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# Subtraction And Addition

The Unreal Engine world theory is based on subtractive geometry. That means you start with a world filled of matter and you proceed to subtract areas, inside of which you can then add other solid patches.

Let's proceed with a graphic example: let's suppose you want to create a room with some sort of metal pipe inside of it. At the beginning your 3D UnrealEd Viewport appears all black,you need to give the Red-builder-brush a shape to make it appear. Click on the cube item in the top left corner of the brushbuilder constructor menu.

1.Below is the result you should obtain:

...Now go into the Texture Browser, and choose a nice texture for the wall of your room (a brick texture would be good)

2.Click on the subtract Button from the brush operations menu(top-right)

Tada!..This is what you should get...

You have just removed the shape of the brush builder from the Solid world.

Now rescale your red-builder-brush using the brush scaling tool.Give it the shape of a (very basic) square pipe or something...

go into the Texture Browser, and choose a texture you find appropriate.

Click on the Add button from the brush operations menu(top-left)

Voila!

You have just added a new shape in a previously empty space.

Note that adding a brush into the solid world (or another solid brush) or removing a shape from the void will have no effect and furthemore could get the engine to make rendering artifacts and errors (see BSP hole)

Now I think you should have a good idea about substractive geometry,your next step should be to learn or to further manipulate the brush builder to create complex shapes, by using the brushbuilders in the toolbox.

### Miscellaneous

(these more advanced point probably need a seperate page)

- Subtracting a brush often causes sheets to vanish. Should be fine after a Geometry rebuild.

### See also

[First Room Photo-Tutorial]

[Basic Brushwork Photo-Tutorial]

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