[vtkusers] lut->SetAlphaRange and lut->SetRange do not work on RGBA images?

[Thomas Kilgus]

Hi everyone,

My problem is that the methods SetAlphaRange and SetRange do not have 
any effect on RGBA images. SetRange works well on gray scale images.

I use the following pipeline to render arbitrary images:
Load the image -> Reslice it via vtkImageReslice -> Apply some 
properties with a lookuptable like Window/Level, opacity, etc. -> render 
the texture with a vtkPolyDataMapper.

Here comes the code (which I think is important for my question):
vtkSmartPointer<vtkLookupTable> lut = 
vtkSmartPointer<vtkLookupTable>::New();

   //built a default lookuptable
lut->SetSaturationRange( 0.0, 0.0 );
lut->SetHueRange( 0.0, 0.0 );
lut->SetValueRange( 0.0, 1.0 );
lut->Build();
   //map all black values to transparent
lut->SetTableValue(0, 0.0, 0.0, 0.0, 0.0);

lut->SetRange(someValue, someValue); //someValue between 0 and 255
lut->SetAlphaRange(someOtherValue, someOtherValue); //someOtherValue 
between 0 and 1

[...] //load the image

vtkSmartPointer<vtkTexture> texture = vtkSmartPointer<vtkTexture>::New();
texture->SetLookupTable(lut);
texture->SetInput(image);

vtkSmartPointer<vtkPlaneSource> plane = 
vtkSmartPointer<vtkPlaneSource>::New();
[...] //generate a plane to render the texture on

vtkSmartPointer<vtkPolyDataMapper> mapper = 
vtkSmartPointer<vtkPolyDataMapper>::New();
mapper->SetInputConection(plane->GetOutputPort());

vtkSmartPointer<vtkActor> actor = vtkSmartPointer<vtkActor>::New();
actor->SetMapper(mapper);
actor->SetTexture(texture);

[...] //pass the actor to the renderer



Does anyone have an idea what could be wrong? Again: the lut is working 
fine for grayscale images, but not for RGBA images...

best regards
Thomas