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"Castle Game Engine" is a free software, open source game engine.

You can use it to develop commercial (including closed-source) games and applications.

Summary

  • "Castle Game Engine" is covered by the "LGPL with static linking exception" license.

  • It is the same license as used by FPC (Free Pascal Compiler) run-time library and LCL (Lazarus Component Library).

  • You can use "Castle Game Engine" to develop any applications, including closed-source.

  • If you modify the "Castle Game Engine" units, you need to share your modifications.

  • The code is covered by our copyrights. E.g. if you distribute the "Castle Game Engine" sources yourself (modified or not), you must clearly specify our copyrights to the original code.

  • Some of the examples graphic data, and some tools (in particular, the upcoming castle-editor), are covered by a more strict GPL license. To put it simply, you cannot make a closed-source fork of our (upcoming) "Castle Game Engine" visual editor.

The above is a quick summary of our licensing terms.

Please note that the text above is not "legally binding". E.g. "the need to share your modifications" mentioned above is a simplification of the license terms. The precise license requirements discuss (non-internal) distribution of the application and other details. The exact legal text of the licenses is linked below, and they are analyzed in depth in many places on the Internet.

Engine core (LGPL)

The core of the "Castle Game Engine" is available on terms of the GNU Lesser General Public License (version 2 or above, whichever you like). See the COPYING.LGPL2.txt for the exact LGPL 2 license text, see the http://www.gnu.org/copyleft/lesser.html for the current version, FAQs etc.

Moreover, we add the so-called "static linking exception" to the LGPL terms. The idea of this exception is to allow statically linking with the engine on the same terms as dynamically linking. Static linking is what usually happens when you compile a program using the engine units (without wrapping the engine in a dynamic library (dll, so, dylib) or Delphi runtime package).

The exact legal text of this "static linking exception" is this:

As a special exception, the copyright holders of this library
give you permission to link this library with independent
modules to produce an executable, regardless of the license
terms of these independent modules, and to copy and distribute
the resulting executable under terms of your choice,
provided that you also meet, for each linked independent module,
the terms and conditions of the license of that module.
An independent module is a module which is not derived from
or based on this library. If you modify this library,
you may extend this exception to your version of the library,
but you are not obligated to do so. If you do not wish to do so,
delete this exception statement from your version.

By "engine core" we mean everything that can be found in this directory (GitHub repository https://github.com/castle-engine/castle-engine/ ) except the exceptions mentioned in the section below.

Visual editor (castle-editor), some example data, some website data (GPL)

The items listed below are covered by a more strict GNU General Public License (version 2 or above, whichever you like). See the file COPYING.GPL2.txt for the exact license text. See http://www.gnu.org/copyleft/gpl.html for more information about this license (including translations of it to various languages) and philosophy of the free software.

In short, it means that: If you use the items listed below (modified or not) to create your own application, then you need to share the code of your entire application, as the complete work must be covered also by the GPL license.

The items covered by the GPL license are:

CASTLE_ENGINE_LGPL symbol (relevant only for Castle Game Engine <= 6.4)

In the older Castle Game Engine versions (6.4 and earlier), if you wanted to use the more permissive LGPL license, you had to compile the engine with the symbol CASTLE_ENGINE_LGPL. This avoided pulling GPL-only NURBS implementation.

This requirement is removed in Castle Game Engine >= 6.5 (more precisely, in the GIT commit https://github.com/castle-engine/castle-engine/commit/82f3c7562db21d498d5000628fedeeefaf5f7550 ). All the engine units, including the (new) NURBS implementation, are now automatically covered by the more permissive LGPL license.