Material physics in context of PBR texturing

This article is divided into two parts. The first section lays out a classification of common materials and describes their physical response to light in context of PBR texturing.
Second part focuses on specific workflows (metallnes, specular), textures and their parameters.

1.   Material physics in context of PBR texturing. 

Common Dielectrics (wood, stone, plastic)

002_CommonDielectrics

1. SPECULAR REFLECTION
– homogenous part of the visible color spectrum is being reflected from the surface of the material => texture has grayscale values
– an average dielectric specular value is 4%, 0,04 Linear, 59 sRGB
– Metalness workflow – this value is not described by any texture, it is defined in shader
– Specular workflow – included in Specular Color texture
– for specular workflow – the texture values must be physically accurate for a specific material (trusted value chart/scans)

2. DIFFUSE REFLECTION
– diffuse reflection is a result of the light ray scattering just below the surface of certain material – the depth is being neglected for common dielectris
– uneven part of the visible light spectrum is being reflected back => texture has RGB values
– it describes a surface color of a certain material = ALBEDO
– the texture must fit in physically realistic range of values (trusted value chart/scans)
– the darkest values shouldn’t go below 30-50 sRGB/0.01-0.03 Lin/12-19%
– the brightest values shouldn’t exceed 240 sRGB/0.88 Linear/94%
– metallness workflow – included in Base Color texture
– specular workflow – included in Diffuse texture

3. ABSORBTION
– common dielectrics absorb the rest of the visible color spectrum that hasn’t been reflected

4. REFRACTION
– no refraction is being calculated for common dielectrics


 Transparent Dielectrics (clear water or glass)

002_transparentDielectrics

1. SPECULAR REFLECTION
– homogenous part of the visible color spectrum is being reflected from the surface of the material=> texture has grayscale values
– an average dielectric specular value is 4%, 0,04 Linear, 59 sRGB
– Metalness workflow – this value is not described by any texture, it is defined in shader
– Specular workflow – included in Specular Color texture
– for specular workflow – the texture values must be physically accurate for a specific material (trusted value chart/scans)

2. DIFFUSE REFLECTION
– there is no light scattering ininside transparent dielectrics
– Albedo = 0 (100% black)
– dirty water or unclear glass are considered a translucent material (article below)
– metallness workflow – included in Base Color
– Specular Workflow – included in Diffuse

3. ABSORBTION
– there is no absorbtion, the light that hasn’t been reflected penetrates through the material

4. REFRACTION
– high values of refraction, great amount of the light gets through the material


Translucent Dielectrics (skin, wax, marble)

003_translucentDielectrics

1. SPECULAR REFLECTION
– homogenous part of the visible color spectrum is being reflected from the surface of the material=> texture has grayscale values
– an average dielectric specular value is 4%, 0,04 Linear, 59 sRGB
– Metalness workflow – this value is not described by any texture, it is defined in shader
– Specular workflow – included in Specular Color texture
– for specular workflow – the texture values must be physically accurate for a specific material (trusted value chart/scans)

2. DIFFUSE REFLECTION
– diffuse reflection is a result of the light ray scattering below the surface of  the certain
material – the depth is not being neglected for translucent dielectrics but it is being processed as a sub-surface scattering
– uneven part of the visible light spectrum is being reflected back => texture has RGB values
– it describes a surface or sub-surface color of a certain material = ALBEDO
– translucent materials has a high value of scattering
– the texture must fit in a physically realistic range of values (trusted value chart/scans, for example crosspolarized skin scans)
– the darkest values shouldn’t go below 30-50 sRGB/0.01-0.03 Lin/12-19%
– the brightest values shouldn’t exceed 240 sRGB/0.88 Linear/94%
– metallness workflow – is included in Base Color
– specular workflow – is included in Diffuse

3. ABSORBTION
– translucent dielectrics absorb just small portion of the non-reflected visible color spectrum

4. REFRACTION
– the visible light spectrum, that hasn’t been reflected back or absorbed, scatters inside the material and penetrates out
– in shading these values are being managed by refraction or sub-surface scattering
– high values of refraction

High values of scattering and refraction of translucent materials are usually defined in shader by Sub-Surface Scattering parameter.
Similar effect is possible to reach by changing a Refraction Rougness parameter

Typical shader parameters:

Diffuse/Base Color – describes a surface or sub-surface color of a certain material. The appearance of the texture depends on the depth at which the scattering is happening.
Radius – the amount of the scattering inside the material, bigger radius = more light scattering inside the material per unit of distance
Weight – ratio of scattering intensity right below the surface (diffuse/base color texture) and scattering deeper inside the material (sub-surface texture)


Conductors – Metals

002_Conductors.gif

1. SPECULAR REFLECTION
!! uneven part of the visible light spectrum is being reflected from the surface of the material => texture has RGB values
– specular values for metals fits in the following range: 70=100%, 0.7-1 Linear, 180-255 sRGB
– the texture values must be physically accurate for a specific material (trusted value chart/scans)
– metallness workflow – is included in Base Color texture
-specular workflow – is included in Specular Color texture

