Physically based shading

Ats Kurvet

MTAT.03.296 Computer Graphics Seminar

Tartu 2014 1

Why use PBS?

• Consistency - across materials and

lighting conditions.

• Speed - of authoring content (better tools,

less variables, intuitive parameters, easier

troubleshooting).

• Easier to achieve “hyperrealism”

• Cheaper.

2

Examples of PBS in action

3

4 http://atskurvet.com/portfolio.html

5 http://www.unrealengine.com/files/downloads/2013SiggraphPresentationsNotes.pdf

6 http://www.fxguide.com/wp-content/uploads/2013/04/blueumbrella1.jpg

7 http://www.cgmag.at/index.php?page=Attachment&attachmentID=5113&h=47ccf5d3bb8c3aada9c32a34d63f957f506ae0a9

8 https://www.fxguide.com/featured/gravity/

Also check out

• Unreal Infiltrator demo:

https://www.youtube.com/watch?v=dO2rM

-l-vdQ

• Order 1886:

https://www.youtube.com/watch?v=2FK8d

gzW0o8

• And probably anything else that looks

good*

9

PBS needs a strong base

• HDR and Tone mapping

• Anti-aliasing (also accounting for specular)

• Per object motion blur

• Depth of field (if using a realistic camera

model)

• PostFX

• …

10

Physics

11

Optics

• Geometrical/ray optics – Basic reflections

– Basic refractions

• Physical/wave optics – Interference

– Diffraction

– Polarization

• Electromagentic wave optics – Maxwell equations

• Quantum optics – Photons: the atomic principle

– Wave-particle duality

12

?

• http://isites.harvard.edu/fs/docs/icb.topic186199.files/images/EdgeDiffraction2-800x533.jpg 13

Visible spectrum

• http://hyperphysics.phy-astr.gsu.edu/hbase/vision/imgvis/specol.gif

• http://upload.wikimedia.org/wikipedia/commons/1/1f/Light_dispersion_of_a_mercury-vapor_lamp_with_a_flint_glass_prism_IPNr%C2%B00125.jpg

14

Lights interaction with matter

15

Refraction and absorption

• The refractive index – Describes how light

interacts with the medium it is travelling in.

– Spectral quantity

– The complex refractive index:

• The real part describes the affect to the speed and thus angle of deviation.

• The imaginary part describes the amount of absorption and color.

http://www.topwallpaperphoto.com/wp-content/uploads/2013/09/Whiskey-Drink.jpg 16

Scattering

• Light splits

into multiple

directions

due to

abrupt micro

scale

changes in

refraction.

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_slides.pdf

17

Absorption, scattering and

emission

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_slides.pdf

18

Emission

• Many different

sources and ways

of generation

• There is a

difference between

white light and

white light

http://www.ni.com/cms/images/devzone/tut/image4_20080109201025.png

19

Reflections on a planar surface

20

Non-Optically-Flat Surfaces

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

21

Subsurface scattering

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

22

SSS and the diffuse term

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

23

Math

24

Radiance - L

• Radiance is the measurement of the quantity of radiation that passes through or is emitted from a surface and falls within a given solid angle in a specified direction. In our case “a single ray of light”.

• Radiance is a spectral quantity.

http://upload.wikimedia.org/wikipedia/en/thumb/6/63/Etendue-Definition.png/400px-Etendue-Definition.png 25

Bidirectioanl Reflectance

Distribution Function

f(l,v)

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

26

?

• BTDF

• BSDF

• BSSRDF

27

Reversability

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

28

Reciprocity & energy conservation

29

Reflectance equation

30

Diffuse and specular term

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

31

Splitting diffuse and specular in the

real world

• Check out the tutorial at: http://filmicgames.com/archives/233

32

Real world examples

33 http://filmicgames.com/archives/547

Real world examples

34 http://filmicgames.com/archives/547

Real world examples

35 http://filmicgames.com/archives/547

Surface Reflectance (Specular

Term)

36

Microfacet theory

• Microgeometry

• Light only reflected when, h=m, where h is the half angle vector and m is the microgeometry normal.

– But only when the surface point is not shadowed or masked by neighboring geometry.

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

37

Shadowing & masking

• Shadowing (left) – accounted for.

• Masking (middle) – accounted for.

• Interreflections (right) – not accounted for.

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

38

Microfacet BRDF

• D(h) – microgeometry normal distribution function

• G(l, v, h) - the geometry function

• F(l, h) - Fresnel reflectance

• 4(n.l)(n.v) – correction factor for transforms between micogeometry space and the macrosurface

39

Fresnel reflectance

• The Fresnel reflectance function computes

the fraction of light reflected from an

optically flat surface.

• Depends on the incomoing angle of the

light and the index of refraction.

• Spectral quality.

• Values between 0 and 1

40

Fresnel reflectance for a variety of

substances

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

41

Characteristic specular reflectance

– F0

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

42

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

43

The Schlick Approximation to

Fresnel

• For microfacet BRDFs (otherwise replace h with n):

44

Microgeometry normal distribution

function - D(m)

• Describes the amount of microgeometry normals m having the same direction as the surface normal n.

