[vtkusers] vtkPoints + InsertPoint

[Bill Chivas]

ok.
And why GetNumberOfPoints() does not work ok?

2010/3/12 David Gobbi <david.gobbi at gmail.com>

> This thread is full of nonsense.  The reason for the error is in the
> details of how InsertTuple works. Let's dissect what happens when you
> do this:
>
> points->InsertPoint(60983688, x, y, z);
>
> First, vtkDataArrayTemplate.txx (which is the actual container) will
> check to see if it has enough memory to hold 60983688 tuples, and if
> not, it will allocate a new array that is the sum of the old size and
> the new size.  Look at the code for
> vtkDataArrayTemplate::ResizeAndExtend(vtkIdType sz).  Whenever
> InsertPoint requires that the array is resized, the new size is always
> more than double the previous size.
>
> The reason VTK does this is that resizing by a factor of two is known
> to be the most computationally efficient way to grow an array when the
> final size is unknown.  It is definitely not a memory-efficient way of
> doing things, but the VTK designers have, quite correctly, chosen
> computational efficiency over memory efficiency.
>
> So, when you are using InsertPoint or InsertNextPoint (or InsertTuple
> or InsertNextTuple), the array will allocate up to twice as much
> memory as you actually need.  And if you call InsertTuple repeatedly,
> then there will be instances where VTK will temporarily require up to
> three times as much memory as you actually need, while it is copying
> from the old array to the newly allocated array as part of the resize
> operation.
>
> You can avoid this by using SetNumberOfPoints() to allocate exactly as
> much space as you need before you add any points.
>
>   David
>
>
> On Fri, Mar 12, 2010 at 8:06 AM, Bill Chivas <noo134 at googlemail.com>
> wrote:
> > Thanks David.
> > I run it without crashing, too.
> > So, i think the problem is somewhere else ...
> >
> > Thanks,
> > Bill
> >
> > 2010/3/12 David Doria <daviddoria+vtk at gmail.com<daviddoria%2Bvtk at gmail.com>
> >
> >>
> >> On Fri, Mar 12, 2010 at 9:36 AM, Bill Chivas <noo134 at googlemail.com>
> >> wrote:
> >>>
> >>> To David:
> >>>
> >>> If it works with only two points, try to put some more (e.g. 10).
> >>> In mine, it crashes in 3 or 4 points.
> >>>
> >>
> >> This runs without crashing for me:
> >> #include <vtkSmartPointer.h>
> >> #include <vtkPoints.h>
> >> #include <vtkMath.h>
> >> int main(int argc, char *argv[])
> >> {
> >>   vtkSmartPointer<vtkPoints> points =
> >>       vtkSmartPointer<vtkPoints>::New();
> >>
> >>   points->SetNumberOfPoints(61000000);
> >>
> >>   int start = 60983588;
> >>   for(unsigned int i = 0; i < 100; i++)
> >>     {
> >>     points->InsertPoint(start + i, vtkMath::Random(0.0,1.0),
> >> vtkMath::Random(0.0,1.0), vtkMath::Random(0.0,1.0));
> >>     }
> >>
> >>   return EXIT_SUCCESS;
> >> }
> >> Thanks,
> >>
> >> David
> >
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