Brushes in a CSG environment can have a few settings for Solidity. The Solidity of a brush defines how it interacts with a player and what effects it has on the environment as the engine is rendering it, and the BSP (Binary Space Partition) as the level is played.
Many of the brushes that are used will be Solid. This means that they not only make BSP Cuts (This is explained in more detail in the article titles Introduction to BSP) but they either block the player (Additive CSG Operations) or allow the player passage (Subtractive CSG Operations). If you would like to know more about CSG Operations, read the articles on Constrictive Solid Geometry in the Introduction to Editing Section of this file.
A Solid Brush is the only type of brush that will create a BSP Cut, and allow the player to collide with the brush, meaning that any brush that is subtractive (creating a place for the player to walk) has to be Solid.
A Non Solid brush type can be added to the level, but it will not block the player in any way. A Non Solid brush will also not make any BSP cuts. This means a few things:
Firstly, it means that a non solid brush is will most often be used as a decoration of sorts. It can only be placed inside a Subtractive Solid Brush. Putting this type of brush anywhere else will have no effect. Why? Because the brush is Non Solid, it doesn´t make any BSP cuts, and without BSP cuts, it will not create an area for the player to move within.
Secondly it means that a Non Solid brush reduces the Node Count of the level because it does not make BSP cuts. This can be important in the final framerate of the level when it is played.
Lastly, it means that a Non Solid brush can be used as a Zone Portal very well. For more information regarding Zones and Zone Portals, refer to the Zones information within this file.
A Semi Solid brush is really a mixture of the Solid and Non Solid brush types. It takes properties of both. It can block a player or actor (e.g. a bullet) and it doesn´t make BSP cuts to the level (From information on Non Solid Brushes, you should see that because it doesn´t make BSP cuts to the level, it will reduce the Node Count in the level). However, because it doesn´t make BSP cuts to the level, you should also see that it can only be used as an Additive Brush. A Semi Solid brush however cannot be used to close off a Zone. It is also a Dark Horse as far as brushes are concerned, as it can cause problems with player collision at times. Semi Solid Brushes should be used with caution and intuition.
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