How i can get a oblique view throught a line in VolumeViewport?

I have find that these can show oblique view in VolumeViewport though tools/examples/volumeAnnotationTools by using this code

viewport = 
      viewportId: viewportIds[2],
      type: ViewportType.ORTHOGRAPHIC,
      element: element3,
      defaultOptions: {
        orientation: {
          // Random oblique orientation
          viewUp: <Types.Point3>[
            -0.5962687530844388, 0.5453181550345819, -0.5891448751239446,
          viewPlaneNormal: <Types.Point3>[
            -0.5962687530844388, 0.5453181550345819, -0.5891448751239446,
        background: <Types.Point3>[0.2, 0, 0.2],

but i could not understand what the viewUp and viewPlaneNormal means here? how can i using this to get my own view like this?

I want to view an oblique view though a curve line to get the oblique, how can i do?any suggest?