[vtk-developers] Scope of VTK and it's potential as a common research language

Sun Jan 31 21:54:31 EST 2010

Jacob,
   In an attempt to move along a very interesting discussion, I would
like to get to the point:

   Would you be happy to use a BSD like license the same as VTK? It
seems to me that this will nicely remove all the complexities raised
in the previous discussion.

Is there a specific reason for preferring LGPL?

Regards
   Andrew

On Mon, Feb 1, 2010 at 6:55 AM, Benoit Jacob <jacob.benoit.1 at gmail.com> wrote:
> 2010/1/31 Benoit Jacob <jacob.benoit.1 at gmail.com>:
>> 2010/1/31 Berk Geveci <berk.geveci at kitware.com>:
>>> I don't agree. Here are two sections:
>>>
>>> 0. Additional Definitions.
>>>
>>> ...
>>>
>>> A “Combined Work” is a work produced by combining or linking an
>>> Application with the Library. The particular version of the Library
>>> with which the Combined Work was made is also called the “Linked
>>> Version”.
>>>
>>> ...
>>>
>>> 4. Combined Works.
>>>
>>> You may convey a Combined Work under terms of your choice that, taken
>>> together, effectively do not restrict modification of the portions of
>>> the Library contained in the Combined Work and reverse engineering for
>>> debugging such modifications, if you also do each of the following:
>>>
>>> ...
>>>
>>>
>>> I read these together to conclude that a binary that is compiled from
>>> any code that includes Eigen is "Combined Work" (a work produced by
>>> combining through the inclusion of templates). If that is the case, 4
>>> would apply and hence the reverse engineering clause. It would be
>>> against the spirit of LGPL to grant templated libraries less
>>> protection than compiled libraries so why would they leave such a
>>> loophole?
>>
>> Section 3 only talks about using material from header files. It never
>> says that this material is assumed to be used by #include'ing it.
>>
>> If VTK uses Eigen headers code, and then application X uses VTK, then
>> the only Eigen code that application X uses is code from Eigen's
>> header files. Actually, all Eigen code is located in header files. So
>> Section 3 still fully applies.
>>
>> One might wonder the extent to which that might be used to abuse the
>> LGPL by claiming that arbitrary files are header files. In the case of
>> Eigen, it's very simple: all our internal source file have the ".h"
>> extension and all our documentation tells to use Eigen by #including
>> files, not linking to anything.
>
> ...and perhaps most conclusively, these files are inherently header
> files at the level of their contents, for example, they have include
> guards, and they define templates without instantiating them which
> would be a NOP if they weren't headers.
>
> Benoit
>
>>
>> As to why this "loophole", I believe that's simply because there's no
>> way around it if the FSF wanted the LGPL to be applicable at all to
>> libraries that have nontrivial code in header files, which includes
>> most C++ libraries including Qt (think QVector). The reason why
>> there's no way around it is that #including is in essence similar to
>> static linking, and doesn't have a runtime analogue.
>>
>> Benoit
>>
>>
>>
>>>
>>> Of course, the copyright holders of Eigen are free to write an
>>> addendum to the license that explicitly states (4) does not apply.
>>>
>>> My vote is to keep Eigen out of core VTK. I wouldn't have any problem
>>> with releasing an application that brings together VTK and Eigen (and
>>> Qt for that matter) because I don't see any issues with LGPL's
>>> requirements but I don't think many of VTK's users that develop
>>> proprietary apps would share my opinion. We have worked very hard to
>>> keep VTK's license free of complications and the value Eigen adds is
>>> not worth wasting all of that effort.
>>>
>>> -berk
>>>
>>>
>>> On Sun, Jan 31, 2010 at 11:14 AM, Benoit Jacob <jacob.benoit.1 at gmail.com> wrote:
>>>> 2010/1/31 Berk Geveci <berk.geveci at kitware.com>:
>>>>> Hi Benoit,
>>>>>
>>>>> That's great! The only clause that still gives me pause is the
>>>>> following from section 4:
>>>>
>>>> Section 4 doesn't apply at all to Eigen ;) see my previous e-mail.
>>>>
>>>> So if this is your only concern then I am very optimistic :)
>>>>
>>>> Benoit
>>>>
>>>>>
