[vtkusers] Why is the image red?

[David Gobbi]

I'd have to say that I don't know.  Are you willing to do some VTK
code profiling with the intel compiler?

   David


On Tue, Dec 15, 2009 at 5:42 PM, Allan James <ar.james at qut.edu.au> wrote:
>
> Hi David,
>
> Doesn't the use of pointer arithmetic within loops like this prevent certain compilers (for example - Intel) from performing automatic vectorization?
>
> I'm pretty sure the Intel compilers will ignore such loops - which is a shame because the performance increase can be significant..
>
> Cheers,
>
> Allan James
> High Performance Computing & Research Support
> Queensland University of Technology
> (07) 3138 9264
> ar.james at qut.edu.au
> http://www.qut.edu.au/its/hpc
>
>><(((º>  ._.·´¯`·..  >++(((º>  ._.·´¯`·..  >++(((º>
>
>>       On Mon, Dec 14, 2009 at 6:02 PM, David Doria <daviddoria+vtk at gmail.com> wrote:
>>       >
>>       > ? ? ?ptr = static_cast<T *>(image->GetScalarPointer(p0, p1, z));
>>       >
>>       > ? ? ?pf = drawColor;
>>       > ? ? ?// Assign color to pixel.
>>       > ? ? ?for (idxV = 0; idxV <= maxV; ++idxV)
>>       > ? ? ? ?{
>>       > ? ? ? ?*ptr = static_cast<T>(*pf++);
>>       > ? ? ? ?ptr++;
>>       > ? ? ? ?}
>>       >
>>       > Couldn't this be simply:
>>       >
>>       > // Assign color to pixel.
>>       > for (component = 0; component <= maxV; ++component)
>>       > ?{
>>       > ?image->SetScalarComponentFromDouble(p0, p1, z, component, drawColor[i]);
>>       > ?}
>>       >
>>       > This function is a template function which is called with
>>       > vtkTemplateMacro. I don't understand what that is doing - isn't the
>>       > whole point of templates that you don't have to declare all of these
>>       > different types explicitly?
>>       >
>>       > Thanks,
>>       >
>>       > David
>>
>>       All of the VTK imaging filters work by doing pointer arithmetic and
>>       direct access to memory.  It's the VTK convention.  It's possibly not
>>       the best way for things to be done, but after you're familiar with the
>>       common pointer arithmetic coding patters it's easy to read this code
>>       and see what it's doing.  Really, if the main problem that I see is
>>       that the variables weren't named very well.
>>
>>       The SetScalarComponentFromDouble() method isn't 100% inlined and it
>>       involves a virtual function call, so it's far too inefficient to use
>>       inside an image filter.  Not to mention that it does a few
>>       multiplications and additions internally.  Yikes.
>>
>>       The big "switch" in the vtkTemplateMacro allows the execute methods to
>>       be templated while the classes themselves aren't.  Because the filter
>>       classes themselves aren't templated, an instance of a vtk filter class
>>       can actually operate on any data type that you feed it.  In contrast
>>       with e.g. ITK where filter instances are type specific.
>>
>>          David
>
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