Subtracted vs. Additive

Created August 2004

Updated 09-22-2004

This tutorial will cover the benefits of using only subtracted brushes to create a basic map. It will also provide some attributes of each brush and how they interact with each other.

Subtracted Brushes

Subtracted brushes should be used as your first choice (on eventually only) choice to create the open space in which the player will walk and see into. Think of term "space" as anything the player can be in. This could include a room, an outside area, or a door way. When you first open the editor, try to think of all of that black stuff as solid rock. You need to make some of that open space for your map, using subtracted brushes. When you create a subtracted brush the player will be able to be inside of it. You will also be able to texture inside of the box.

The benefit of using only subtracted brushes is the engine doesn't have to change the calculations of rendering the brushes. If you additive brushes, the engine will have to go into another mode of rendering. When ever you have two subtracted brushes, that are snapped to grid, with faces that are touching each other, the parts of the faces that touch will disappear. Example: You have one brush for a room, one for a door way, and one for a hallway. When you arranged these so their faces are touching each other, you will now have a passage way that would allow you to start in the room, walk through the door way, then into the hallways.

Additive Brushes

Additive brushes are used to add to a subtracted space. They can be used for objects such as stairs, columns, and walls. These should be use sparingly through out the map. Static Meshes are a better solution for the objects I just mentioned. There will be times when a static mesh from a game package won't give you the certain size or style you want. This is when you can create that object with additive brushes. Once you get familiar with brushes and other related subjects, you will want to change these additive brushes into static meshes using the editor. This is a subject by itself, so we won't cover it here.

When you have two additive brushes with faces that touch each other, those faces will disappear and they won't be rendered. This is also true when you have a subtracted face touch a additive face. In addition, you should never add an additive brush inside a subtracted brush until the subtracted brush has been built in the editor.

Example Map-Subtracted vs. Additive Brushes

View in 3D windows, all brushes selected, subtracted brushes only.

View in 3D windows, all brushes selected, subtracted & additive brushes.

Top View in the Editor

The map on the left is built entirely out off subtracted brushes. I created a subtracted brush for each space the player can walk into or see into The map on the right is made from subtracted and additive brushes. I first created 1 big subtracted brush, then added a smaller additive brush to be the building. I also added some additive brushes for the walls and porches. I then used subtracted brushes to create the rooms and the space the player can see into (example: above the outside walls).

Open the map in the editor and to get and idea of how the brushes interact with each other how you interact with them in the editor. Select a face for both types,. Notice in the rooms, that the door way creates an open space where the two faces meet. You can't select that face for the door way. If you select the face in the room, the whole face will be selected and you can apply 1 texture to it.. You can get a better idea by selecting the brushes that have touching faces in a 2d view by holding the Ctrl key and selecting them or in the 3d view by holding the Ctrl + Shift keys and selecting a face. This should give you a clearer pictures of what happens when the faces of brushes meet.

The brushes in the A group are snapped to grid and aligned so the faces are touching by not intersecting each other. This is how all subtracted and additive brushes should be aligned in the map, no exemptions. This will prevent bsp holes, which are holes to the outside of the map. When these occur, parts of your map will disappear and you will be able to see the black space and blue grid lines that are outside of your map.

The brushes in the B group are snapped to grid, but aren't aligned so the faces are touching. These brushes are intersecting other brushes. This is an example of how NOT to align brushes. It will certainly lead to to bsp holes occurring. There should never be a reason to do this. If you are having problems fitting a brush where it needs to go, you need to learn how to clip the brush to make it the size you want.

The brushes on the bottom are examples of C=Brush that is snapped to the 512 grid, D=Brush that is snapped to a grid smaller than 512, and E=Brush that is NOT snapped to any grid.

The C brush is aligned to the 512 grid, which is what I wanted. If you changed the grid size to 512, you will notice the map grid and the brush are perfectly aligned. This is normally how you will want to work with brushes. The grid size will be different, 16 is considered to be the standard grid size to work with.

The D brush some how is not aligned with the grid size I want to use right now, but it is snapped to some other grid size. If you changed the grid size to 1 and zoomed into a corner of the brush, you will notice it is snapped to that grid (and possibly other sizes as well). While this is good if you were aligning things on that grid size, but not if you are using 512, like I am right now. To snap a brush (including the red builder brush), hold the Ctrl key and select the brush, it will now be highlighted. Right click with your mouse on one of it's corners and it will snap to what ever grid size is selected. Be sure to select the grid size first, then select the brush, then right click on the corner. I have found when you change the grid size after a brush has been selected, it will align it off grid.

The E brush is completely off any grid. You can confirm this setting the grid to 1 and zooming to one of it's corners. To get this back onto a grid, follow the same directions I gave for the D brush.

The two most important things you can learn from this tutorial is this... 1. Always have additive and subtractive brushes snapped to grid (usually 16, but it's not required). 2. Always align the brushes so that the faces are touching, but not intersecting. This should prevent all bsp holes from ever happening.

Click here to download the sample map.

Written by cwv_Odedge