General Concepts 102: Lesson 3
Detail Brushes
Now that you have a basic understanding of compiling, let's look at the VIS process in a little more detail.
Earlier, we said that each portal is the largest possible convex shape in any given space in a room. So what happens when we add a bunch of objects to a room? Well, the largest possible convex shapes in that room become extremely small - making a very lengthy compile. The problem is that despite all this extra time taken in the VIS process, because the portals are all so close together, it typically doesn't help framerate. Thus, VISing that area is just a waste of time.
Thus, iD (the original creators of the Q3A engine which the JK2 engine is based on) added in a feature to differentiate brushes - ones that the VIS process does and does not need to pay attention to. A standard brush, one which the VIS process uses to calculate visibility, it called a Structural Brush. A brush which VIS ignores is called a Detail Brush.
95% of the time, you want to make every brush that doesn't touch the void into a detail brush. Hold control and press the M Key (CTRL+M) to Make a brush into a Detail Brush. You can also right click on the 2D View and select "Make Detail.. If you make a brush that touches the void into a detail brush, you will cause a leak in your map. Remember, the VIS process is what determines leaks - since a detail brush is invisible to the VIS process, it will interpret it as a hole in your map.
Note that 5% of the time you will want non-void-touching brushes to be structural. This is useful if you want to "force a split" - causing portals to be made where the engine wouldn't normally draw them if the brush was a detail brush. This will be covered in a much later tutorial if you want more information.
The most severe case of "forgetting the detail brushes" that I have heard of is thus: a man made his map without making any brushes detail. Around the 20th hour of the compile process, he read on a message board that he should make all of the brushes in his map not touching the void into detail brushes. He then restarted his compile. This time, it completed in 16 minutes.
As you can see, while detail brushes aren't necessary to the compile process, they certainly help you, the map builder, in building your maps. The faster your compile, the more time you have to work on making it just perfect!