[scikit-image] linking to GPL code from BSD code (was: BM3D)

[Juan Nunez-Iglesias]

Thanks Josh. That was a very informative click-hole, and I now fully agree with your assessment that a Python import in a BSD package would not violate the GPL clause. I would caution two things:

- If scikit-image required GPL-licensed code to function, then any downstream distribution would need to bundle both together, and might thus fall foul of GPL restrictions. Therefore I would argue that if we include GPL dependencies, we should make them optional.

- If we were to vendor GPL code with scikit-image, I believe that would also violate the GPL if we did not license the resulting library as GPL.

Having said that, if we stick to optional dependencies, e.g. on PyFFTW to speed up our FFT code if it is present, then I think we would be in the clear. And this is a very attractive proposition.

Thanks,

Juan.
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