dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Jan 16, 2009 at 08:20:40AM +0100, Johan Hake wrote:
> Ali!
> 
> > def numpyArrayToDolfinMatrix(a):
> >     nx = a.shape[0]
> >     ny = a.shape[1]
> >     m = dolfin.Matrix(nx, ny)
> >     m.set(a, numpy.arange(0, nx, dtype=numpy.uint0), numpy.arange(0,
> > ny, dtype=numpy.uint0))
> >     m.apply();
> >     return m
> >
> > Can you confirm that from the c++ side, just a buffer pointer is
> > passed and there is no loop involved?
> 
> For the DOLFIN library this depends on which linear backend you are using. 
> This is given by:
> 
>   dolfin_get("linear algebra backend")
> 
> For the uBLAS and MTL4 backend we do a loop. For the PETSc and Epetra backend 
> we just send the pointer to the underlying library. The last ones support 
> distributed data, which DOLFIN also intend to use, so there will probably be 
> some loop in these libraries too.
> 
> > I'm actually trying to convert a c++ image data structure to dolfin
> > matrix, why shouldn't dolfin have a design to dynamically accept this
> > sort of input? 
> 
> Well, the DOLFIN library is built to produce the matrices by assembling 
> variational problems defined by forms. The DOLFIN linear algebra part is 
> built to facilitate this, and to be able to use different backends.

It sounds like you should consider using a dense matrix
representation which means using the class uBLASDenseMatrix.

-- 
Anders


> > Here is how the c++ image data is converted from/to 
> > numpy in our project:
> >
> > http://code.google.com/p/wrapitk/source/browse/trunk/ExternalProjects/PyBuf
> >fer/itkPyBuffer.txx
> >
> > I'm trying to solve some variational image processing problems where
> > the data term in the PDE comes from the image buffer. Just to make
> > sure, is dolfin matrix the right way of inputing the image data or am
> > I missing something?
> 
> I think the best way to find out is to state your question in mathematical 
> terms. Who knows maybee someone on the list are trying to solve the same 
> problem, I am not...
> 
> Probably you want to read in the data in a Function and use this in your 
> variational formulation. 
> 
> Johan
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