Surface Material Physics for Shader Writers

8/13/2019 Surface Material Physics for Shader Writers

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Surface Material Physics

for Shader Writers

Chas. Boyd

DirectX Graphics Architect

Windows Gaming & Graphics Technology

Microsoft Corporation


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Motivation

Lighting and surface effects areclearly discernable to end-users

Enables novelty for them, anddifferentiation for you

Enable a title to have a distinctlook

See other talks on lighting models,this one focuses on surfaces


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Overview

Surfaces vs. LightingSurface Types

Metal

Rough

Specular

Subsurface

Layered Materials

Maya, SOFTIMAGE, Renderman


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Assumptions

For all surfaces:

Other effects such as shadows will

be applied in the lighting modelThese are independent of thesurface effects described here


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Surfaces and Lighting

Key participant in visual Look

Interacts with lighting environment

in various waysNeed to take both into account inany given situation

Design content to have surfacesthat look best in the lightingenvironment they will be used inand vice versa


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Fundamentals of Surfaces

Surfaces often contain multipleconstituents

Absorptive dies, reflective layers,scattering particles of various sizes

This talk proceeds from simplest tomore complex surface structures


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Polished Metal Surface

Process

Light comes in

Some colors are absorbedRemaining light reflects

Atomically smoothmirror finish

Structured highlightsimage of environment are visible inthem


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STM Micrograph


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Polished Metal


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Map-Based Technique

Apply environment map forreflection

Scale it by metal colorAccounts for some colors absorbed

That is the final color for the screen

C = envmap*base

Gamma Note: no effect


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Modulate Environment Map


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Procedural Technique

Use very high specular exponent

~100 or higher

Accurate model for single constantcolor round light source

C = light*dot(N,H)^100 * base

Gamma Note: picking exponent


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Use in Games

Works for gold, copper, bronzesome anodized effects

Ideal for weapons

Gun metal blue

Not appropriate for enameled metal

More metallic than any metallic auto-

body enamelNor for oxidized e.g. verdigris

These are too rough for this model


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Rougher Surfaces

Smudges, scuffs, dust, oxides

At high magnification we see:


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Rough Surface

Reduced

Specular

Diffuse

Scattering


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Map-based Technique

Roughness varies per-pixel ingloss map e.g. alpha of base tex

Scale down environment map

c = gloss*envmap*base

Blur environment map in areas oflow gloss

MIP LOD or post-process effect

MIP LOD is per-pixel in ps.2.0

Blur is partial integral of map


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Use procedural specular

spec = light*dot(N,H)**25

but with same gloss term

c = gloss*spec*base

Decrease exponent to achieve blur


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Both Techniques

Add diffuse scattering term


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Increased Roughness Surface


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Fully Rough Surface


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Fully Rough Surface

No Specular

Diffuse

Scattering


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Rough Surfaces

Concrete

Sand

ClothPlaster

Marble, non-polished

Pavement

Stone


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Moon Photograph


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Moon Rendering


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Rough Surfaces

Diffuse reflector

Lambertian or Oren-Nayar effects


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Map-based Technique

Blur map across very large scale

Integrate entire hemisphere


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Cube Map And Its Integral


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Diffuse only


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Model only Diffuse term

For directional light:

c = light * dot(N,L) * baseOr use hemisphere light

Gamma Note:Correct using sqrt or 1-(1-x)**2

for non-CAD applications


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Layered Surface

Varnish

Dye Layer


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Layered Surface Larger

Varnish

Dye Particles


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Layered Surface

Thin layer of smooth clear material

Over rough dye particles

Underlayer is rough becauseparticles may be suspended

Plastics

Linoleum

Enameled metalcar finish

Varnished surfacewood


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Map-Based Technique

Envt. map scaled by gloss term

spec = gloss*envmap

Added to diffuse term:c = dot(N,L)*base + spec

Base color does not tint highlight

Gamma Note: add should be linearCorrect to linear, add, convert back

if have enough precision and time


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Add Environment + Diffuse


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Specular Only


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Base color does not modulatespecular, only light color does

spec = light*dot(N,H)**100

c = dot(N,L)*base + spec*gloss

Top layer + bottom layer


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Summary

Polished specular*gloss*base

Rough diffuse*base

Varnished spec*gloss

+ diffuse*base


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Fresnel Effect

At top layer interface

Some light is reflected,

Remainder is transmitted through

Depends on surface material

And on incident angle!

