[vtk-developers] Changing floating point Types in VTK

Max Sagebaum max.sagebaum at scicomp.uni-kl.de
Mon May 11 04:22:45 EDT 2015

Dear developers,

we had asked for your opinions about a typedef for the floating point
types in vtk and if we make this change if it would be accepted. Until
now we have received no answer. We would appreciate it, if we could get
some insights about the acceptance of the changes we want to make.

Best regards,

Max Sagebaum 

On Mon, 2015-04-20 at 09:04 +0200, Max Sagebaum wrote:
> Dear developers,
> we are currently using parts of the VTK Version 5.10 for our CFD
> Postprocessing Tool. We have successfully modified theses parts to
> facilitate an AD-Tool (Algorithmic Differentiation) to differentiate
> these code parts (intention is to not just get absolute values for our
> CFD-results, but also evaluate sensitivities/gradients). Caused by
> further developments in our tool we would like to go with newer
> VTK-Versions, which means repeating the changes in the code parts
> facilitating AD. We would like to implement these changes into the
> VTK-trunk, to avoid reprogramming and to offer the possibility of using
> AD to everyone. We would like to explain the changes in short and ask
> you to judge if such changes are likely to be accepted or if there is an
> interest in making VTK “AD-able”. If not, we would avoid the presumably
> laborious implementation and would not put it into the Git/Develop loop.
> There are two main changes we had to make. To get the gradients of a
> program, the AD-Tool defines new AD floating point types which take care
> of the derivative computation. In order to use these AD types in VTK we
> needed to create two typedefs for float and double. Then every usage of
> float and double in VTK was replaced by the new typedefs. With this
> change, it is also possible to other types in VTK. E.g. complex values.
> The second change we made, was to rename every 'static_cast' call to a
> new function 'perform_cast'. Basically 'perfom_cast' just calls
> 'static_cast'. If you use other types than float or double 'static_cast'
> will complain that the conversion from eg. 'MyFloatType' to 'int' does
> not exist. 'perfom_cast' was specialized for the AD types and therefore
> the casts could be done. During this process we checked most of the
> occurrences of 'static_cast' and validated that the usage will not break
> the differentiation.  
> There are two other solutions to our approach. We could implement
> conversion operators for the AD types, which enables 'static_cast' to do
> the conversion. But this is mostly discouraged by the tool developers.
> If the conversion operators are implemented, the compiler starts to
> implicit convert the AD types and very weird things can happen.
> The second approach would define these operators as explicit which would
> prevent the compiler from implicitly converting them. But then VTK would
> need to be compiled with C++11 enabled or if the user changes these
> types he also has to enable C++11 if necessary. 
> Is the any interest on the developers or users side to use VTK with AD
> or other floating point types?
> Thanks for your opinions and judgment!
> Max Sagebaum

Max Sagebaum

Chair for Scientific Computing,
TU Kaiserslautern,
Bldg/Geb 34, Paul-Ehrlich-Strasse,
67663 Kaiserslautern, Germany

Phone: +49 (0)631 205 5638
Fax:   +49 (0)631 205 3056
Email: max.sagebaum at scicomp.uni-kl.de
URL:   www.scicomp.uni-kl.de

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