[vtkusers] vtkPoints + InsertPoint

[David Gobbi]

The call to GetNumberOfPoints() is returning MaxId+1, which is exactly
what it is supposed to do.

points->InsertPoint(609839, x, y, z);
points->InsertPoint(609838, x, y, z);
points->GetNumberOfPoints() -> 609840 = 609839+1

The lowest Id is always zero, and the maxId is 609839, so the number
of points is 609840.

  David


On Fri, Mar 12, 2010 at 8:41 AM, Bill Chivas <noo134 at googlemail.com> wrote:
> 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>
>> >>
>> >> 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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