[vtkusers] Volume Render

[Darshan Pai]

I was trying most of the volume render functionality . I just wanted some
input on this effect I see.

I threshold  a volume dataset between 400 - 699 using vtkImageThreshold.
InValue set at 255 , out value set at 0

I feed this to the VolumeRaycastMapper. The function is set to a
vtkVolumeRayCastCompositeFunction.

In the property I set the opacity as follows
 vtkPiecewiseFunction *oTFun2 = vtkPiecewiseFunction::New();
  oTFun2->AddSegment(  10, 0.0, 200, 1);
  oTFun2->AddSegment(  200, 1, 201, 0);

This means that all scalar values higher than 201 should be clamped to
opacity 0 . Since the threshold volume has only 2 values 255 and 0 , the
output should be a blank screen . But I see some volume structures .

The question is why do I see volume elements even though the opacity for 0
and 255 are both at 0 ?

Regards
Darshan
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