[vtkusers] Uneven colors after the vtkButterflySubdivisionFilter

[Bill Lorensen]

In ITK we use traits to turn runtime tests into compile tests. This might
help, And specialization might help to provide clamping for those types
that need it.




On Sun, Apr 21, 2013 at 4:14 PM, David Gobbi <david.gobbi at gmail.com> wrote:

> Yes, but performance could equally well be a non-issue.  No way to be
> sure without trying a fix and doing some timings.  I strongly suspect
> the performance hit would be negligible, given the large amount of
> code in InterpolateTuple().
>
>  - David
>
>
> On Sun, Apr 21, 2013 at 1:43 PM, Bill Lorensen <bill.lorensen at gmail.com>
> wrote:
> > Performance could be an issue.
> >
> >
> > On Sun, Apr 21, 2013 at 3:39 PM, David Gobbi <david.gobbi at gmail.com>
> wrote:
> >>
> >> Hi Bill,
> >>
> >> I traced the code that is interpolating the scalars, and it led me to
> >> the following function in vtkDataArray.cxx, which is called from
> >> vtkDataArray::InterpolateTuple().  In fact, that is the only place
> >> this function is called from.
> >>
> >>   template <class T>
> >>   inline void vtkDataArrayRoundIfNecessary(double val, T* retVal)
> >>   {
> >>     *retVal = static_cast<T>((val>=0.0)?(val + 0.5):(val - 0.5));
> >>   }
> >>
> >> It might be good for us to add a clamp here immediately before the
> >> cast, rather than to trust that the weights will never sum to a value
> >> greater than one.
> >>
> >> Either that, or vtkButterflySubdivisionFilter can be modified so that
> >> it normalizes the weights before it interpolates the scalars (i.e. it
> >> would be approximating when computing the points, but would use
> >> normalized weights when computing the scalars).  My preference,
> >> though, would be to modify vtkDataArray in order to make the fix more
> >> general.
> >>
> >>  - David
> >>
> >>
> >>
> >> On Sun, Apr 21, 2013 at 10:12 AM, Bill Lorensen <
> bill.lorensen at gmail.com>
> >> wrote:
> >> > Maybe I have it this time.
> >> >
> >> > Multiple problems:
> >> > 1) First number of points must be used, not number of ploys
> >> > 2) I think color interpolation is overflowing/underflowing the
> unsigned
> >> > char
> >> >
> >> > Solution. Use FloatArray for colors. After subdivision,  convert the
> >> > floats
> >> > to unsigned chars, clamping between 0 and 255.
> >> >
> >> > Attached is an example c++ that seems to work.
> >> >
> >> >
> >> >
> >> > On Sun, Apr 21, 2013 at 11:46 AM, Bill Lorensen
> >> > <bill.lorensen at gmail.com>
> >> > wrote:
> >> >>
> >> >> Oops. I still see problems after that change.
> >> >>
> >> >>
> >> >>
> >> >> On Sun, Apr 21, 2013 at 11:45 AM, Bill Lorensen
> >> >> <bill.lorensen at gmail.com>
> >> >> wrote:
> >> >>>
> >> >>> Found the problem.
> >> >>>
> >> >>> You want to set the point data, but your colors array is filled for
> >> >>> NumberOfPolys. Replace NumberOfPolys with NumberOfPoints and the
> >> >>> results
> >> >>> look good.
> >> >>>
> >> >>>
> >> >>>
> >> >>> On Sun, Apr 21, 2013 at 10:43 AM, DivyaS <div.anand141 at gmail.com>
> >> >>> wrote:
> >> >>>>
> >> >>>> Bill,
> >> >>>>
> >> >>>> Butterfly subdivision gives me better results(shape) to the actual
> >> >>>> data
> >> >>>> i'm
> >> >>>> working on. Is there no other way we can get this working? you were
> >> >>>> talking
> >> >>>> about assigned scalar float, single component values, I would like
> to
> >> >>>> give
> >> >>>> it a try.. could you please give me some details on that.
>



-- 
Unpaid intern in BillsBasement at noware dot com
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