How do I license code when rewriting code in another language? [duplicate]

Question

I took the C code from the png2theora example tool of Xiph.Org Theora which has a copyright header:

/********************************************************************
 *                                                                  *
 * THIS FILE IS PART OF THE OggTheora SOFTWARE CODEC SOURCE CODE.   *
 * USE, DISTRIBUTION AND REPRODUCTION OF THIS LIBRARY SOURCE IS     *
 * GOVERNED BY A BSD-STYLE SOURCE LICENSE INCLUDED WITH THIS SOURCE *
 * IN 'COPYING'. PLEASE READ THESE TERMS BEFORE DISTRIBUTING.       *
 *                                                                  *
 * THE Theora SOURCE CODE IS COPYRIGHT (C) 2002-2009,2009           *
 * by the Xiph.Org Foundation and contributors http://www.xiph.org/ *
 *                                                                  *
 ********************************************************************

  function: example encoder application; makes an Ogg Theora
            file from a sequence of png images
  last mod: $Id$
             based on code from Vegard Nossum

 ********************************************************************/

The COPYING file points is a 3-clause BSD license.

I have taken the code apart into several pieces which I have rewritten in Vala code classes (so in a different programming language).

I also wrote several vapi files to bind to the C APIs (for libtheora, libogg and libpng).

I did several heavy changes to the code:

• Replaced return code based error handling by exception handling
• Rewrote command line parsing from getopt to GLib OptionGroup
• Reorganized the code into several classes
• Replaced stdio file functions by gio classes
• Added some additional error handling

So I have almost completely rewritten the original code, but it should still work essentially like the original tool from the user perspective.

Now I would like to release this code (preferably under an AGPLv3 license).

How do I correctly attribute the original authors copyright / license in this case?

Answer 1

Under some interpretations, re-writing code to a different language constitutes a derivative work. Under others it doesn't.

If you hold your work not to be a derivative work (in the copyright sense of the word), you're done; you don't have anything to do with the original work anymore, and you can license whichever way you like without attributing the original authors.

I you do hold your work to be a derivative work, which is the safer, and IMO correct route, you have to comply with the 3-clause BSD license. The first two are the most relevant:

• Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

The most practical way to comply in my opinion is to include the BSD notice in the project, and have a central notice that remarks it was based on the original work, link to the original work and original authors file, and then do your standard stuff for the new license.