Re: [vcg-devel] Major changes ahead....

[Gael G. <gae...@gm...>]

UPDATE: eventually I managed to workaround the second incompatibility
I listed about Transpose (but that does not means you should not care
about updating them ;), see my remarks )
So, it remains only the dot product as the unique incompatibility.

gael.

On Wed, Oct 29, 2008 at 5:36 PM, Gael Guennebaud
<gae...@gm...> wrote:
> Dear all,
>
> for your information all the changes I made in vcg related to matrices
> and points are enabled only if you define VCG_USE_EIGEN.
>
> Since 2 minutes or so, meshlab compiled with that token on the
> following three platforms:
> - linux, gcc 4.3
> - linux gcc 4.0.1 (= mac's version + epsilon)
> - msvc 9.0
>
> This token is now defined by default in meshlab. If you really have
> big troubles, then edit the files shared.pri and meshlab/meshlab.pro
> to remove it and, please, send me you compiler errors.
>
> In order to allow for a smooth transition, the goal was to try to keep
> a source compatibility with previous matrix/point classes once
> VCG_USE_EIGEN defined. In practice this was not entirely possible, and
> here is the list of the major issues you might have while enabling
> VCG_USE_EIGEN:
>
> - first of all make sure your code compile without it. In particular
> we had to change Point*::Zero() to the more explicit SetZero(). Remind
> that p.Zero(); set p to zero. Once VCG_USE_EIGEN is defined, p.Zero()
> will still compile but it will do nothing ! Indeed in Eigen, Zero is a
> static member function which returns the expression of a zero matrix
> or vector. For consistency I've also changed the few other Zero()
> functions which actually performed a "set *this to zero", a big regexp
> replacement should do the job in most cases.
>
> - the second major difference is that the dot product is no longer
> defined via the operator*. Instead use p1.dot(p2). Perhaps we can also
> add a global function vcg::dot(p1,p2) if some of you would prefer such
> a syntax. Fortunately those errors are caught at compile time: you
> should first get some weird messages stating that it cannot convert a
> Eigen::Product<...> to a scalar followed by the very explicit:
> invalid_vector_vector_product__if_you_wanted_a_dot_or_coeff_wise_product_you_must_use_the_explicit_functions
>
> - the third and last incompatibility is with respect to m.Transpose()
> and vcg::Transpose(m). I cannot provide such functions because
> Transpose is the name of a class in Eigen. The quick fix is to replace
> them by either:
>  * m.transposeInPlace() if you really want to transpose the matrix
> you have in hand (it returns nothing).
>  * m.transpose() if you only want to use the transpose of m (it does
> not change m, and returns a transposed expression)
> If you want to be a bit smarter, you should also check for code like that:
>  Matrix33 mt = m;
>  mt.Transpose();
>  m * mt;
> which should be replaced by the more efficient and intuitive:   m *
> m.transpose();
> Note that those 2 functions also work if VCG_USE_EIGEN is not defined.
>
> - finally I removed a couple of things which seemed to be used only
> internally by vcg math classes like MatrixDiag*, and LinearSolver.
> Again fell free to contact me if that's a major problem for you or if
> you are unsure about how to work around.
>
> These 3 modifications are the only ones I had to do to make meshlab
> compatible. So you have some troubles please report them to me.
>
> Once you've done that you can start using Eigen's API which is
> available trough all Matrix and Point types.
>
> Of course, this is only the first step towards this major change, more
> to come later...
>
>
> cheers,
> gael.
>
>
>
>
> On Tue, Oct 28, 2008 at 9:44 AM, Paolo Cignoni
> <pao...@is...> wrote:
>> Dear Developers,
>>
>> as you could have noted, some major changes happened to our VCG library:
>> the currently very chaotic, buggy, and incomplete math folder is going
>> to be replaced by
>> a clean, documented and efficient template library for linear algebra
>> (vectors, matrices,
>> and related algorithms): Eigen  ( http://eigen.tuxfamily.org )
>>
>> You should have seen a new folder in the vcglib repository, eigen, this
>> folder
>> is a SVN direct link to the repository of eigen, so you have no write
>> permission
>>  on that folder (you have to be a eigen developer to commit stuff there).
>> More details on these changes later.
>>
>> In the meantime, if something stop compiling, dont panic :)
>>
>> I wish to thank warmly Gael Guennebaud, a core developer of eigen, for
>> suggesting and implementing this change
>> (and obviously for all his point related contribs inside meshlab :) )
>>
>> Cheers
>>
>> Paolo
>>
>> --
>> Paolo Cignoni  -- Senior Researcher
>> Visual Computing Laboratory - ISTI - CNR
>> http://vcg.isti.cnr.it/~cignoni
>>
>> ISTI - CNR
>> Via Moruzzi 1,
>> 56124 Pisa
>> ITALY
>>
>>
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