A texture map is a two-dimensional image file that can be applied to the surface of a 3D model to add color, texture, or other surface detail like glossiness, reflectivity, or transparency. Texture maps are developed to directly correspond to the UV coordinates of an unwrapped 3D model and are either devised from real-life photos or hand-painted in a graphics application like Photoshop or Corel Painter.
Texture maps are usually painted directly on top of the model's UV layout, which can be exported as a square bitmap image from any 3D software package. Texture artists usually work in layered files, with the UV coordinates on a semi-transparent layer that the artist will use as a guide for where to place specific details.
Color (Or Diffuse) Maps
As the name would imply, the most obvious use for a texture map is to add color or texture to the surface of a model. This could be as simple as applying a wood grain texture to a table surface, or as complex as a color map for an entire game character (including armor and accessories).
However, the term texture map, as it's often used is a bit of a misnomer — surface maps play a huge role in computer graphics beyond just color and texture. In a production setting, a character or environment's color map is usually just one of three maps that will be used for almost every single 3D model.
The other two "essential" map types are specular maps and bump/displacement, or normal maps.
Specular maps (also known as gloss maps). A specular map tells the software which parts of a model should be shiny or glossy, and also the magnitude of the glossiness. Specular maps are named for the fact that shiny surfaces, like metals, ceramics, and some plastics show a strong specular highlight (a direct reflection from a strong light source). If you're unsure about specular highlights, look for the white reflection on the rim of your coffee mug. Another common example of specular reflection is the tiny white glimmer in someone's eye, just above the pupil.
A specular map is typically a greyscale image and is absolutely essential for surfaces that aren't uniformly glossy. An armored vehicle, for example, requires a specular map in order for scratches, dents, and imperfections in the armor to come across convincingly. Similarly, a game character made of multiple materials would need a specular map to convey the different levels of glossiness between the character's skin, metal belt buckle, and clothing material.
Bump, Displacement, or Normal Map
A bit more complex than either of the two previous examples, bump maps are a type of texture map that can help give a more realistic indication of bumps or depressions on the surface of a model.
Consider a brick wall: An image of a brick wall could be mapped to a flat polygon plane and called finished, but chances are it wouldn't look very convincing in a final render. This is because a flat plane doesn't react to light the same way a brick wall would, with its cracks and coarseness.
To increase the impression of realism, a bump or normal map would be added to more accurately recreate the coarse, grainy surface of bricks, and heighten the illusion that the cracks between bricks are actually receding in space. Of course, it would be possible to achieve the same effect by modeling each and every brick by hand, but a normal mapped plane is much more computationally efficient. It's impossible to overstate the importance of normal mapping in the modern game industry — games simply could not look the way they do today without normal maps.
Bump, displacement, and normal maps are a discussion in their own right and are absolutely essential for achieving photo-realism in a render. Be on the lookout for an article covering them in depth.
Other Map Types to Know
Aside from these three map types, there are one or two others you'll see relatively often:
- Reflection Map: Tells the software which portions of the 3D model should be reflective. If a model's entire surface is reflective, or if the level of reflectivity is uniform a reflection map is usually omitted. Reflection maps are grayscale images, with black indicating 0% reflectivity and pure white indicating a 100% reflective surface.
- Transparency Map: Exactly like a reflection map, except it tells the software which portions of the model should be transparent. A common use for a transparency map would be a surface that would otherwise be very difficult, or too computationally expensive to duplicate, like a chain-link fence. Using a transparency, instead of modeling the links individually, can be quite convincing as long as the model doesn't feature too close to the foreground, and uses far fewer polygons.