dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

The previous email was sent a bit premature...

[snip]

> > I have also thought of this :)
> >
> > Then you can take an ordered mesh, iterate over the vertices, and use the
> > present implementation of the eval function (which in its present shape
> > is a piecewise constant interpolator isn't it?), to fill the new _vector,
> > all done in the constructor. Then you have a discrete function in the
> > first place :)
>
> Is it possible to set the pointer to _vector data directly where we
> can pass the pointer to image data? If this works, no loop will be
> involve in data conversion and it will be super fast.

Yes but you do not know how the mesh is ordered.

> It seems _vector.set()
> (http://www.fenics.org/pub/documents/dolfin/dolfin-progr-reference/d6/da4/c
>lassdolfin_1_1GenericVector.html#bc4ecede55f2846ecadae02426df8f63) accepts
> the pointer to each row, is that right? 

There is no such thing as a row in a vector. It stores the data in a 
contigouos way (if not distributed).

> Is there a better way of assigning the whole data at once?

Maybe but you then have to be sure that your data is arrange in exact same 
manner as the mesh.

> > Note that this will only work for a piecewise linear lagrange
> > FunctionSpace.
>
> It should be ok, it would be equivalent of having the image function
> initialised with Lagrange element, but much more faster.
>
> > But haveing this in place you can always interpolate to other
> > FunctionSpaces. However these interpolations will only be linear of
> > course.
>
> It is not possible to interpolate a function to another space using
> non-linear interpolators?

If your raw image data is represented by a linear FunctionSpace any 
interpolation using this space would be linear.

Johan

> To Anders: Do you think if this is more practical and efficient than
> the other discussed approach? Would this be general enough to work
> with an arbitrary PDE of any finite element type?
>
>
> -Ali
>
> > Johan
> >
> >> -Ali
> >>
> >> > -Ali
> >> >
> >> >>> >> Another way of doing this could be by the use of an existing
> >> >>> >> FunctionSpace:
> >> >>> >>
> >> >>> >>   UnitSquare dummy_mesh(1, 1);
> >> >>> >>   PoissonFunctionSpace V(dummy_mesh);
> >> >>> >>   ImageFunction v(image, V);
> >> >>> >>
> >> >>> >> Then, in the constructor of ImageFunction, V.element and V.dofmap
> >> >>> >> can be used to create another FunctionSpace which has a mesh
> >> >>> >> created using image:
> >> >>> >>
> >> >>> >>   ImageToFunction(Image image, const FunctionSpace& V)
> >> >>> >>   {
> >> >>> >>     // Create the function space
> >> >>> >>     UnitSquare mesh(image.get_size()[0] - 1, image.get_size()[1]
> >> >>> >> - 1); FunctionSpace IV(mesh, V.element(), V.dofmap());
> >> >>> >>
> >> >>> >>     // ...
> >> >>> >>   };
> >> >>> >>
> >> >>> >> The problem with this approach is that it involves the use of a
> >> >>> >> dummy mesh.
> >> >>> >>
> >> >>> >> A mesh-independent constructor added to FunctionSpace could help.
> >> >>> >> Alternatively, if a (protected) default (empty) constructor is
> >> >>> >> added to FunctionSpace, ImageFunctionSpace:FunctionSpace can have
> >> >>> >> a mesh-independent constructor. However, the FFC-generated
> >> >>> >> function spaces, eg PoissonFunctionSpace, still need a mesh.
> >> >>> >>
> >> >>> >> Hope this makes the problem more clear now.
> >> >>> >
> >> >>> > Create the mesh and the FunctionSpace subclass inside the
> >> >>> > ImageFunction constructor. Neither the mesh nor the function space
> >> >>> > need to be visible outside.
> >> >>>
> >> >>> Again, there is no mesh-free ctor for FunctionSpace, and it doesn't
> >> >>> come with a default ctor so that the subclass can implement a
> >> >>> mesh-free ctor. I quote the above again:
> >> >>
> >> >> You don't need a mesh-free constructor for FunctionSpace, see above.
> >> >>
> >> >>> ' A mesh-independent constructor added to FunctionSpace could help.
> >> >>>  Alternatively, if a (protected) default (empty) constructor is
> >> >>> added to FunctionSpace, ImageFunctionSpace:FunctionSpace can have a
> >> >>> mesh-independent constructor.'
> >> >>
> >> >> It's not needed if you do as I suggest.
> >> >>
> >> >> --
> >> >> Anders
> >> >>
> >> >> -----BEGIN PGP SIGNATURE-----
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> >> >>
> >> >> iEYEARECAAYFAkmdMN8ACgkQTuwUCDsYZdGnOQCdFBJSD4FymLnVPbheRt63aJJa
> >> >> yyoAn3KDuOmwd8ZX5YR1KucbafvieNBc
> >> >> =lpyI
> >> >> -----END PGP SIGNATURE-----
> >> >>
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