• Values must me greater than 0.

• Scalar.

• Determines the size, brightness, and shape of the specular highlight.

• Isotropic and anisotropic surfaces.

https://lva.cg.tuwien.ac.at/ecg/wiki/lib/exe/fetch.php?hash=ac8b26&media=http%3A%2F%2Fwww.cs.utexas.edu%2F~fussell%2Fcourses%2Fcs384g%2Fprojects%2Fraytracing%2Fray_examples%2Fanisotropy_ball.jpg & http://blog.selfshadow.com/publications/s2013-shading-course/karis/s2013_pbs_epic_notes_v2.pdf

45

Examples of NDFs

• http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

46

Geometry function - G(l, v, m)

• Represents the probability that surface

points with a given microgeometry normal

m will be visible from both the light

direction l and the view direction v.

• Scalar.

• Values between 0 and 1.

47

Limitations of the Microfacet model

• Does not account for pronounced wave optics effects (such as diffraction and interference), those are usually solved with ad hoc methods.

• Geometry features in the scale range of wavelength of light, or larger features becoming smaller due to foreshortening at grazing angles.

• More complex microgeometry.

48

Subsurface Reflectance (Diffuse

Term)

49

Diffuse Term

• Gets the left over light that is not reflected straight off the surface(specular term).

• Spectral.

• Values between 0 and 1.

• Extra considerations to look at:

– Specular reflectance increases at grazing angles so the diffuse value must decrease.

– Affect of roughness (microgeometry features larger than that of the sub-surface scattering distance)

50

Lambert

• Most widely used. A constant value.

51

Other terms

• Subsurface single-scattering.

• Multiple-bounce surface reflectance.

• Might want to account for these in some

ways no.

52

Illumination model

53

General Lighting

• Integrate BRDF against all incoming light

from all directions.

• Solving this requires global illumination

models such as Monte Carlo ray tracing.

54

Image-Based Lighting

• Typically represented as environment maps, can also be represented by spherical harmonics for example.

• Works well for smooth surfaces, needs to be sampled for arbitrary BRDFs.

• Perceptually flexible.

55 http://www.randomcontrol.com/images/products/arion2/importance_sampling.gif

Area Lights

• Have both intensity and area

• In reality all light sources have

dimensions, this avoids issues like

specular intensity reaching inifinity.

• Faster shadow calculations than

environment maps.

• Physically more correct.

56

Area light examples

57 http://blog.selfshadow.com/publications/s2013-shading-course/pixar/s2013_pbs_pixar_notes.pdf

Punctual Light Sources

• Infinitely small, infinitely bright, physically not correct.

• Computationally convenient.

• Effects are defined by:

– Clight - intenisty of light in RGB.

– lc – light direction vector

• More common used variants of punctual lights are: point, directional, and spot lights.

• Unbound range

58

Energy conservation.

59 http://www.guerrilla-games.com/presentations/Drobot_Lighting_of_Killzone_Shadow_Fall.pdf

Ambient light

• Low-frequency lighting.

• Can be represented by a constant or more

complex structures such as lower order

spherical harmonics.

• Mostly used by games

• Often breaks materials

60

Extra 0: material blending & layering

• http://blog.selfshadow.com/publications/s2013-shading-course/rad/s2013_pbs_rad_slides.pdf

61

Extra 1: Disney BRDF explorer.

• http://www.disneyanimation.com/technolog

y/brdf.html

• Can load GLSL code as an arbitrary

BRDF.

• Can load reference data to compare your

model to real world measurements.

62

Extra 2: Crytek & Ryse things.

• My lighting portfolio:

http://atskurvet.com/portfolio.html

• PBS article about Ryse:

http://www.makinggames.de/index.php/ma

gazin/2391_ryse__the_transition_to_physi

cally_based_shading

• Cryengine presentations:

http://crytek.com/cryengine/presentations

63

Thank you!

64

Reference

• http://blog.selfshadow.com/publications/s2013-shading-

course/hoffman/s2013_pbs_physics_math_notes.pdf

• http://www.makinggames.de/index.php/magazin/2391_ryse__the_transition_to_physically_based_

shading

• https://www.fxguide.com/featured/game-environments-parta-remember-me-rendering/

• http://interplayoflight.wordpress.com/2013/12/30/readings-on-physically-based-rendering/

• http://www.disneyanimation.com/technology/brdf.html

• http://wiki.nuaj.net/index.php?title=BRDF

• http://www.cs.princeton.edu/~smr/papers/brdf_change_of_variables/brdf_change_of_variables.pdf

• http://renderwonk.com/publications/s2010-shading-

course/snow/sigg2010_physhadcourse_ILM.pdf

• http://www.guerrilla-games.com/presentations/Drobot_Lighting_of_Killzone_Shadow_Fall.pdf

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