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For a game engine I am writing, I am planning to use a heavily cut down and thoroughly modified version of the Java logging library logback. Basically I am using their code for lookups and instantiation of new loggers (so, maybe some 5% tops of the code, not that it matters).

Q: What does this mean for my engine; What does the LGPL license force me to do?

Addendum:

  • I am not native speaker and hence have issues understanding all that legal mumbo-jumbo which forms the license.
  • The question also is specifically about LGPL v2.1, as that is the license of the library I abuse.

Addendum2: to clear some confusion; the phrase '... 5% ... not that it matters', refers to the fact that it does not matter how much or how little code it is in the end, it still is licensed code and hemce has to be treated in such a matter.

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  • "maybe some 5% tops of the code, not that it matters" – It absolutely does matter. If you didn't copy all of their work, and it's only a small part of your project, then you can argue that you made a "de minimus" copy and therefore are not required to comply with any of LGPL's terms. thetradesecretsofintellectualproperty.com/videos/… – Abhi Beckert Sep 22 '15 at 2:16
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    5% of a library is not "de minimus". On the other hand, there is no need to copy these 5%. You can instead rewrite it. – gnasher729 Sep 22 '15 at 6:20
  • @gnasher729 it's not that simple. 100% use of a work can be "de minimus" if the work you're including it in is vastly larger in size than the work you're copying. For example, the iconic swiss clock is copyrighted (arstechnica.com/apple/2012/09/…), but I am still allowed to make a movie that has a swiss clock hanging on the wall - it's a de minimus use even though I copied 100% of the copyrighted work. I'm not saying that this is de minimus work (ask a lawyer), just that the size of the copying matters. – Abhi Beckert Sep 22 '15 at 21:46
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The license terms of the LGPL license effectively distinguish two different models:

  1. You make changes to the LGPL code, or you copy portions of the LGPL licensed code into your own project. In this case, the license requires you to make your code available under the LGPL or GPL license as a library that can be used by other programs.
  2. You use the LGPL licensed library in your application by linking to it. In this case, you must provide a way for your users to replace the library, but otherwise you are free to choose your license terms.
    Providing a way to replace the library is easiest if you link dynamically to it or of you publish your code under an open source license.

In your case, your best option is to create a library of the cut-down version of logback and publish that under the LGPL. Then your game can link to this new library for its logging.

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