Volume rendering is a very effectived method to help user to find problems inside a very complicated structure. Compared with surface based visualization methods

it keeps more detail information in the final 2D image. This advantage just brings up another big problem. Usually some important information is obscured or overwhelmed by the data standing closer to the observer. To solve this problem

two algorithms are put forward in this paper. Cutting the structure with an arbitrary convex polyhedron to bring forward the focused part. By using the hardware supported stencil function in OpenGL

the cutting operation become very fast and easy. Another method is the definition of multiple transfer functions. The integration of these two methods makes the user's inspection more flexible. To achieve nearly real time response

several optimization methods are taken in this paper. The rendering time in each slice is decreased to the minimum due to the combination of stencil reference values by using binary tree. The integration of the stencil operations between the front and back faces of polyhedron speed the algorithm greatly by saving lots of complicate normal computations.