This is the second diffuse texture, with the tiling areas also marked out (Fig.21).
The next major step to be taken is creating the normal map. First thing to do is create a height map from the diffuse map we already created. This is by no means realised by just de-saturating the colour texture and running a filter; there is more to it than that. A close study of the volumes defined by the colour texture is necessary. We start by isolating large volumes like door frames, window frames and large grills. Finer details like wood fibre, rust, or chipped paint, which do not have a lot of volume associated to them, should be kept separate so that they do not compete with the large details when converting the height map to normal map. Doing so will result in a final texture that is unnatural and cartoony.
Here's how the height map looks (Fig.22). Basically it's a greyscale map that defines the depth of each detail. Contrast is very important as it is the element that will add depth to the resulting normal map.
This is how the resulting normal map looks after using the nVidia Normal Map Filter on the previously created height map (Fig.23). You can also use Crazybump or Xnormal for the same purpose. This normal map can be further enhanced in Photoshop before being used. If some details need to be more pronounced, they can be duplicated on the normal map and the resulting layer can be set to Overlay. This will increase the depth of that area.
The same method has been used for the second texture. Here is the height map and the normal map for that texture in one image (Fig.24).
For some areas I have also added normal map details that were not taken from the diffuse map. Here are a few such details that have been added to the previous normal maps with Overlay to add variation to them (Fig.25).