Semi Solid Brushes are a special brush class, somewhere between Non Solid brushes, and Solid Brushes. It is a special class, because it takes attributes from both of those brush types. It will block a player, and it will block actors, but at the same time, it will not create BSP cuts, and it will not block a zone.
Semi Solid brushes do not create BSP cuts, nor do they call the CSG Additive Operation (meaning that the area within them is technically still void, though it blocks the player from entering it) which means that a Semi Solid brush can only be an additive brush. This brush cannot be used to create a new room within a solid space, it can only be used to create a blocking shape within an area that the player can already get to.
As a Semi Solid brush does not make any BSP cuts, it can well have a beneficial affect on framerate. Even though the engine will still have to display the brush, because it does not make BSP cuts, it often means that the brush itself is simpler for the engine to display, and that in turn means that the area around it can be simpler for the engine to render.
Semi Solid brushes are most commonly used as either decorations inside an area, or they are used to create parts of a level that do not require BSP cuts (for example, a pillar, or piping running along a wall). This is because of the lower BSP complexity that they confer upon the level due to lack of BSP cuts (This really is very important!).
To place a Semi Solid Brush into your level, shape the Active Brush and place it where you want the Semi Solid Brush to be, then click the Add Special button. (shown below).
This will open a dialogue window (shown below). Select Semi Solid from the Solidity options presented, and click OK. This will then create the Semi Solid Brush.
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