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This document is designed to help those who need to quickly get up to speed in Unreal Engine 4's physically based Materials system. It assumes that you are at least somewhat familiar with Material creation in the Unreal Engine, at least with Unreal Engine 3 or later. If you are completely new to Materials in Unreal, you may prefer to start with the Essential Material Concepts page.

Our goal here is to give you just the "plain-language essentials" on how to make standard Materials using this system, as it can be confusing even for those with experience in previous Unreal Engine iterations. We will only be focusing on those aspects of Materials that are involved with our physically based approach. For a complete rundown of the available inputs, check out Material Inputs .

What Does Physically Based Mean?

Physically based shading means we approximate what light actually does as opposed to approximating what we intuitively think it should do. The end result is more accurate and typically more natural looking. Physically based Materials will work equally well in all lighting environments. In addition, the Material values can be less complex and interdependent, resulting in a more intuitive interface. These benefits are applicable even for non-photorealistic rendering as evident in the latest films from Pixar [4] and Disney [3] .

For these reasons and more, Unreal Engine 4 has adopted a new physically based Material and shading model. For an in depth, technical look at UE4's physically based Material and shading model, please refer to my SIGGRAPH presentation [2] .

Material Parameters

In terms of the "physically based" aspect of our Materials system, there are really only 4 different properties with which you need to be familiar. These are:

• Base Color

• Roughness

• Metallic

• Specular

Physically based values can be measured from real world Materials. Some examples will be given below.

Base Color

Base Color defines the overall color of the Material, taking in a Vector3 (RGB) value where each channel is automatically clamped between 0 and 1.

If taken from the real world, this is the color when photographed using a polarizing filter (polarization removes the specular of nonmetals when aligned).

Measured BaseColor values for nonmetals (intensity only):

Measured BaseColors for metals:

Roughness

The Roughness input controls how rough or smooth a Material's surface is. Rough Materials scatter reflected light in more directions than smooth Materials, which controls how blurry or sharp a reflection is (or how broad or tight a specular highlight is). A Roughness of 0 (smooth) results in a mirror reflection and roughness of 1 (rough) results in a diffuse (or matte) surface.

Mapping Roughness

Roughness is a property that will frequently be mapped on your objects in order to add the most physical variation to the surface.

Metallic

The Metallic input controls how 'metal-like' your surface will be. Nonmetals have Metallic values of 0 and metals have Metallic values of 1. For pure surfaces, such as pure metal, pure stone, pure plastic, etc. this value will be 0 or 1, not anything in between. When creating hybrid surfaces like corroded, dusty, or rusty metals, you may find that you need some value between 0 and 1.

You will notice we have added a little bit of Roughness in this example. This was purely an artistic choice.

Specular

When you're editing a non-metallic surface material, there are times when you'll want to adjust its ability to reflect light, specifically, its Specular property. To update a Material's Specular, input a scalar value between 0 (non-reflective) and 1 (fully reflective). Note that a Material's default Specular value is 0.5.

Commonly, if we modify Specular, we do so to add micro occlusion or small scale shadowing, say from cracks represented in the normal map. These are sometimes referred to as cavities. Small scale geometry, especially details only present in the high poly and baked into the normal map, will not be picked up by the renderer's real-time shadows. To capture this shadowing, we generate a cavity map, which is typically an AO map with very short trace distance. This is multiplied by the final BaseColor before output and multiplied with 0.5 (Specular default) as the Specular output. To be clear this is BaseColor = Cavity*OldBaseColor, Specular = Cavity*0.5.

For advanced use, this can be used to control the index of refraction (IOR). We have not found this to be necessary for 99% of Materials. Below are Specular values based off of measured IOR.

Measured Specular Values:

References

1 . Lagarde, Feeding a physically based shading model

2 . Karis, Real Shading in Unreal Engine 4

3 . Burley, Physically-Based Shading at Disney

4 . Smits, Reflection Model Design for WALL-E and Up

5 . Hable, Everything is Shiny