How to get the picture every frame before rendering?

     I am new in CS.
     In my programe I want to get the rendering picture before it is rendered on screen every frame, but i don't know how to get it! Please help me !!!
   
    Thank you very much!!!

  I finally found the way to get all the vertices of portal ,the code is followed:

 
   iMeshWrapper* portal_mesh = engine->FindMeshObject("_p_door01");
   csRef<iPortalContainer> portal_container = portal_mesh->GetPortalContainer();
   iPortal* portal = portal_container->GetPortal (0);
   
   int portalcount = portal->GetVertexIndicesCount();
   printf("the portal vertex is :%d\n",portalcount);
   const csVector3 *portalver = portal->GetVertices();

               for(int i=1; i<=portalcount;i++)
   {   
        printf("the portal's vertex is :%f,%f,%f\n",portalver->x,portalver->y,portalver->z);
        portalver ++;  // there we can make the pointer to the next vertex
   }