dolfin team mailing list archive

[A Navaei <axnavaei@xxxxxxxxxxxxxx>]

2009/2/19 Garth N. Wells <gnw20@xxxxxxxxx>:
>
>
> A Navaei wrote:
>>
>> 2009/2/19 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.
>>
>> Consider a 2D image data mapped on a structured grid, with size
>> (number of nodes in each direction) of (sx, sy), the value of image
>> brightness at coordinate (i, j) is given by I(k), where:
>>
>> k = i + sx * j
>>
>> Is this not enough information to create the mesh?
>>
>>>> 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).
>>
>> No idea why it says rows: virtual void dolfin::GenericVector::set
>> (const double *block, const uint *num_rows, const uint *const *
>>        rows).
>>
>
> You can set as few or as many entries as you wish at once. 'block' contains
> the values you want to set, 'num_rows' are the number of entries you wish to
> set, and 'rows' are the positions into which the entries should be inserted.

I hate to say this, but _vector is private!


-Ali

>
> Garth
>
>
>>>> 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.
>>
>> As long as we work with complete rectangular images, which is the
>> frequently used case, it should be fine. Given this, how is it
>> possible to directly assign the image data pointer to _vector?
>>
>>>>> 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.
>>
>> Right, that's yet another problem.
>>
>>
>> -Ali
>>
>>> 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-----
>>>>>>>> Version: GnuPG v1.4.9 (GNU/Linux)
>>>>>>>>
>>>>>>>> iEYEARECAAYFAkmdMN8ACgkQTuwUCDsYZdGnOQCdFBJSD4FymLnVPbheRt63aJJa
>>>>>>>> yyoAn3KDuOmwd8ZX5YR1KucbafvieNBc
>>>>>>>> =lpyI
>>>>>>>> -----END PGP SIGNATURE-----
>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> DOLFIN-dev mailing list
>>>>>>>> DOLFIN-dev@xxxxxxxxxx
>>>>>>>> http://www.fenics.org/mailman/listinfo/dolfin-dev
>>>>>>
>>>>>> _______________________________________________
>>>>>> DOLFIN-dev mailing list
>>>>>> DOLFIN-dev@xxxxxxxxxx
>>>>>> http://www.fenics.org/mailman/listinfo/dolfin-dev
>>>
>>>
>> _______________________________________________
>> DOLFIN-dev mailing list
>> DOLFIN-dev@xxxxxxxxxx
>> http://www.fenics.org/mailman/listinfo/dolfin-dev
>
>
>