   Whats the difference between PVS and Portals?
  I''ve read a bunch of stuff, I probably dont understand it, but potential visibility set determines what is visible from the point of view, and so does portals. SO what exactly are the main differences between the two?Can you use them together? Why or why not? If not, which one is better to use and why?Thanks a lot for clearing things up. 
  There are other PVS techniques besides portals. Portals is just one of them. ;  Generally speaking, when someone says "PVS" they mean a precalculated potential visible set (Quake style.)  A list of the possibly visible sectors is stored in every sector.  That list is the PVS.  Internally Quake uses portals to determine its PVS. Portal culling is done in realtime rather than preprocess.  Each portal serves as a viewport into another sector.  All of the objects in the sector a portal is looking into are culled using that portal.  Portals are view dependent and the PVS just depends on what area you are in.  ;  Thanks, I think I get it.prtal rendering is actually a PVS technique. But in teh general sence when people say PVS they mean precalculating what can be seen from a given sector, and when they say portals they mean calculating what sectors can be seen from which sector in real time.So with PVS (in the general sence) you avoid a lot of runtime processing (determing which sectors are visible) because its all been already calculated. So then whats the point of portals?Thank yees. ;  The PVS is used to check which 'rooms' should be considered for rendering, because they're potentially visible. But depending on where you stand in a room, a potentially visible room may not be completely visible. Portals are a way to determine what parts of the PVS set not to draw.edit: example; there are 2 rooms connected by an open door. You are in room one, but are against the wall and can only see a small part of the other room. The part of the room you cant see doesnt have to be rendered. Objects inside the not-visible part of the room don't have to be rendered either (as long as you don't have realtime lighting anyway)   [edited by - Sponzie on February 17, 2004 12:43:39 PM];  A PVS is the set of all areas that are potentially visible from a point inside an area. A portal is just an object that defines a window in which an area can "see" another area.Think of a single room with a window looking outside. If inside the room is area A and the outside world is area B. the PVS for Area A is area B. Likewise the PVS for area B is area A. The portal is just simply the window itself. Once the pvs is calculated and is known, the portal(s) are not needed anymore as far as rendering goes. In the case of a portal engine, the portals are kept around because the PVS is not precomputed and not stored in advance. It''s more or less calcualted at runtime. It''s also not really a PVS at this point because since the players position in a given area is known, the specific set of polygons that are visible through the portal(s) can be calcualted.A portal is just a tool used to calculate visibility. In the case of a PVS the set is precomputed and is pretty static. It cant really be changed easily. If all of this is done at runtime, you can create new portals on the fly and on the next run the visbility will be calculated.-=[ Megahertz ]=- ;  ok, getting clearer. Now ...quote:there are 2 rooms connected by an open door. You are in room one, but are against the wall and can only see a small part of the other room.Suppose the next room has like 100 floating boxes with high poly counts. How would I know which boxes can be seen in the next room from my current position?  ;  Using the ''door'' (the portal) as a viewing window into the other room, you compute a frustum to use for culling -> all of the boxes that are totally outside this viewing frustum don''t need to be rendered. If you try it out on paper first it may become clearer. ;  So you have to use a PVS with a portal. You cant do that with only PVS correct?And how bout the boxes that are behind boxes and at the same time in the frustum?Thanks for your patience by the way. ;quote:Original post by transformation  So you have to use a PVS with a portal. You cant do that with only PVS correct?You could use a PVS without portals, but it won''t be very effective. The PVS is a coarse list of all geometry that can possibly be visible from the current sector. In practice, only a small subset of that list is actually visible. Portals refine this list at runtime, trying to throw out as many invisible nodes as possible.quote:Original post by transformation  And how bout the boxes that are behind boxes and at the same time in the frustum?Occlusion culling.