[vtkusers] VTKWiki, VTK/Examples/Histogram

[David Doria]

On Tue, Jan 5, 2010 at 1:09 PM, Dean Inglis <dean.inglis at camris.ca> wrote:
> Ive revised the example Histogram.cxx.  If you pass
> it a 3 channel image or less, the image gets broken
> up into its constituent components using vtkImageExtractComponents
> and a histogram is generated for each component.
> While experimenting with a 3 channel jpeg ( VTKData/Data/beach.jpg)
> I tried to limit the yscale of the vtkXYPlotActor to the maximum frequency
> range,
> but the maximum was always a large count corresponding to the value 0.
> Hence the plot gets a large spike at 0 and the remaining frequencies appear
> relatively small. If you set IgnoreZero to true for vtkImageAccumulate, it
> will still increment the bin count for value 0.  Is this an error?
>
>           if( !ignoreZero || double(*subPtr) != 0. )  // this is fine if
> ignore zero is true and the value is zero
>             {
>             // Gather statistics
>             sum[idxC] += *subPtr;
>             sumSqr[idxC] += (*subPtr * *subPtr);
>             if (*subPtr > max[idxC])
>               {
>               max[idxC] = *subPtr;
>               }
>             if (*subPtr < min[idxC])
>               {
>               min[idxC] = *subPtr;
>               }
>             (*voxelCount)++;
>             }
>           // compute the index
>           outIdx = static_cast<int>((static_cast<double>(*subPtr++) -
> origin[idxC]) / spacing[idxC]);
>           if (outIdx < outExtent[idxC*2] || outIdx > outExtent[idxC*2+1])
>             {
>             // Out of bin range
>             outPtrC = NULL;
>             break;
>             }
>           outPtrC += (outIdx - outExtent[idxC*2]) * outIncs[idxC];
>           }
>         if (outPtrC)  // regardless of whether ignore zero is true, it seems
> that the bin for value 0 gets incremented anyway
>           {
>           ++(*outPtrC);
>           }
>
> I was figuring that histogram->GetOutput()->GetScalarRange()[1] would return
> the max frequency (in the case of unsigned char data), and it does, its just
> in this
> particular case when the frequency is maximum for value 0, ignorezero doenst
> do what
> I hoped it would do.  Altering the last if condition would fix this wouldnt
> it?:
>
>         if (outPtrC && (!ignoreZero || double(*subPtr) != 0.))
>           {
>           ++(*outPtrC);
>           }
>
> Dean

Dean -

I don't know the answer to your question - but I separated the example
to demonstrate the BarChar separately:

http://www.cmake.org/Wiki/VTK/Examples/Histogram
http://www.cmake.org/Wiki/VTK/Examples/HistogramBarChart

Let me know if you have an objection.

Thanks,

David