[vtkusers] Uneven colors after the vtkButterflySubdivisionFilter

[David Gobbi]

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.