1. The Diffuse Map:
The last part of this tutorial will present a simple way to create the textures for this vehicle. I will use, as an example, the exterior texture of this asset to show an easy and effective way to create textures in layers.
One important thing that I always keep in mind when texturing is that, a lot of the time, the texture has to be corrected or changed during the creation process. If I just paint all the details on one layer in Photoshop, later on a simple colour change can be impossible to perform without redoing most of the texture. That is why the texture should be done in layers, by keeping similar types of details grouped together for easy editing.
The base is a layer set which contains one layer for each material/colour on the texture. Naming these layers in a comprehensive way is a good idea and will make things easier.
In the next layer set up we have the big details in the texture. I used simple shapes and some layer styles to draw the grills. The headlight lenses are taken from a photograph; desaturated and pasted as a layer set to Overlay.
After adding the big details, it is time to add some subtle material variation. As this vehicle is new and clean, I only add one layer with a metallic texture set to overlay. To keep everything subtle, the layer opacity is lowered. Also, there is a mask to hide the metallic texture for areas that do not require it. Masking is a good method of hiding detail and it is preferable to deleting as it can be reversed.
The texture still looks flat and lacks depth. To add some variation to the huge areas of blue car paint, I add a new layer of a lighter colour, and by using its mask I blend it with the original one like so:
The texture is getting better but it is not done yet. Adding further detail can be achieved by using the ambient occlusion map baked earlier. Adding it on top of the colour texture, with the layer set to Multiply,will add a lot of detail to the texture, making it fit for use.
This is how the texture looks after adding the AO map.
2. The Specular Map:
A specular map defines the intensity of the specular highlight, effectively describing how reflective a texture is. Creating this texture is fairly simple and involves using the color texture as a base and modifying it. This is yet another reason to keep the color texture in layers.
Every type of material reflects light differently, so each area will have to be adjusted to reflect this. Very shiny materials like glass or chrome will have a very light color on the specular texture, usually white. On the other hand, matte materials will be defined by dark colors on the specular map. Pure black is to be avoided as having an area without any specular highlight will make it look very flat in the game's engine because there will be no highlight to show off the normal map detail. These are the two extremes; the rest of the materials will have their specular color somewhere in between. There are no "standard" values, so the artist has to create this texture by trial and error. Unless specified by design or technical documents, the absolute value of the specular color is not as important as the general contrast of the map. One trick used to enhance the look of metallic textures is to change its specular to a color that complementary to the diffuse map. So here, a reddish specular on a blue color texture will give a nice metallic effect. Another good idea is to enhance the contrast of the small surface details on the specular map to make the specular easier to read. This is the case of the next image, even thought the noise in the texture might not be visible due to the screenshot's resolution.
2. The Normal Map:
The normal map base is the texture generated in the last chapter. On top of this I will add the details that have been added in the colour texture. To extract a normal map from the diffuse texture, there are two free programs that do the job well. The first one is nVidia Normal Map Filter which is part of the nVidia Photoshop Plug-ins http://developer.nvidia.com/object/photoshop_dds_plugins.html
. This plug-in uses a height map to generate the normal map. A height map is easy to create by using the colour texture as a base. Each pixel will have a greyscale value that represents its elevation. The lighter it is, the more elevated/extruded it is. This value is a local one in the sense that each pixel has its elevation defined in relation to its neighbours. A flat white height map will result in a flat normal map as there is no variation in elevation. Basically, the process of converting a diffuse texture to height map means desaturating the original layer and modifying it to better reflect the elevation of that area.
This is the height map created for the exterior of the car:
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