Fresnel term fis proportionreflection

grazing angles: 100% reflected

normal angles: 5% reflected


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Fresnel Mid

Material

Air

60% reflected

40% transmitted


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Fresnel Normal

Material

Air

10% reflected

90% transmitted


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Fresnel Grazing

Material

Air

90% reflected

10% transmitted


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Fresnel Term Approximation

Use f = 1 - cosq= (1 - E dot N)Raise to power for betterapproximation

f = (1cosq)4[Schlick] used 5thpower

Hack curve fit to Glassner

F l V t Sh d


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Fresnel Vertex Shader

m4x4 oPos,v0,c8; // xform pos. to output spacem4x4 r0,v0,cW // xform pos. to world space,

m3x3 r1,v3,cW // xform normal to world space

add r0,-r0,c14 // compute eye ray

dp3 r3.x,r0.xyz,r0.xyzrsq r3.xyz,r3.x

mul r0.xyz,r0.xyz,r3.xyz// normalize it

dp3 r0,r0,r1; // dot eye ray with normal

add r0,c1111,-r0 // complement colormul r1,r0,r0 // **2

mul r0,r1,r1 // **4

mul r0,r0,c1 // scale for fresnel

add oD0,r0,c0 // bias and emit as diffuse

F l Pi l Sh d


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Fresnel Pixel Shader

ps.1.1

; def v0 // eye ray

def c0 // 5% biasdef c1 // 95% scaletex t0 // normal map

dp3 r0,v0_bx2,t0_bx2; // dot eye ray with normal

mul r1,1-r0,1-r0 // complement and squaremul r0,r1,r1 // **4mad r0,r0,c1,c0 // scale & bias


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Fresnel Comparison

45o 90o0o

1- cosq

Measurement(1- cosq 4


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Fresnel Term


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Fresnel Affects Diffuse Layer

Only light that was not reflectedilluminates diffuse dye layer

In single light case, incident anglecan be used to compute f

Then fis used to blend (lerp)between specular and diffuse

Not as useful for multi-light case

diffuse layer is illuminated by lightfrom all directions in that case


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Metallic Flake

Can used BEML with randomGaussian distribution ofperturbation pixels

Can use similar trick for glassbuildings

point sampled displacement map gives

each window a separate normalOr use filtering to get curved windows

then must mask seams with alphatest


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MicroFlake

Use procedural specular term withlow exponent ~8

Feasible with DirectX 8.* pixel shader

Still need enviroment map for carfinish enamel gloss coat


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Metallic Flake Finish

Varnish

Dye Layer

Flakes


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Sub-Surface Effects

Absorption

Light has to get inside

Re-emission

Light has to get out again


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Subsurface Absorption

Can be impacted by Fresnel effects

Only non-reflected light penetrates

Simple term to model

Can be impacted by structure ofsurface


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Skin

A layer of ketchup covered bytissue paper

Probably more translucent than realketchup


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Anisotropic Effects

Most visible in specular case

Brushed metal

Cloth fabricsWire spools


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Anisotropic Hair

Hair Tufts

snip a lock, put it in scanner

Eyelashes

use alpha channel mask

per-vertex anisotropic

no per-pixel direction variation

Anti-aliased lines are too thick


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Key References

Nelson Max

Christophe Schlick

Peter Shirley


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Empiricism

Shading is not fully understood

Most efforts not based on purephysics

Equations are fit to measured data

Equations are chosen to look good


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Empirical Process

Discern an effect

Measure how it varies

Curve fit some equation to itTweak it until it looks good


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Empirical Process

Measure

Fit Eqn

Plot

Render

Evaluate

Tweak


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Nelson Max

Horizon mapping: shadows forbump-mapped surfaces

Basis of last years GDC talk on real-

time self-shadowing bump mapsAvailable as sample in DX8.1 SDK

Most work is applicable and

accessible


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Christophe Schlick

A Customizable Reflectance Modelfor Everyday Rendering

Uses his procedurally generated BRDF

Generic Lighting Model

Single shader for specular vs diffuse, andisotropic vs anisotropic surfaces

Reduces state changes?Requires 2.0 shaders


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Peter Shirley

Real-time Ray-tracing

Hemisphere lighting model

A Practitioners Assessment ofLight Reflection Models

Demonstrates process of identifyingan effect and defining a technique to

represent itIncludes good background material


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Books

Foley & Van Dam, Feiner, Hughes

Still lots of good background material

Fundamentals

Principles of Digital ImageSynthesis by Andrew Glassner

Broad sampling of topics

Excellent detail in key areas

Concordance for much of literature


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Miscellaneous

Graphics Gems

Game Programming Gems

Real-time Photorealistic RenderingEric Haines


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Call to Action

Pick a unique Look and do it

Toon several methods

Cheesy unlit or flat shadedRetro std FF pipelines

Radiosity soft lighting only

Shadows horror movieGritty ultra realistic

Uses photos for concept art

Documents

Surface Material Physics for Shader Writers