I guess you have read the Vis for dummies! Structural brushes block the view beetween portals but it's difficult to exactly tell how the portals are in game. In corridor map it's quite easy to have the portals exactly as you want them to get high FPS, it's more difficult outside.
Doesn't matter if you standing infront of a high wall, things on the other side might get draw depending on how the portal are made. I've got a simplified picture with two portals and a thin wall, As a player you cant see to the other side but the engine will because the portal can see to the other side. Don't get fooled by my arrow that goes from the player and up into the other portal, it doesn't really have anything with the player to do. It's allways what the portal that the player are inside that decides what will be drawn.
A portal face is like a window and any other portal that can be seen from that window will be drawn. You might not think that the portal can see anything but if you have a map where you got a lot of high portals up to the skybox then they can all see eachother unless you have some structural roofs that touches the sky box.
Hint/vis and regular structural brushes should help you to make splits that creates new portals and prevents the straight line of sight between portals.
Picture two is an example with three portals, because the structural brushes have split the map into 3 portals, portal no 1 will never see portal 3 because you can't draw a straight line from anywhere on portal 1 to portal 3. Portal 3 won't be drawn when the player are in portal 1, when the player are in portal 2 all portals will be drawn and if the player are in portal 3, portal 1 won't be drawn.
Also remeber that lod terrain, models etc does not block the view, If you use lod then you might need to put caulk brushes under it to get portals that won't see to much. If you make a map with ground only made out of lod and then start making houses and stuff above and really plan for VIS above ground, then you might get terrible fps anyways because a lot of the portals will go down under the lod, and there they will be able to see eachother because nothing blocks the view.
Structural brushes block the view, the reason to use detail is that a lot of brushes will never block the view. If you make a map and put in a 1x1x1 unit brush that are structural then it will split your map into several new portals. But those portals will still be able to see eachother, that's why you should make it detail. The same goes for a stair, rails, nice small stuff that you make out of brushes etc.
: Is there anything written about Portals which is understandable for a noob?
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