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By Gunnar Radeloff

Adding Dirt 1

This tutorial outlines various methods of mapping your models to look realistic - in essence, Dirtying up the otherwise shiny "super real" look of computer generated imagery.

Level: Medium

This tutorial makes extensive reference to project files and materials. To download the relevant files click HERE. 907Kb.

Scope mapping

Perhaps the most useful method of creating additional detail to your scenes - scope mapping allows the user to add additional materials to a single object/level. Each successive material takes precedence over the previous material. eg. The material at the bottom of the select window is placed on TOP of the material underneath in the model. (ie. Each material is layered successively, the lower in the select window, the closer to the top or front of the material)


This is useful for adding grafiti to walls - rust stains, drains in the ground - or overlay any object or material.
To effectively use scope mapped materials - one generally makes use of an alpha channel or seperate image that is used to "cut" the material (Unless a simple square parallel map suffices) This alpha channel can be applied to;

  • The Scope channel - which will simply cut out the extent of the mapping.
  • Or the Fade channel - which actually cuts the object that is mapped

Obviously we can map any image to any channel in RS3D - but for the purposes of this exercise we shall deal with scope and fade mapping. To achieve the "cutting" action can also be brought about by a variety of means;

  • Using the clip material in the standard /image templates
  • Using an Image Alpha channel like in a 32 bit Targa file -
  • Or having a seperate Greyscale image to the color image.

Fade mapping with the CLIP function

The clip material uses a color range to determine what extent should be cut. This material is best used for areas of solid color. If the mapped material has a blue background - we can set the clipping range to encompass all the blues to cut out. refer to:

example project file: #RS ROOT/Tutorials/dirt/projects/iron.r3d


In this scene, the blue in the image is used as a mask. (Below) We can set any colour as a range - ideally it would be a colour that is not used in the image. This method generally produces a hard edge and is probably the least desirable.


Scope mapping with two images

The next scene example project file: #RS ROOT/Tutorials/dirt/projects/scope01.r3d

Uses two seperate images. The diffuse colour image D_manhole.jpg and an Alpha image (Cutout) called A_manhole.jpg. Its often a good idea to prefix all your textures with B_ for Bump, A_ for Alpha etc.


This simple scene has mapped two materials onto one object. The foundation is the concrete material that surrounds the whole floor. On TOP of that (ie below it in the select window) we have put our manhole texture. Note the structure of the manhole texture.

It is very simple : The Scope channel is set to our alpha channel image (Shown above) Note the Black designates complete transparency - and the white solid areas of colour. Any grey between these values would give a soft transition between the two.

The color channel is set to the color image of the manhole. Note that the colour image does not need to be cut out. The alpha image or channel does this for us.

The end result is a manhole sitting in concrete on one object. The scope channel is very useful for adding all manner of materials on top - Windows, graphiti, leaves, grime etc.


Dripping rust often congregates under beams and appendages - adding a simple scope rust material over and over again can add whole dimensions to your scene - all from either an image map or simply a dark color mapped on top of another material with 0.25 % scope.

Materials useful for Dirty mapping:


Adding Noise over materials

Using a low scope - one can "dirty" otherwise bland areas or disguise repetitive materials by simply adding a noise texture. Click HERE to see the values (Starting point)

Faking shadows

Using a simple greyscale gradient image, we can add shadows under eaves, or add subtle radiosity like effects to objects. Adjust the scope level to increase or decrease the effect of the material.
project: #RS ROOT/Tutorials/dirt/projects/gradient.r3d


Angle based scope

This material takes into account an objects ANGLE to the materal mapping - hence we can add "dust" to the top of objects simply by applying this material downward. Often having a white down material and a dark up material can effectively simulate both sun burn on top and grime underneath. project: #RS ROOT/Tutorials/dirt/projects/dust.r3d

dirty bumps

Darker bumps

This material uses the height of the bump to add grime into the gaps. It essentially only burns the darker regions of the bump material - but can be good for tiles and very rough materials.
project: #RS ROOT/Tutorials/dirt/projects/dirtybump.r3d

Perpendicular scope

This can be used to fine tune materials that should only fall on a certain area of an object - used in conjuction with a finite mapping method - this is particularly useful for additional dust and grime.
project: #RS ROOT/Tutorials/dirt/projects/smscope.r3d


Truly Effective materials make use of the whole gammut of available channels : Specularity, Color, Fade, Bump, Transparency etc. Through clever use of materials we can keep geometry to a minimum and maximize the realism of a scene. Lets have a look at a scene created earlier and examine how it works.

Click HERE to go to next tutorial in Dirty mapping :

Page updated on Tuesday, 25 February, 2003 . For feedback / model submissions or articles - please email us.
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