>>>>> "You may convey a Combined Work under terms of your choice that, taken
>>>>> together, effectively do not restrict modification of the portions of
>>>>> the Library contained in the Combined Work and reverse engineering for
>>>>> debugging such modifications ..."
>>>>>
>>>>> I am guessing that this applies to Eigen and it allows anyone the
>>>>> right to reverse engineer a proprietary application for the purpose of
>>>>> debugging Eigen. Am I right? If that is the case, this would be a
>>>>> concern because this section does not impose a clear limitation on
>>>>> what reverse engineering means. To track a particular bug in Eigen,
>>>>> which is intertwined with the original code, you may need access to
>>>>> some top-secret-data-structure that the developers do not intend to
>>>>> release. I would expect that this would be unacceptable to some of the
>>>>> users of VTK.
>>>>>
>>>>> Sorry for making this complicated but such is life when mixing
>>>>> open-source with proprietary app development.
>>>>>
>>>>> -berk
>>>>>
>>>>> On Sun, Jan 31, 2010 at 12:36 AM, Benoit Jacob <jacob.benoit.1 at gmail.com> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> [Re-sending now that I am subscribed to the list which is subscribers-only]
>>>>>>
>>>>>> I'm one of the developers of Eigen. Reading your e-mails and your
>>>>>> concerns, I updated a bit the FAQ, see especially:
>>>>>> http://eigen.tuxfamily.org/index.php?title=Licensing_FAQ#How_about_static_linking.3F_2
>>>>>>
>>>>>> Let me summarize things a little (just elaborating on what Marcus said).
>>>>>>
>>>>>> Executive Summary:
>>>>>>
>>>>>> The LGPL version 3 is fully applicable to template libraries and there
>>>>>> simply is no issue at all with static linking, not even if VTK uses
>>>>>> Eigen and then some application links statically to VTK.
>>>>>>
>>>>>> Explanation:
>>>>>>
>>>>>> First of all, the old LGPL version 2.1 did indeed have lots of issues
>>>>>> with template libraries. That's why we don't use it for Eigen.
>>>>>>
>>>>>> The LGPL version 3 provides 3 different ways in why LGPL-licensed
>>>>>> libraries may be used: these are Sections 3, 4, and 5.
>>>>>>
>>>>>> In the case of Eigen, the only of these sections that you have to read
>>>>>> is Section 3. It covers the case of template libraries. Eigen is to be
>>>>>> used entirely under Section 3.
>>>>>>
>>>>>> Section 3 is very simple, it's essentially the same thing as a
>>>>>> 2-clause BSD license. That doesn't mean that LGPL=BSD, because there
>>>>>> still is the fact that the LGPL prevents proprietary forks of Eigen
>>>>>> itself.
>>>>>>
>>>>>> I would now like to address this concern:
>>>>>>
>>>>>>>> I disagree with this not being an issue. Adopting something licensed
>>>>>>>> under LGPL as a
>>>>>>>> core library in VTK is a bad idea since it would preclude anyone
>>>>>>>> building a statically
>>>>>>>> linked application using VTK - unless they were distributing all of
>>>>>>>> the components
>>>>>>>> required to relink.
>>>>>>
>>>>>> The "issues with statically linking to LGPL libraries" refer to
>>>>>> Section 4, specifically 4.d.1. As I said, Section 4 simply doesn't
>>>>>> apply to Eigen. The confusion comes from the fact that most LGPL
>>>>>> libraries are used under Section 4 (e.g. Qt is) but Eigen is just a
>>>>>> special case as it is a pure headers-only library with nothing to link
>>>>>> to. Thus Eigen is used entirely under Section 3, thus bypassing
>>>>>> Section 4 entirely, thus bypassing the issues with static linking in
>>>>>> particular. Subsequently, it just doesn't matter that VTK is a binary
>>>>>> library and not a headers-only library, because Section 4 just never
>>>>>> was used in the first place.
>>>>>>
>>>>>> More details and explanations in the FAQ that Marcus already linked to:
>>>>>> http://eigen.tuxfamily.org/index.php?title=Licensing_FAQ
>>>>>>
>>>>>> I hope that this addresses your concerns and am available if you need
>>>>>> more explanations, or disagree with mine!
>>>>>>
>>>>>> Thanks,
>>>>>> Benoit
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>>>>>>
>>>>>
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>>>>
>>>
>>
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Andrew J. P. Maclean
Centre for Autonomous Systems
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