PBR Terminology

Energy conservation

The energy conservation states that an object can not reflect more light than it receives.

For example material with sharper and bold highlights appears more reflective than the one with smooth and dull highlights, however it is the variance in microsurface (roughness map) that results in a sharper o smoother highlights while the specularity remains the same.

Albedo

Albedo, commonly known as diffuse map. Contains the information about diffuse reflection of the specific material.

Common dielectrics albedo value ranges between 30-240 (12-94%) sRGB, 0.01 – 0.88 Linear

For transparent dielectrics and metals the albedo value is  0 (100% black)

Microsurface

Microsurface in PBR is being described by the microfacet theory which supposes that a surface is composed of small-scaled planar detail surface of varying orientation called microfacets. Each of these small planes reflects light in a single direction based on its normal.
The surface irregularities cause light diffusion which resuts in blurred reflections – scattered light rays are not reflected in parallel.

Rougher surfaces will have larger and dimmer looking highlights.
Smoother surfaces wil keep specular reflections focused and sharp which can appear to look brighter. However the amount of the reflection remains the same.

Fresnel

Fresnel is the percentage of light that a surface reflects at grazing angles.

All types of materials become 100% reflective at grazing angles so the fresnel should be set to 1 in the shader.

F0 Reflectance

F0 is fresnel reflectance at 0 degree (when the light hits a surface straight on or perpendicularly) there is a percentage of that light that is reflected back as specular reflection.

It is the F0 reflectance value that we are concerned with in regards to authoring specular color texture.

Using the Index of Refraction (IOR) for a surface, you can derive the amount that is reflected back which is referred to as F0.

Trusted IOR values from internet:

www.refractiveindex.info

To turn an index or refraction (IOR) to F0 use the following equation:

Disclaimer: all the numerical values  are in Linear space.

IORtoF0equation

For a practical sample, take the index of refraction of water (1.33) and put it inside the equation:

IORtoF0equationWater

The result (0.02) is the specular color for water.

In specular color texture dielectrics will have a grayscale values and metals will have RGB values.

The F0 range for common Dielectrics and Metals:

Dielectrics: 0.02 – 0.05 Linear

Metals:  0.5 – 1.0 Linear

 

PBR Texture Convert tool for Mari

These two Mari gizmo tools are made for converting textures between three different texturing workflows (metalness, specular, Arnold4)

Download links:
Metalness (PBR) to Specular (PBR) texturing workflow
Metalness (PBR) to Arnol4 (ai standard) texturing workflow

Created in Mari 4.1. using ExtensionPack 4R2

Basic principle is described on the picture below:

GizmoMtlToSpc_01

  1. Install gizmos by copying downloaded files into Mari gizmo folder that is usually here:
    On WindowsMY_DOCUMENTS / MARI / GIZMOS
    On Mac~ / MARI / GIZMOS
    On Linux: ~ / MARI / GIZMOS
    Or any Folder Path you specified for Gizmos via the MARI_GIZMO_PATH Environment
  2. plug your metalness textures.
  3. Switch between outputs in the node properties.

Metalness (PBR) workflow

This PBR workflow is suitable for Arnold5 or Mantra renderers.

Metalness wf texture set:

BASE COLOR
RGB – Linear
Contains albedo color for dielectrics and reflectance value for metals.
Derived from real-world measured data

ROUGHNESS
Grayscale – Linear
black = smooth, white = rough

METALNESS
Grayscale – Linear
black = dielectric, white = raw metal

Specular (PBR) workflow

This worflow is also PBR and allows you to have better control over your specular values for dielectrics.

Specular wf texture set:

DIFFUSE
RGB – Linear
Contains only albedo color no reflectance value.
Albedo color for dielectrics is derived form real-world measured data
Metals have no albedo color, use 100% black.

SPECULAR
RGB – Linear
Contains reflectance value for metals and dielectrics.
Values are derived from real-world measured data.

GLOSSINESS
Grayscale – Linear
black = rough, white = smooth

Arnold4 (ai standard) workflow

This workflow is not PBR but you can easily convert your PBR metalness workflow textures (Mari,Substance) and convert them using Mari texture convert gizmo into required output for Arnold4 renderer.

Arnold4 texture set:

DIFFUSE
RGB – sRGB
Contains only albedo color for both, dielectrics and metals, no reflectance value.
Albedo color for dielectrics is derived from real-world measured data, metals had no albedo color (use 100% black).

SPECULAR
RGB – sRGB
Contains only reflectance value for metals derived from real-world measured data. For dielectric is specular value in this texture set to 100% white and is being determined by the fresnel texture (below)

ROUGHNESS 
Grayscale – Linear
black = smooth, white = rough

FRESNEL
Contains fresnel (F0 reflectance) value for dielectrics:
Linear = 0.04, sRGB = 59/4%
metals = 100% white