This tutorial will provide an overview of how this scene was textured using a combination of various metal textures and masks in conjunction with Composite maps.
Although it looks as though there is an abundance of geometry, the scene is comparatively simple and incorporates a limited number of components. The focal point is the two ducts situated left of the centre. The heat exhaust is identical to its counterpart barring the small trowel shaped lip, as indeed are the two hatches in the upper left. In fact all of the girders, cross struts, pipe work and cylinders are instances of an original version. It is worth mapping/unwrapping any geometry before duplicating it as the UVs are preserved in subsequent copies, hence it need only be done once per object.
Here is an un-textured render of the scene showing just the geometry and lighting (Fig.01). There are a couple of Omni lights that correspond to the small lamps and glow from the heat exhaust, but the principal light source is created by an Area Spot (see Fig.02 for settings).
As the two ducts are the focal point in the scene I opted to unwrap these in order to localise detail in specific areas.
Fig.03 shows the texture assigned to the ducts, both of which have been unwrapped into the one template. Section 1 represents the interior of the right duct with section 2 being assigned to the opposite one. The worn areas to the left show the lower parts of the duct interior which are visible to the camera.
The areas labelled 3-4 correspond to the large curved section that runs through the middle. The number "3" corresponds with the top of the right curve where there is a highlight, whilst number 4 represents the left side.
For a still such as this it is worth concentrating on those areas visible to the camera. As a result I focused the details into the most conspicuous areas marked by the red dots, namely the rusted sections from various textures in the 3DTotal collection.
To create a highlight along the top I extracted a section from a metal door which I color corrected to match my template, before converting it into a Specular map (Fig.04).
The panel above the ducts with two small hatches was another area I chose to unwrap due to its proximity to the camera.
Fig.05 shows the template in the centre and a few of the textures used in its creation. To create the scratches I copied and pasted the white areas from the Dirt map and set the Blending mode to Difference. You may also notice I have used the same door texture from Fig.03 to create the base metal and rusted section in the lower half, which is apparent in the upper corner of the right hatch (ringed in white).