OpenCASCADE

Description
OpenCASCADE Technology, OCC or OCCT for short, is a collection of C++ libraries that together constitute a professional computer aided design (CAD) kernel for modelling 2D and 3D objects with a computer, and to build specialized tools for manufacturing, simulation, or visualization. OpenCASCADE is the heart of the geometrical capabilities of FreeCAD.

The geometrical classes of OCCT are mostly implemented and made available in FreeCAD through the Part Module, on which most other workbenches depend. It also provides internal functions to read and write different file formats like STEP and IGES, and to perform 2D projections, which can be used to create technical drawings in a workbench such as TechDraw.

OpenCASCADE should not be confused with OpenSCAD, which is a different open source project to build 3D models.

OpenCASCADE is free software governed by the terms of the GNU Lesser General Public License (LGPL) version 2.1 with an additional exception.

Installation
OpenCASCADE is a core component of FreeCAD, so if you get FreeCAD from one of the links in the Download page, you will have it installed, and no further installation is necessary.

However, if you would like to develop applications that use OCCT, or would like to contribute C++ code to FreeCAD, then you need to install the development files of OCCT. In this case, the procedure is explained in Compiling for each of the main systems, Linux, Windows, and MacOS.

Community edition
A "community edition" of OpenCASCADE, abbreviated OCE, was released in 2011, based on the official OpenCASCADE sources (OCCT) of version 6.5. In theory the community edition OCE should be compatible with the main version OCCT in most aspects, while having some additional code contributed by the community.

However, this alternative distribution stopped active development around 2017, and lagged behind the main version in terms of features and bug fixes. For this reason, since FreeCAD v0.17, FreeCAD is compiled exclusively with OCCT, and OCE is not tested.

In some older Linux distributions, FreeCAD is compiled against OCE 0.18, equivalent to OCCT 6.9.x, causing various issues that have been solved already in the main OCCT 7.x releases. If this is the case, try removing OCE, and installing OCCT instead. If this is not possible, use the AppImage to get a modern FreeCAD with an updated OCCT version.

History
The Cas.CADE geometric kernel was originally closed source, but it became open source under its current name around the year 2000. Shortly after, the FreeCAD project was started, with the oldest files being dated to January 2001. Read more in History.

OpenCASCADE version 6.6 and earlier were governed by its own "OCCT public license", which made it not entirely "free software". This was solved with the release of OCCT 6.7 (2013), when it adopted the LGPL2 license.

Related

 * Main website, opencascade.com
 * Development portal, dev.opencascade.org
 * OpenCASCADE Technology (OCCT), latest release
 * Glossary, Open CASCADE
 * OpenCASCADE community edition (OCE), repository