Extract opacity value at 3d point (in a form of world coordinate) on vtkImageData?

Given that ijk[3] is the world coordinate of a point and xyz[3] is transformed by (connector->GetOutput()->TransformPhysicalPointToContinuousIndex(ijk[0], ijk[1], ijk[2], xyz);

I have tried the following ways:

  1. volume->GetProperty()->GetScalarOpacity(…);
  2. volume->GetProperty()->GetScalarOpacity()->GetValue(…);

I have no idea how to input parameters in these function. Can anyone help me?