LOWPlain-text 3D file; contains no executable code. Texture paths inside the file could theoretically reference remote paths, but this is not a practical attack vector.
Format details
in a nutshell- Generic autoconf output / shell script (ac) - GNU autoconf uses .ac extension for configure.ac scripts (M4 macro text); entirely different context, common on Linux systems.
- AC Audio Codec file (rare) - Some legacy encoders used .ac for audio codec data; very rare.
Programs that open AC files
Windows3 apps
Blender
Open-source File > Import > AC3D (.ac) - requires enabling the AC3D import add-on in Blender preferences first (Edit > Preferences > Add-ons, search 'AC3D'). 
macOS2 apps
Linux1 app
Technical details
deep spec| Encoding | Plain ASCII text; human-readable and editable in any text editor |
| File header magic | First line is `AC3Db` (5 bytes + newline); version token `b` identifies the current format; older files use `AC3Da` |
| Container | Line-based token format with sequential MATERIAL blocks followed by hierarchical OBJECT / kids blocks |
| Compression | None (plain text, no packing) |
| Scene complexity | A single `.ac` file can contain a complete scene with multiple named objects, sub-objects, and LOD (level of detail) groups |
| Typical file size | 1 KB - 50 MB (simple objects to complex multi-object scenes) |
| Texture handling | External only; texture image files referenced by filename path and not embedded inside the `.ac` file |
| Scene hierarchy | Parent/child nesting via OBJECT and kids blocks; supports arbitrarily deep grouping |
| Material definition | Per-object MATERIAL blocks define RGB ambient/diffuse/specular colour, shininess, transparency, and texture mapping |
| Supported platforms | Windows, Linux, macOS (AC3D and Blender are cross-platform) |
| Primary use | Flight simulation add-ons (FlightGear, X-Plane scenery), open-source game assets, lightweight 3D scenes |
| Format version identification | Header token encodes version: `AC3Db` = current; `AC3Da` = older revision |
| Released | 1992 (AC3D first version by Andy Colebourne); Inivis acquired/maintains from ~2000s |
| Latest version | AC3D 8.x; format version token 'AC3Db' in file header (current) |
| Specification | www.inivis.com |
AC conversions
Community Q&A
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