polygon from cube cut by plane

HI I am doing volume rendering here.
I want to get the polygon which is a cube cut by the plane parallel with the view plane. How to calculate it quickly?

“the polygon which is a cube” does not make sense. Cubes are not polygons and polygons are not cubes.

“the plane which is parallel to the view plane” does not make sense either, since there is no “view plane”.

Can you reword your question?