[vtkusers] Problem mapping scalars to colors using a vtkLooku pTable

Nithiananthan, Sajendra Sajendra.Nithiananthan at rmp.uhn.on.ca
Tue Jul 3 10:16:23 EDT 2007

> Hello Geofram and others,

>Now, if only a vtkColorTransferFunction would allow you to specify an
opacity for each scalar... Are there any 
>classes that do such things?


I don't think there's a class that does it directly, but you can define a
transfer function for colour, and one for opacity, then use that to
construct the lookup table.  eg:

  vtkColorTransferFunction *colourTF = vtkColorTransferFunction::New();
  colourTF->AddRGBPoint(0, r0, g0, b0);
  colourTF->AddRGBPoint(255, r255, g255, b255);
  vtkPiecewiseFunction *opacityTF = vtkPiecewiseFunction::New();
  opacityTF->AddPoint(1, 0);
  opacityTF->AddPoint(255, 1); 

  vtkLookupTable *tbl = vtkLookupTable::New();
  tbl->SetRange(somemin , somemax); 
  for(int i=0; i < 256; i++) {
    double rgba[4];
    rgba[0] = colourTF->GetRedValue(i);
    rgba[1] = colourTF->GetGreenValue(i);
    rgba[2] = colourTF->GetBlueValue(i);
    rgba[3] = opacityTF->GetValue(i);


Hope that helps,

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