The rust and grime that has built up around the two hatches was extracted from the upper right texture whilst the small holes were taken from the Total Textures: V17 - Urban Extras DVD.
Varying Coordinates and Composite Maps
If modular design forms an integral part of your scene and it is necessary to create numerous sections of identical geometry then creating duplicates is an obvious choice. It also makes sense to map these before copying them; however, the drawback is that when you apply a single texture they all look the same. One way to combat this is to alternate the UVs of each component so that they do not reference the same part of the texture. If you are tiling the texture, which may be necessary to achieve the correct scale, this will not necessarily solve the problem, however using a Composite map will help.
The girders, for example, are long and thin and as these were Box mapped it was necessary to tile the texture lengthways to avoid stretching due to the template ratio. However this created a visible repeat and so to alleviate the problem I used a Composite by overlaying a second texture, but this time without any tiling.
Fig.06 illustrates this principal: The base texture is tiled (Layer 1) but there is an obvious issue so a second layer is added without any tiling (Layer 2). This is set to a suitable blending mode within the Composite map rollout (see example layout in the Material Editor) in order that we see both textures (Overlay in this case). As you can see on the bottom right the result is far less problematic now. The second texture has gone some way towards concealing the tiling issue and created some much needed variation.
Fig.07 shows the method applied within the context of our scene using one of the many girders. Layer 1 is the base texture, which is tiled by a value of 3 along the U and V axis, and you can observe the issues along the girder on the far right (1). When the second texture is applied and set to Spotlight at 34% you can see the resultant effect (2). In this instance the second texture has been tiled by a value of 2 instead of being left at the default of 1. Normally you would avoid this, but as long as there are no obvious tiling issues this is fine. The shapes of the girders permitted this deviation, but were we dealing with a large expanse this would have proved problematic.
I used this same technique on the majority of the geometry, with the exception of the aforementioned ducts and hatches, as well as the cylindrical tanks that are scattered along the right side of the shaft. I used a single texture for the tanks, but in order to avoid any obvious symmetry I rotated a couple to expose a different part of the texture (Fig.08). In order to add more variety I pasted the texture into a Composite map and then added a second texture set to Overlay, which was then assigned to a selection of the tanks (1). As illustrated by the girders in Fig.07 this helps mask any uniformity as well as adding a different tint.
The red and green geometry highlighted in Fig.09 show the sections that employ this technique.
The final image can be seen here with additional smoke and heat waves courtesy of some post-production in Photoshop (Fig.10).
To see more by Richard Tilbury, check out Digital Painting Techniques: Volume 4
Digital Painting Techniques: Volume 5
Digital Painting Techniques: Volume 7
Beginner's Guide to Digital Painting in Photoshop Elements
Beginner's Guide to Digital Painting in Photoshop
Photoshop for 3D Artists
and Prime - The Definitive Digital Art Collection