| Extension | 123DX |
| Full Name | Autodesk 123D |
| Type | CAD |
| Mime Type | application/octet-stream |
| Format | Binary |
The 123D Design application uses the 123DX file format, which is a Computer Aided Design (CAD) graphics format. Developed by Autodesk, 123D Design was intended as an easy-to-use CAD application aimed at entry-level and enthusiast designers.
123D Design has since been retired, with users moving to other CAD applications such as Fusion 360. The 123DX format is a ZIP container format containing various data files describing the properties of the various objects within the CAD file. Elements such as lines, curves, and 3D volumes such as cubes and cylinders are all stored within human-readable text files wrapped within a compressed ZIP file containing the 123DX file extension.
As mentioned, the 123D Design software has been superseded by other, more capable CAD software, meaning support for editing the file is limited. The original 123D design software can still be downloaded and contains numerous options to export data to standard formats, such as STEP and STL.
The STL, or Standard Triangle Language, file format has been around since the '80s and is supported by most modern 3D modelling applications. In particular, the STL format has become something of a standard format within the area of 3D printing, despite the rise of more modern formats such as 3MF.
STL files come in two flavours: binary or text format. The text format is human-readable, making it easy to perform manual edits, but can lead to very large file sizes. As you can imagine, the binary format is much more compact and represents the majority of the STL files you are likely to encounter.
The format itself is very simple; the 3D model is represented by a sequential list of triangles, with each triangle defined with 3 points consisting of their X, Y, and Z positions, along with an additional vector that contains the face normal (the direction the face is pointing). There is an additional attribute block for each triangle; however, this is largely unused and thus unsupported.
