The Surface Properties handle all settings related to the surface of the material, such as the surface roughness and bump. They mainly describe if the surface is polished or rough.
Roughness allows you to add tiny imperfections and miniscule details on a surface to make it reflect light in a more diffuse way. The roughness of a surface can range from 0 (perfectly smooth surface) to 100 (pure diffuse). A roughness value of 100 corresponds to a “lambertian” or perfectly diffuse model. It is important to remember that if you want to have very smooth, mirror-like surfaces or clear glass, the roughness should be set to a low value (i.e. 0 to 10).
A lambertian surface (left), medium rough surface (center) and an smooth mirror surface (right).
You can also set a black and white texture to control the roughness. Brighter values in the texture create a higher roughness (a more diffuse surface). When using a texture, the roughness number just applies to the maximum roughness needed. For example, if you use a black and white checker map as a roughness texture, and you set the roughness to 30, then the white parts of the texture will result in 30 roughness while the black parts will represent 0 roughness. If you now change the roughness to 70, the white parts of the texture will result in 70 roughness and the black areas will still represent 0 roughness.
It is important to understand how roughness controls the falloff between the 0° and 90° colors, i.e. which of these colors will be most visible. When roughness is low and ND is set to a higher value (Nd 5 or higher), the 90° color will be more visible. As the roughness increases, the 90° color will gradually lose its influence and only the 0° color will be visible. This will happen even with higher Nd values.
This parameter, much like the Roughness parameter, allows you to simulate grooves and imperfections on a surface, but at a much larger scale. For example, you can use a Bump / Normal map to simulate wood grain. Keep in mind that Bump / Normal mapping only simulates these grooves on the surface - it does not actually create the geometry in any way, like Displacement does.The "bumpyness" effect depends on the direction and size of the emitter(s) in your scene - a small emitter placed at a low angle to your surface will give a stronger illusion of depth because of the low angle and the sharp shadows that a small emitter creates. On the other hand, a large emitter placed above a flat surface will create a very small illusion of depth.
How to use it
Specify a Bump texture and the Bump strength. Brighter values in the texture will create bumps on the surface and darker values will create indents. You can use a color map as a bump map but only the grayscale information of the map will be used. Bump mapping can be a sensitive parameter and standard values should be around 1 – 25.
A bump map with more contrast will produce a stronger effect, so the required bump strength depends on the particular map and the look you are after.
There is an additional option to specify a Normal map as a Bump texture (the little N icon next to the texture icon). Click this icon to specify to Maxwell that the map you loaded in the Bump parameter is a normal map.
While a grayscale bump map can simulate only the up/ down direction of the grooves, a normal map has the additional advantage of specifying an angle, or the direction of the grooves. A normal map is a RGB texture, not grayscale. Each channel (Red, Green, Blue) specifies an angle and the strength for the bump. It is recommended to use a Normal map if you want to give the impression of very strong bumps/grooves on a surface, or even to create a brick wall. A bump map is usually enough for smaller bumps, but if you are trying to simulate bumps that are very large, a bumpmap may not be enough.
When loading a normal map in the texture picker, the options Flip X, Flip Y, and Wide specify how the normal map was created. The most common standard is ‘Flip Y’, so this is selected by default. Consult the application you use to create normal maps to find out which of these options it uses to generate the maps.
Normal mapping (left) versus Bump mapping (right)
Creating Normal maps
Most modeling applications have an option to create a normal map from a detailed model, and there are also applications that allow you to convert a grayscale bump map into a normal map.
This parameter controls how directional the surface reflections should be. Anisotropic reflections occur on a surface with micro grooves or details that run in one dominant direction. Like an old music LP with grooves that run in an organized circular pattern. These types of surfaces reflect light back in a specular way in the direction of the grooves, and in a more diffuse way in the direction perpendicular to the grooves. Many common materials that have been polished show anisotropic reflections instead of the usual isotropic reflections (that blur equally in all directions when increasing roughness).
You can specify the anisotropy strength (0 for isotropic surfaces – 100 for full anisotropy). You can also set a grayscale texture to control the anisotropy strength: brighter values in the texture specify higher anisotropy. When using a texture, the numeric control has no influence.
Specify the anisotropy angle; the main direction of the reflected light. You can also set a grayscale texture to control the anisotropy angle. Brighter values in the texture specify a larger angle. When using a texture, the numeric control has no influence.
An interesting way to use an angle map is to create the type of anisotropic reflections seen on surfaces that have grooves running in a circular pattern. The map should have a circular gradient that gradually increases in brightness.
When applied to the top of a cylinder for example, you will get the typical anisotropic reflections of a surface that has been polished with a spinning polisher.
Angle map (left) and the resultant render(right)