That is a very complicated case. Hint brushes are there to produce additional portals.
To understand hint brushes you must first understand how BSP works. Here is the top view of one of my current levels (not finished) :
The numbers represent different 'areas' (the corridor counts as one). Inbetween all areas are portals (which are normally walls) that seperate the areas from each other. And now there are some rules:
-the postion of the camera determines the current area
-the current area is rendered
-all areas that the current area could POSSIBLY see are also rendered (even if they are not really visible!)
now what does that say us? Here is an example:
I am in area number 3. number three can see 2, 4, 5 and 6, because the walls seperating 3, 5 and 6 are not as high as the whole room. (i hope you understand this...ok i will add a picture
)
But why? for a human it is easy to see that when i am standing at the bottom of the room (which normally is the case) i cannot see those three areas.
But the problem is that the BSP tree does NOT know WHERE you are in a specific area, it just knows THAT you are in this area (btw this is why BSP trees are so fast - they save a lot of computing) You could be in the middle you could be on the highest place of the area you could be in the darkest corner, THE BSP-TREE DOES NOT KNOW THIS! Because of that, it renders every possible solution to what you could be seeing. (and this is the problem with outdoors, the BSP tree says that you could basically be seeing anything on the whole map, meaning everything that is not cut by the farclip plane is rendered!)
So whow do we solve this? We need a new portal! TADAA!! *enter: Hint Brush*
But not only 1 in fact we need TWO hint brushes, because the compiler must first know where the are starts and then it must know where it ends. Otherwise we cannot be sure if the compiler creates the areas exactly how we want it.
(the green lines represents the hint brushes, you will see later why i gave the new area the number
the new area number 8 can see areas number 3,4,5 and 6, and becasue we now have an area inbeetween, area number 3 cannot see 4, 5 and 6 anymore. That means we have optimized this part of the level as when we are now in area 3 only 2, 3 and 8 are rendered and not the whole room as before.
I also added another hint brush to prevent, if there was some kind of bug and i could somehow see from 3 to 1, that the whole room would be rendered.
This way only the northern part of the room is rendered when this bug would appear. Also when i am in that small part (the one that goes north a bit) of the corridor, i assume that area number 7 is not renderd, but i am not very sure there...
Ok if you understood that all it should help you.
One thing to remark is that they are really difficult to use and that in most cases you do something wrong and then they don't work. (i mean i do something wrong in most cases, and then i don't know why it didn't work, wasn't meant as an offense)