Base Material Properties
A 3D material is a collection of attributes that aim to simulate the visual 'physical properties' of real-world or imaginary objects. Each attribute determines how the light bounces off a surface for a specific method for reflecting light (diffuse, specular, reflection, sss). Any attribute can be colored, parametrized, textured or painted in 2D or 3D, inside or outside the shading software.
Understanding how material attributes function are important for the texture artists who have to set up the materials, but also later on in production. First, material properties have a big influence on how the light will react with the surfaces, at the lighting stage of a 3D production. Then it influences how the different lighting methods will be split into render passes during the rendering process. Finally, it will be important for the compositing stage to reassemble the different render passes into the intended image by the compositing artists.
Diffuse Color
The diffuse color is the main attribute in the material, it is generally what people call the 'color' of an object, unless it is overpowered by another attribute (reflectivity, sss) or lighting. The diffuse color is generally painted for every 3D object, to make it unique within the project, matching the artistic direction.
Specularity
The specular level is the amount of direct reflection of the of a light source on an object. It is the difference between a mat finish and a shiny finish, but it is not the reflection of the environment itself.
Reflectivity
The reflectivity is the amount of reflection of the real world environment, around the material. It acts like a mirror, and can be colored to act like a filter has been on top of it. Reflections are a percentage value represented with numbers between 0 and 1, with any level of precision.
Refraction
The refraction is a complex property, essentially derived from the simple concept of transparency. But where transparency is only a simple opacity slider, refraction is a deviation of light rays and photons. This in turns creates a myriad of interactions between the geometry shape and the material's refraction index.
In essence, it's main attribute is the transparency of the object, but it is controlled by many other attributes, such as the index of refraction (degree of deviation) and the roughness (porosity) of the surface it is applied onto. Reflections can also have a significant impact on the appearance of glass surfacesm determining what goest through and what bounces. The color of refraction can also tint the surface, unless the transparency is 100%, leaving no room for diffuse effect.
Luminosity
This is how much light is emitted by the surface of the object. No other object than a light source should have luminosity in order to preserve realism. Doing otherwise breaks the energy conservation rule of physics which will make the render look un-natural. Ex. lamps, neons, fire, fireflies...
Sub-surface Scattering (SSS)
Sub-surface scattering is the amount of light comming into a surface and bouncing inside of it. It acts like a localized luminosity source inside the object, like your hand with a flashlight behind.
Basic Deformation Maps
Bump Maps
The bump map is the oldest, but fastest and best supported method, for simulating small details on a surface using textures. It does not actually modify the silhouette of a 3D object, but alters the way light bounces off, simulating a false depth onto the object. The bump map must be a greyscale image, of the same size as the diffuse map.
Displacement Maps
The displacement map takes the concept of the bump map further, but actually moving the geometry points (vertex) of a 3D object according to the texture map. The only problem with this method is that is takes a lot of memory, because the mesh will most likely need to be 'virtually subdivided' at render time, for a smooth result.
Here are the most common 3D applications that create displacement maps to make 3D sclupted details:
- Autodesk Mudbox
- Substance painter
Normal Maps
A normal map is similar to a bump map, but it uses a colored map, where each color value (R, G, B) represent a different dimension of bump mapping. This recreates a better version of a bump map, albeit slightly heavier, that reacts better with light. This type of map cannot be created by eye. It must always be generated thru a 2D/3D software.
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