[vtkusers] can i get the rgba from vtkvolume?

[yhq at sjtu.edu.cn]

in my program I have writen:  
    mymapper->SetInput(ImageReslice->GetOutput()); 
    volumeProperty->SetColor(ctfun);
    volumeProperty->SetScalarOpacity(tfun);
    volumeProperty->SetAmbient(0.3);
    volumeProperty->SetDiffuse(0.7);
    volumeProperty->SetSpecular(0.5);
    volumeProperty->SetSpecularPower(0.5);
    volumeProperty->SetInterpolationTypeToLinear();
    volumeProperty->ShadeOn();
    newvol->SetProperty(volumeProperty);
    newvol->SetMapper(mymapper);//////
    vtkVolumeTextureMapper * mymapper;
and in   vtkVolumeTextureMapper::Render() I want to use GetRGBArray (),but I got the NULL; why  
how can I get the rgbaarray in the render()?