What is an .ac file? AC3D models, FlightGear, and the Mac

Text you can read

You can read and write an .ac file in a text editor. This is a single red triangle:

AC3Db
MATERIAL "red" rgb 1 0 0 amb 0.2 0.2 0.2 emis 0 0 0 spec 0.5 0.5 0.5 shi 10 trans 0
OBJECT world
kids 1
OBJECT poly
name "triangle"
numvert 3
0 0 0
1 0 0
0 1 0
numsurf 1
SURF 0x10
mat 0
refs 3
0 0 0
1 1 0
2 0 1
kids 0

The first line is always AC3D followed by a hexadecimal version number, here b, which is 11. According to the specification, a program may refuse a file newer than it knows. Next comes a palette of materials (MATERIAL) with colour, highlights and transparency. Then come the objects: world is the root of the scene, group a group, and poly an object with a mesh. An object has points (numvert) and surfaces (numsurf). Each surface starts with a SURF line with flags, mat picks a material from the palette, and the lines after refs are point numbers with texture coordinates. Every object ends with a kids line saying how many child objects follow.

An object's position and rotation (loc and rot) are relative to its parent, so a model is a tree of parts. Each object can have one texture, written as a file name. The images aren't in the .ac file; they have to be where the file expects them. The format has no unit, and no animation either.

Is it a model, or configure.ac?

GNU Autoconf's configure.ac files, which programs are built from, use the .ac extension too – there are six on my Mac. According to FileInfo, the Ancestral Quest genealogy program uses it as well. Terminal tells you whether you have an AC3D model:

head -c 4 model.ac

An AC3D file prints AC3D. A configure.ac starts with a line such as AC_INIT(…).

Two-sided surfaces and the mesh check

The SURF line packs two things into one hexadecimal number. The low digit is the type: 0 is a polygon, 1 a closed line, 2 a line. The high digit is a sum: 1 means a shaded surface and 2 a two-sided one, so SURF 0x30 is a shaded, two-sided surface.

Two-sided surfaces have a catch when they become an ordinary mesh. The Assimp library, which my app reads .ac files with, doubles every two-sided surface and flips the copy. I tried it on a cube: with one-sided surfaces it has 12 triangles and the mesh check is clean; with two-sided ones it has 24 triangles, and the check finds 12 edges shared by more than two faces. Converting to STL would carry the doubled faces over too.

So switch two-sidedness off before printing – in AC3D, in Blender, or right in the text. This command rewrites SURF 0x2… to SURF 0x0… and SURF 0x3… to SURF 0x1… in a copy of the file:

sed -i '' 's/^SURF 0x3/SURF 0x1/; s/^SURF 0x2/SURF 0x0/' copy.ac

On my test cube, the mesh check came out clean again. Lines don't become part of the mesh at all.

Where AC3D is used

• FlightGear, the open-source flight simulator, keeps its aircraft and scenery models in .ac files. The Cessna 172P alone is made of 84 .ac files, and the largest, the interior with the instrument panel, is 4.9 MB.
• X-Plane reads its own OBJ format. But AC3D, with a plug-in X-Plane offers its developers, can open and save it, so X-Plane scenery gets modelled in it.
• MATLAB uses AC3D files in its Aerospace Toolbox examples and points to FlightGear's aircraft models.

Inivis also lists 3D printing, Google Earth and Second Life.

How to open AC on a Mac

macOS doesn't know AC3D. On macOS 27.0.1 the system has no file type for it, the Model I/O framework doesn't load it, and neither Preview, Quick Look nor the Finder shows it.

• AC3D from Inivis has a Mac version. It's paid, with a trial.
• Blender imports and saves .ac files with the Blender-AC3D add-on (version 7.2, for Blender 2.63 to 4.3).
• Preview3D, my app, opens .ac files on a Mac in its window, in Quick Look and as Finder thumbnails. On iPad, iPhone and Apple Vision Pro too. So you can browse the parts of a FlightGear aircraft as thumbnails without starting Blender.

Preview3D reads AC3D through the Assimp library. I tried it in the same build the app uses, with files I wrote from the specification. It opened a tree of objects with the parts in the right places, polygons with five vertices, and a file with the newer AC3Dc header. Assimp works out the newer subdiv line (not yet in the 2003 specification): a cube with subdiv 2 came out rounded, with 192 triangles. A file with points and no surfaces doesn't open. The app shows the geometry in one plain colour: it doesn't use the file's materials or textures, and it doesn't draw lines. Model info has the dimensions in the file's units and the triangle count, plus a mesh check. On iPad and iPhone it stands the model on your desk in AR, fitted to the desk.

Converting AC to STL

On a Mac, Preview3D converts from File → Export model as… (⌥⌘E) to STL, OBJ or PLY, without uploading anything. Materials, textures and lines are dropped. First switch off two-sided surfaces, as described above, though. Models from simulators are often built from many separate parts, and the mesh check shows them as loose parts. The format has no unit, so check the size in the slicer.

Common questions

What is an .ac file?

The plain-text format of Inivis's AC3D modeller: materials, a tree of objects, points and surfaces. The FlightGear flight simulator, for example, uses it.

Is my .ac file a 3D model?

head -c 4 model.ac prints AC3D for a model. A configure.ac file belongs to GNU Autoconf and isn't a 3D model.

Why does the mesh check report edges shared by more faces?

The model has two-sided surfaces (SURF 0x2… or SURF 0x3…). They're doubled when read, and the flipped copies share edges with the originals. Switch two-sidedness off before printing.

Does a Mac open .ac without another app?

No. macOS 27 has no file type for AC3D, and neither Preview nor Quick Look shows it.

How do I convert AC to STL?

On a Mac, in Preview3D with File → Export model as…, or in Blender with the Blender-AC3D add-on. Then check the mesh and the size.

Where do I find models in AC format?

Mostly in FlightGear aircraft. The Cessna 172P, for example, is on GitHub under the GPL.

Related pages

• The .obj file – another simple text format for meshes
• The .3ds file – the 3D Studio format from the same era
• The .stl file – the printing format AC is converted to

Sources

• Inivis: the AC3D file format – header, materials, objects, SURF flags
• Inivis: AC3D – platforms
• Assimp: reading AC3D – two-sided surfaces, subdiv
• Blender-AC3D – the Blender add-on
• X-Plane Developer: the AC3D plug-in
• MathWorks: AC3D files in Aerospace Toolbox
• FlightGear: the Cessna 172P on GitHub
• FileInfo: the .ac extension – other programs using the extension

Building an aircraft or scenery for FlightGear and want to browse .ac parts quickly on your Mac? Write to me at apps@xfanta.com and tell me what would help.