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The Dirac software source code is licenced under the Mozilla Public License Version 1.1. A plain text version of this licence is available in copying.txt. Much more detail may be found at Mozilla & Netscape Public licences. Legal documents can be difficult for non-lawyers to read. An annotated version of the licence is available at Mozilla & Netscape Public licences that contains informal explanations of various sections of the licence.

The provisions of the Mozilla Public License allow for relicensing under other licences that are specified in the licence preamble at the beginning of each source-code file. For the Dirac project the licences under which relicensing is possible are specified to be the GNU General Public License Version 2.0 and the GNU Lesser General Public License Version 2.1. These may be downloaded from GNU; copies are also provided in Annex A and Annex B of copying.txt. The rationale for allowing re-licensing under GPL & LGPL is to permit the widest possible use of this technology whist retaining the advantages of the MPL license.

The Dirac approach to licensing

We wish Dirac to be used as widely as possible, royalty free. We thought long and hard in selecting the MPL version 1.1 (hereafter simply the MPL) as the licence for Dirac. We chose it because it's a well-known and widely used licence, and also because it contains practical provisions for dealing with patent issues in what is a highly patented field.

As a defensive measure the BBC has applied for patent protection for some techniques that are, or may be, used within Dirac. Our purpose in doing so is to provide protection for Dirac from spurious patent suits by other parties. Under the terms of the MPL we have licensed these patents irrevocably and royalty free for use within the Dirac software. Our aim is to increase the likelihood that Dirac succeeds, and is used.

However, no licence can grant rights to use third-party patents that are not owned by the licensor. This problem applies to all technologies, whether proprietary, Open Source, or (as illustrated in the case of JPEG) Open Standards, and is a risk that is faced by everyone. We're working hard to avoid using third-party patents altogether, and this is one reason why the Dirac algorithm may yet change. The licence itself cannot address this issue.

Our intention is that Dirac code be used as widely and as freely as possible. This is why we have allowed re-licensing under the terms of the GPL and LGPL licences. Had we not allowed this, as alternative licences under the MPL, GPLed projects could not have used the Dirac software. The option to re-license under the LGPL allows even commercial software to link to Dirac libraries. With the re-licensing provision in the MPL, GPL and LGPL projects may use the Dirac software as is. In this case the patent licence granted under the MPL still applies explicitly. Some projects may prefer to ensure that all their code is distributed only under the GPL or LGPL licence. This is also permitted under the MPL licence. If a project does this then the usual provisions of the GPL or LGPL licence applies and these simply do not involve patent issues (they relate only to copyright). However, in practice, users are still free to use the BBC patents royalty free.

Our philosophy is that the Dirac licence is merely a tool to allow people to use and work on Dirac: the licence is not a complete expression of our commitment to Dirac. The MPL does have some limitations, but it was our view that it was better to use a licence that did most of what we wanted than invent a new one that might be viewed with suspicion and which would not have been subject to the same level of scrutiny as the MPL. If we find that the MPL is an impediment to free and open use of Dirac then we will be happy to consider using alternative or additional licences to achieve its widespread, royalty free use.