BSP Trimming Pt. 1 – Basics (Info and How to)

Welcome to this tutorial about BSP trimming.
In this tutorial I will explain some things about decorative trimming and trimming appliance from what I use on personal experience.
Examples with screenshots have been given to practice and copy any of the information explained below.

Note: This tutorial might differ from other ways of trimming. This is pure my personal way.
I hope you enjoy.

Please find below reference guide so it may navigate you if needed.

Part. 1 – Basics:

1. Trimming, what is it?
1.1 BSP trimming and Static Mesh trimming?
2. How to apply trimming.

(To come!) Part 2. – Advanced:

3. Shapes and objects
3.1 Styles and Materials for trimming


1. Trimming, what is it?

The exact definition of trimming is: “Decorating the fraying”. Which is exactly what this tutorial describes.
A more softer definition could be: “Decorating a surface” but its not. Its actually the decoration of “edges, fraying, cutoffs” so a surface holds more beauty and
will therefore look smoother.


1.1  BSP trimming and Static Mesh trimming

screenshot.60 screenshot.61

Left: BSP Brush trimmed                               Right: Static Mesh

Trimming BSP and Static Mesh trimming is different for a logical reason.
A Low poly map is most likely benefited by BSP trimming, while a map build most reliable on Static Meshes is trimmed like using bars/objects, like below:


It takes us back to the basic differences between BSP usage, and Static Mesh usage
Trimming (Adjusting) a BSP surface will mean a fully Vertex customizable option in Trimming by using editing, extruding, clipping an adjusting in BSP in the Geometry Tools.
You can trim whatever surface of the brush you want without holding on to a certain amount of Polygons and will always start with the same amount of Polygons on whatever basic shape is being used, which are used to play a major part in determining how smooth and fast the gameplay will be.
A high amount of Polygons (like Static Meshes) and a result of overusage in meshing can mean extreme rendering and heavy Framerate loss.

Static meshes (The word “Static” already says it)  cannot be vertex animated, only moved, scaled, or reskinned in realtime, and will therefore always hold the same amount of Polygons! Saying: The less polygons, the better!

For a balanced map, the question should always be asked: What surface can benefit BSP trimming, and what surface is in need of Static Mesh trimming.


2.1 How to apply BSP trimming and when to apply BSP trimming?
Grid and recommended view options:
– Grid 16
– Top view.
– To prevent BSP errors and unaligned trimming. The most important thing would be,
Stay on the grid.


Floor BSP trimming:

A standard trim is about 32 pixels. In a Grid 16, that is about 2 grid units in width.
2×32 cubes.


  • Lets say I want to trim this basic size cube floor brush which is exactly 256x256x256


  • Create another cube brush that will become the first side of our floor trimming. dimensions: 32x256x32


  • A perfect trim will meet every angle in a 45 degree cutoff so that every axis of the material will walk over each other smoothly.
    For achieving this, we will use clipping. Start the Geometry Tools, select the “Red Square” named Object which reveals the Vertices of the builder brush and
    click on the Toggle Modifier Window


  • Now its time to clip this brush into a first usable trim brush. Place the first clipmarker at the very left verticle of the brushscreenshot.67
  • Place the second clipmarker at a 32px distance located at a 45″ angle facing inside from the brush so a smooth 45″ angled line appears.screenshot.68
  • Select the builder brush again and all the clipmarkers at the same time. In the Geometry Modifier window, leave all other clipping options blank and click Apply.
    Note: The red stripe in the middle of the stripe that connects both Clipmarkers is called the Normal.
    Could it be that the normal is on the other side then that you wanna trim?
    , then you should select the option bFlipNormal before hitting Apply


  • If done correct, a 45 angle should be cut off your builder brush like below:


  • Repeat the step above for the same at the opposite side.
  • The result should look like this.


Basically, this is our trim. Subtract it, and add it and our trim appears.
Repeat this step for all sides and voila.

screenshot.72 screenshot.73

Thats it, this is the easiest and most common way of floor trimming. This is just a simple cube.
Imagine doing a whole map:


We will continue on other shaped brushes and trimming already clipped 45 angled floors and walls in Part 2: 3. Other shapes and CSG’s


Accelerate your BSP trimming! ~

If you need multiple brushes of similar trimming (For example, the cube brush we just trimmed) we can use an Intersection of the brush we just trimmed to create a replica of the existing brush to the builder brush.

  • Select the Cube brush, make a new brush of a dimension equal or bigger then the brush we already made. (In this instance 256x256x256)
    So make a new Brush with dimensions: 256x256x256 and just place it on your trimmed brush (This is taken from the side view)


Perspective view:


  • Press the button CSG Intersect


What’t heck just happened? The builder brush makes a replica of all brushes inside of area we just placed it in.
In this case the cube we just made, it has become one complete brush. And can be added multiple times in a row.


Wall BSP trimming:

Wall (not ceiling) trimming is just slighty different from the floor trimming since almost all levels have walls that vertically interact and therefore in most cases, dont need to be cut off diagonal.
For a smart trimming of walls, The Extruding option in the Geometry tools would help considerably for creating trimming on walls, pillars, and more decorative stuff for deco geometry.

This picture is taken with light only:



  • Start with creating a brush next to the floor brush we already had to keep perspective on the floor > wall.
    Dimensions: 256x256x32.
    This will serve as layer 1 for our wall.


  • Open Geometry tools and select Poly “First red Arrow button”. In your Geometry Modifier window, select Extrude (Picture)

screenshot.82 screenshot.83

  • Select the Upper Face of the Builder brush. 

screenshot.86 screenshot.87

  • Extruding simply said attaches another brush to an existing Brush face. This can be done with Additive and Subtractive brushes too.
    Lets extrude this brush by 128for now.
    In the Extrude options, fill in: Length: 128 and leave Segments as it is, since we only want to extrude 1 brush segment on top of this face of 128px. This serves at the second layer for our wall.screenshot.89
  • And click Apply

screenshot.91 screenshot.90

  • Select the Upper Face of this brush again, and extrude it again, this time, by 32


  • Select the Upper Face of this brush again, and extrude it again, this time, by 128again


  • Finally, Select the Upper Face of this brush again, and extrude it again by 32 


This is a very basic wall setting with a 3 based trimming section and simply just flat, there is no interesting geometry going on here.

Without trimming, and a basic two material setting, it would have looked like this:


Or, worst case scenario, not even any breakup material at all.


Although the geometry itself isnt very interesting, the use of different materials makes it look clean and still pretty interesting for an unlit version.
There is a use of 2 grey materials that cover the whiles, while both the trims itsself cut off texture flow. Looks good.


This wraps up the basic concept of trimming.
In Part 2 I will explain some harder and more advanced ways of trimming, like diagonal trimming, arched geometry, and more.
Coming soon!

I thank you for your time.


5 thoughts on “BSP Trimming Pt. 1 – Basics (Info and How to)

  1. Pingback: BSP Trimming Part 1 – Tutorial added. « DarK

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