Physically based shading Ats Kurvet MTAT.03.296 Computer Graphics - - PowerPoint PPT Presentation

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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

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Physically based shading

Ats Kurvet

MTAT.03.296 Computer Graphics Seminar

Tartu 2014

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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.

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SLIDE 3

Examples of PBS in action

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http://atskurvet.com/portfolio.html

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

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

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http://www.cgmag.at/index.php?page=Attachment&attachmentID=5113&h=47ccf5d3bb8c3aad a9c32a34d63f957f506ae0a9

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

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SLIDE 9

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*

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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
  • …

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Physics

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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

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?

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

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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

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Lights interaction with matter

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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

  • f absorption and color.

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

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Scattering

  • Light splits

into multiple directions due to abrupt micro scale changes in refraction.

  • http://blog.selfshadow.com/pu

blications/s2013-shading- course/hoffman/s2013_pbs_p hysics_math_slides.pdf

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Absorption, scattering and emission

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

course/hoffman/s2013_pbs_physics_math_slides.pdf

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Emission

  • Many different

sources and ways

  • f generation
  • There is a

difference between white light and white light

http://www.ni.com/cms/image s/devzone/tut/image4_200801 09201025.png

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Reflections on a planar surface

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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"

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Subsurface scattering

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

course/hoffman/s2013_pbs_physics_math_notes.pdf original source: “Real-Time Rendering, 3rd edition"

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SSS and the diffuse term

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

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Math

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Radiance - L

  • Radiance is the

measurement of the quantity of radiation that passes through

  • r 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/thu mb/6/63/Etendue-Definition.png/400px- Etendue-Definition.png

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Bidirectioanl Reflectance Distribution Function

f(l,v)

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

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?

  • BTDF
  • BSDF
  • BSSRDF

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Reversability

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

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Reciprocity & energy conservation

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Reflectance equation

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Diffuse and specular term

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

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Splitting diffuse and specular in the real world

  • Check out the tutorial at:

http://filmicgames.com/archives/233

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Real world examples

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http://filmicgames.com/archives/547

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Real world examples

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http://filmicgames.com/archives/547

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Real world examples

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http://filmicgames.com/archives/547

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Surface Reflectance (Specular Term)

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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

  • r masked by neighboring geometry.
  • http://blog.selfshadow.com/publications/s2013-shading-course/hoffman/s2013_pbs_physics_math_notes.pdf
  • riginal source: “Real-Time Rendering, 3rd edition"

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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
  • riginal source: “Real-Time Rendering, 3rd edition"

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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

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Fresnel reflectance

  • The Fresnel reflectance function computes

the fraction of light reflected from an

  • ptically flat surface.
  • Depends on the incomoing angle of the

light and the index of refraction.

  • Spectral quality.
  • Values between 0 and 1

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Fresnel reflectance for a variety of substances

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

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Characteristic specular reflectance – F0

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

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

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The Schlick Approximation to Fresnel

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

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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%2Fcs3 84g%2Fprojects%2Fraytracing%2Fray_examples%2Fanisotropy_ball.jpg & http://blog.selfshadow.com/publications/s2013-shading-course/karis/s2013_pbs_epic_notes_v2.pdf 45

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Examples of NDFs

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

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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.

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Limitations of the Microfacet model

  • Does not account for pronounced wave
  • ptics 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.

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Subsurface Reflectance (Diffuse Term)

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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)

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Lambert

  • Most widely used. A constant value.

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Other terms

  • Subsurface single-scattering.
  • Multiple-bounce surface reflectance.
  • Might want to account for these in some

ways no.

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Illumination model

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General Lighting

  • Integrate BRDF against all incoming light

from all directions.

  • Solving this requires global illumination

models such as Monte Carlo ray tracing.

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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.

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

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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.

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Area light examples

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

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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

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Energy conservation.

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

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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

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Extra 0: material blending & layering

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

course/rad/s2013_pbs_rad_slides.pdf

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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.

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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

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Thank you!

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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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