2. DIFFUSE REFLECTION
– there is no light scattering inside conductors
– Albedo = 0 (100% black)
– in metallness workflow this value is not described by a texture, it is defined in a shader
– specular workflow – is included in Diffuse texture

3. ABSORBTION
– metals absorbs all visible spectrum that hasn’t been reflected
4. REFRACTION
– conductors has no refraction


Microsurface

002_microsurface.gif

In theory, both diffuse and specular reflection are dependent on the surface irregularities (roughness/glossiness) In practice though, the effect of roughness on diffuse reflection is much less visible then on the specular reflection (the scattering  inside the material is already happening in diffuse reflection).
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 as shown on picture above.
The light is being distributed according the law of reflection:  Angle of incidence = angle of reflection
Not all microfacets will contribute in reflection of the light some of them will be blocking it.
The surface irregularities cause light diffusion which results in blurred reflections – scattered light rays are not reflected in parallel.

In metallness workflow the microsurface is authored in the Roughness texture
In specular workflow the microsurface is authored in the Glossiness texture

002_microsurfacePic.jpg

img source: https://marmoset.co/

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 if it is the same material.



2.  PBR Texturing workflows, textures and their parameters

Metalness Workflow

Base Color


image source: Allegorithmic (pbr guide)

Contains RGB values
being defined in sRGB color space

DIELECTRICS – diffuse reflection – albedo

lower contrast in tonality, lower saturation (in compare to the photo ref)
– the darkest values shouldn’t go below 30-50 sRGB/0.01-0.03 Lin/12-19%
– the brightest values shouldn’t exceed 240 sRGB/0.88 Linear/94%

– it can contain Micro Occlussion details from Cavity map
– it can not contain Ambient Occlusion information
– the texture values must fit within a physically realistic range of values (trusted value chart, scans)

CONDUCTORS/METALS – specular reflection

Values for the average conductors varies between: 180-255 sRGB/0.5-1 Linear/70-100%
the texture values must be physically accurate for a specific material (trusted value chart, scans)

Roughness/Microsurface


image source: Allegorithmic (pbr guide)

contains only grayscale  values
being defined in Linear color space

– describes the surface irreularities that cause light diffusion, this changes
the light direction but the intensity of the reflection remains the same.

– rough surfaces has broader and more blured reflections while smooth
surfaces has smaller and sharper reflections.

– the artist has full creative controll to describe all features of the material like an exposure to the elements, corosion, greas, all kind of damage etc
– the texture can also contain some fine surface displacement information obtained from the high frequency displacement, normal or bump map

– approximate basic values of the most common materials is possible to obtain from roughness information found in any trusted chart or scans. However these values are highly influenced by the state of the required material and thus they will always vary a lot.

black = smooth surface
white = rough surface

Metalness


image source: Allegorithmic (pbr guide)

contains only grayscale  values
being defined in Linear color space

– this textue is a mask
– it distinguishes between dielectris and conductors


Specular Workflow

Diffuse


image source: Allegorithmic (pbr guide)

contains RGB values
being defined in sRGB color space

DIELECTRICS – diffuse reflection – albedo

lower contrast in tonality, lower saturation (in compare to the photo ref)
– the darkest values shouldn’t go below 30-50 sRGB/0.01-0.03 Lin/12-19%
– the brightest values shouldn’t exceed 240 sRGB/0.88 Linear/94%

– it can contain Micro Occlussion details from Cavity map
– it can not contain Ambient Occlusion information
– the texture values must fit within a physically realistic range of values (trusted value chart, scans)

CONDUCTORS – diffuse reflection – albedo

– conductors have no diffuse reflection thats why we need to use a value of 0 (100% black) in the texture for metals

Glossiness/Microsurface


image source: Allegorithmic (pbr guide)

contains only grayscale  values
being defined in Linear color space

– describes the surface irregularities that cause light diffusion, this changes
the light direction but the intensity of the reflection remains the same.

– rough surfaces has broader and more blured reflections while smooth
surfaces has smaller and sharper reflections.

– the artist has full creative controll to describe all features of the material like an exposure to the elements, corosion, greas, all kind of damage etc
– the texture can also contain some fine surface displacement information obtained from the high frequency displacement, normal or bump map

– approximate basic values of the most common materials is possible to obtain from the inverted roughness information found in any trusted chart or scans. However these values are highly influenced by the state of the required material and thus they will always vary a lot.

black = rough surface
white = smooth surface

A glossiness textue is basically an inverted Roughness texture

Specular color


image source: Allegorithmic (pbr guide)

contains RGB values
being defined in sRGB color space

DIELECTRICS + CONDUCTORS – specular reflection

avarage specular value for common dielectrics: 4%, 0,04 Linear, 59 sRGB

specular value range for conductors: 180-255 sRGB/0.5-1 Linear/70-100%

– Specular values for dielectrics and for conductors must be physically acurate ( trusted value chart, scans)

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