dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Thu, Feb 19, 2009 at 08:45:38AM +0000, A Navaei wrote:
> 2009/2/19 Anders Logg <logg@xxxxxxxxx>:
> > On Wed, Feb 18, 2009 at 07:56:59PM +0000, A Navaei wrote:
> >> 2009/2/18 Anders Logg <logg@xxxxxxxxx>:
> >> > On Wed, Feb 18, 2009 at 02:04:35PM +0000, A Navaei wrote:
> >> >> 2009/2/17 Anders Logg <logg@xxxxxxxxx>:
> >> >> > On Tue, Feb 17, 2009 at 03:28:08PM +0000, Garth N. Wells wrote:
> >> >> >>
> >> >> >>
> >> >> >> A Navaei wrote:
> >> >> >> > The following minimal test for Function in c++ reveals some bugs. I
> >> >> >> > guess this example can help me with dealing with the current issues of
> >> >> >> > ImageFunction.
> >> >> >> >
> >> >> >> > (1) interpolate.py does not work when a Function is created in c++ and
> >> >> >> > wrapped (see comment [2]). It seems that the bug is originated from
> >> >> >> > the copy constructor (see comment [3])
> >> >> >> >
> >> >> >> > (2) In order to perform the interpolation, why is it necessary to
> >> >> >> > create another Function and then copy it?
> >> >> >> >
> >> >> >> > (3) Signature checkes seem not working properly (see comment [1]). The
> >> >> >> > signature-based assignments are error-prone anyway, why the
> >> >> >> > object-oriented approach is not used?
> >> >> >> >
> >> >> >>
> >> >> >> Signatures are used to permit reading/writing Functions to a file. They
> >> >> >> are indeed error prone, so I believe that we reached a consensus a short
> >> >> >> while ago that we would remove pre-compiled elements.
> >> >> >>
> >> >> >> Garth
> >> >> >
> >> >> > Instead of signatures, I'd recommend that you define a simple form
> >> >> > file for each of the different types of FunctionSpace you need, for
> >> >> > example:
> >> >> >
> >> >> >  element = FiniteElement("CG", "triangle", 1)
> >> >> >
> >> >> >  v = TestFunction(element)
> >> >> >  u = TrialFunction(element)
> >> >> >  a = v*u*dx
> >> >> >
> >> >> > If you put this in a file named My.form and compile it with FFC using
> >> >> > -l dolfin, you will get a class named MyFunctionSpace that you can
> >> >> > then instantiate using just a mesh:
> >> >> >
> >> >> >  MyFunctionSpace V(mesh);
> >> >> >
> >> >> > Create one form file for each of the different types of FunctionSpace
> >> >> > that you need, name the files to something suitable and use the
> >> >> > generated code. That way you won't need to worry about signatures,
> >> >> > dofmaps and finite elements.
> >> >>
> >> >> Effectively, I've been using the very same method all this time, it
> >> >> does not work.
> >> >
> >> > Yes, it does. It's used in about 20 of the demos.
> >> >
> >> >> The copy constructor fix never worked. I've been trying to explain
> >> >> this in many different ways, but the right attention was never paid to
> >> >> this. Let's see if the sandbox example can convince you this time.
> >> >>
> >> >> A Function instance still cannot be returned by reference (or value).
> >> >> Returning as shared_ptr seems to work initially, but eventually it
> >> >> generates segmentation fault -- see attached.
> >> >
> >> > Yes, it can. There's absolutely no problem to return a Function by
> >> > reference. See the updated sandbox demo.
> >> >
> >> > The only problem is when you want to copy a Function which is only
> >> > defined in terms of an eval() operator. In those cases the Function
> >> > cannot be copied.
> >> >
> >> > If you do the following:
> >> >
> >> > class MyFunction : public Function
> >> > {
> >> > public:
> >> >
> >> >  MyFunction(const FunctionSpace& V) : Function(V) {};
> >> >
> >> >  void eval(double* values, const double* x) const
> >> >  {
> >> >    values[0] = sin(x[0]);
> >> >  }
> >> > };
> >> >
> >> > MyFunction f(V);
> >> > Function g = f;
> >> >
> >> > Do you then expect g to return sin(x)? It would be possible to
> >> > implement this but it would require g to keep a pointer to f so that
> >> > the eval() in g may call the eval() in f.
> >>
> >> Yes, we eventually want to get the image data in eval() and obviously
> >> the work around:
> >>
> >>  Function g(V);
> >>  g.vector();
> >>  _f = g;
> >>
> >> does not call eval(). I don't think if it is possible to do this
> >> without amending the Function class? The denial of changing the
> >> visibility of the member variables to protected is making this
> >> unnecessarily more and more complicated. I am attaching the updated
> >> sandbox test.
> >>
> >>
> >> -Ali
> >
> > I've already said that we can make the _function_space protected in
> > the Function class. I've done so now and also added a new
> > interpolation function. See if that helps.
> 
> I should have missed that, I didn't notice the message from hg to
> dolfin-dev about this (still cannot find it). I'm afraid, while this
> would help, it's not still enough. Let's revise the design of an image
> function again:
> 
> My guess is that the following constructor for ImageFunction is
> necessary and sufficient:
> 
> Image image;
> FiniteElement element;
> ImageFunction v(image, element);

Why not simply

  Image image;
  ImageFunction v(image);

?

The class ImageFunction could take some optional arguments for how to
represent the function, like degree, continuous or discontinuous etc.

> The user should be able to specify the input image, and the type of
> finite element of interest. The mesh completely depends on image size
> and dimension and the user does not need to define it externally.
> DofMap is also a function of Mesh, and involving it in the constructor
> may be redundant. What do you think?

I suggest avoiding FiniteElement and DofMap. Use generated subclasses of
FunctionSpace instead.

> 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.

-- 
Anders

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