In Revit 2019, the renderer was upgraded with some new appearance parameters. Previous versions of the Revit Materials are now commonly called “Legacy Materials”. An earlier version of Autodesk Raytracer, the built-in rendering engine for Revit, utilized a more color and image-based algorithm that was largely based on Nvidia’s Mental Ray for these legacy materials. Mental Ray was the built-in rendering engine for Revit until it was discontinued by Nvidia in 2017, thus forcing Autodesk to create a new, home-grown engine. Legacy materials utilizing the older appearance parameters will be marked with an alert symbol in the corner.
The latest, upgraded version of Raytracer now utilizes parameters for materials not previously available in Revit. The new materials are sometimes called Advanced Materials. Below is an example of some of the new options when creating or modifying an advanced material in Revit. The new materials are also called Physically-Based Render (PBR) Assets. PBR refers to the concept of using realistic shading/lighting models along with measured surface values to accurately represent real-world materials. This was an important step for Autodesk, as it aligns the Raytracer engine to something much closer to workflows utilized by computer graphics artists for some time.
Revit has four basic categories for its PBR assets. Each one has similar options, but they differ in some which cannot be edited. These differences will tell Raytracer how light is supposed to interact with the underlying material. Therefore, knowing these differences will be crucial to getting the most out of your photorealistic rendering. It would depend on what results you are looking for as to which material to apply to your geometry. For example, light bounces off a metal door handle very differently than it will brick, and even more so a window pane. The four categories that Revit now offers are:
Metal
Metal has a fixed amount of reflectance (100%) which cannot be edited. What this means is that with a roughness value of 0 it acts as a perfect mirror.
Non-Metal
A Non-Metal has a reflectance value of 0.00-0.08 (0-8%). For most materials a value of 0.04 (4%) is enough.
Transparent
Transparent materials have an absorption distance and an index of refraction instead of reflectance (IOR). IOR has a fixed value for different materials and determines how much the path of light is refracted or bent when passing through a material. The absorption distance is the distance light must travel to reach the selected base color of the transparent material. If the thickness of the material stays the same a shorter absorption distance will result in a darker color, beyond that the color will not change. If the absorption distance is longer than the thickness of the object the base color will not be reached.
Base & Top-Coat
Special materials have a base color with highlights and a top coat with a reflectance of 0.00-1.00 (0-100%). Think of a car paint material, inside of Revit. This carbon fiber material below has a base color which is black and highlights (specular) which represent the strands or fibers. It has its own roughness and the weight determines the visibility of the specular highlights. The normal map is a standard plain weave. Anisotropy is used so that there is also a checkered effect of the weave pattern in this case a plain weave which corresponds with the specular map. The top coat has its own reflectance (0-1, 0-100%) and roughness. When the reflectance of the top coat is 100% you will not see the base coat anymore because the top coat acts as a perfect mirror. (note that some of the maps have been scaled up to make the effects more visible)
Revit PBR’s consist of one of those four basic category's and three values/images. For our example, Cherry, this would be:
- Non-Metal
- Color/Diffuse (sometimes called Albedo)
- Roughness map
- Normal/Height map
Material Categories and parameter maps
- Category
As mentioned before, categories are important for PBR materials as this tells Revit what kind of material that you’re planning to bounce light from. The choice of material category should be relatively simple it's either a metal or not a metal. Neither can be transparent and then there is the last category base & top coat, which should only be chosen if you try to recreate a material which consists of distinct layers such as a car finish, or parallax ground cover.
- Roughness map
The roughness map defines the surface of your material, this can be done using a slider, in which everything will have the same roughness. 0 is smooth and 1 is rough, this translates to black=0 and white=1. So, a completely white image in the roughness input will result in a completely smooth surface, depending on the category this could be a mirror (metal).
- Normal/Height map
The choice between a normal or a height map depends of the amount and scale of detail. Consider what is more important: the overall height difference of the material or smaller, fewer protruding details. So, if the overall silhouette is more important than intricate details use a height map in all other cases use a normal map. A normal map also has a depth amount slider, this defines the total height difference and should be used to tweak the look of the material.
- Anisotropy
Should only be used to simulate certain specific effects. Otherwise it might affect your roughness/reflectance in an unwanted way. If roughness is 1 (white) and anisotropy is between 0.01-1 (1%-100%) your material will not reflect light in a completely diffuse way. The color input should be white, unless certain parts must stand out more than other parts, an image (specular) can be used to make certain parts appear darker or lighter.
In the above ways, the new Revit PBR materials are inching ever closer to those enjoyed by multi-media communities such as 3D Max, and help provide much more realistic renders within Revit!
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