I am just a beginner for OPENGL. Now I need to modify a existing (volume rendering )code, to add a geometry(say a cube) to this rendering volume(like medical image). I know this can be done by depth test. But I failed. The result seems strange. Inside the cube there is sth which should not be.

The method should be to modify the raycast part, changing the ray start point and end point. But I am still confused...

Can somebody help with this or give me any example of the solution like this type? Thanks..