vtkImageData的GetScalarPointer参数范围和SetExtent的范围相关

根据VTK源码的

vtkIdType vtkImageData::GetTupleIndex(vtkDataArray* array, int coordinate[3])
{
  vtkIdType incs[3];
  vtkIdType idx;

  if (array == nullptr)
  {
    return -1;
  }

  const int* extent = this->Extent;
  // error checking: since most accesses will be from pointer arithmetic.
  // this should not waste much time.
  for (idx = 0; idx < 3; ++idx)
  {
    if (coordinate[idx] < extent[idx * 2] || coordinate[idx] > extent[idx * 2 + 1])
    {
      vtkErrorMacro(<< "GetPointer: Pixel (" << coordinate[0] << ", " << coordinate[1] << ", "
                    << coordinate[2] << ") not in current extent: (" << extent[0] << ", "
                    << extent[1] << ", " << extent[2] << ", " << extent[3] << ", " << extent[4]
                    << ", " << extent[5] << ")");
      return -1;
    }
  }

  // compute the index of the vector.

  // Array increments incorporate the number of components, which is not how
  // vtkDataArrays are indexed. Instead, compute the tuple increments.
  {
    incs[0] = 1;
    incs[1] = (extent[1] - extent[0] + 1);
    incs[2] = incs[1] * (extent[3] - extent[2] + 1);
  }

  idx = ((coordinate[0] - extent[0]) * incs[0] + (coordinate[1] - extent[2]) * incs[1] +
    (coordinate[2] - extent[4]) * incs[2]);
  // I could check to see if the array has the correct number
  // of tuples for the extent, but that would be an extra multiply.
  if (idx < 0 || idx > array->GetMaxId())
  {
    vtkErrorMacro("Coordinate (" << coordinate[0] << ", " << coordinate[1] << ", " << coordinate[2]
                                 << ") out side of array (max = " << array->GetMaxId());
    return -1;
  }

  return idx;
}

最后idx索引的计算需要减去extent后，计算得到。