QGIS as 'official' GRASS viewer (was Re: [GRASS5] d.legend and d.out.png)

[Glynn Clements]

Moritz Lennert wrote:

> > | In addition, as a special exception, the QGIS Development Team gives
> > | permission to link the code of this program with the Qt library,
> > | including but not limited to the following versions (both free and
> > | commercial): Qt/Non-commerical Windows, Qt/Windows, Qt/X11, Qt/Mac, and
> 
> So there is a free windows version of Qt ? I thought the free versions
> were only for Unix, Linux and MacOSX ?

There is a "free for non-commercial use" version of Qt for Windows,
but the terms are pretty restrictive (i.e. they don't just prohibit
use in commercially-distributed software, but also impose restrictions
on its use in "in-house" software within a business).

> > GRASS libs are unfortunately under pure GPL, so GRASS provider+plugin
> > for QGIS cannot be distributed in binary form for Windows.
> 
> Why not ? Is it because QGIS on windows is not GPL ? (Sorry I am a bit
> ignorant in these issues ?)

It's essentially because the Windows version of QGIS isn't
sufficiently free.

You can link GPL code against libraries which aren't themselves GPL,
provided that they are "compatible" with the GPL, i.e. they can be
used and redistributed without imposing additional conditions. IOW,
you can only use libraries with less-restrictive licences (e.g. LGPL,
MIT, BSD without advertising clause) unless they are part of the OS
itself.

Note that QGIS' "Qt exemption" doesn't help here, as that only applies
to the QGIS code; the GRASS libraries don't have such an exemption. 

[FWIW, I wouldn't be in favour of such an exemption; I'd rather the Qt
supporters made the effort to (natively) port the GPL version to
Windows. Given that a Windows version exists, Qt is likely to have
been written in such a way as to facilitate cross-platform
portability.]

However: if you can compile the GPL version of Qt on Windows (e.g. 
using Cygwin or SFU) such that the resulting DLL is binary-compatible
with the other Windows versions of Qt, nothing would prevent users
from using a different version of Qt at run time. OTOH, I suspect
that's likely to be a remote possibility.

-- 
Glynn Clements <glynn.clements at virgin.net>