Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 1

Global IlluminationGlobal Illumination

Part I Part I -- PhotorealismPhotorealism

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Why Global Illumination?

Achieve more photorealistic images

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All images are from MentalRay’s website

Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 2

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Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 3

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6From “Alexander”

Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 4

7From “The Day After Tomorrow”

8From “Crysis”

Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 5

9From “Attack of The Clones”

10From “Attack of The Clones”

Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 6

11From “Assassin’s creed”

Photorealism

(As much as possible) physically accurate simulation of illumination in a scene.Main missions:

Glossy and mirror-like ReflectionTranslucency & transparencyShadowsDiffuse inter-reflectionCaustics( b) f tt i

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(sub)surface scatteringRefraction & DispersionTextures..and many more

Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 7

TranslucencyTransparency is the material property of allowing light to pass through.Translucent materials allow light to pass through them only diffusely: they cannot be seen through.

The light scatters inside the material before leaving the object.Also Called “Subsurface Scattering”

Requires a more realisticRequires a more realistic Diffuse model

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Soft ShadowsHard shadows are created as a result of discrete (aliased) illumination.In reality, shadows are softer due to light transport from other lighted materials in the scene.

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Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 8

Diffuse Inter-reflectionLight scattered from a non-specular object illuminates (bleeds unto) other objects in the scene.Diffuse models:

Lambertian (CG Course model) - uniform in all directionsOren-Nayar Model – for rough surfaces.Torrance-Sparrow, Schlick and more.

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CausticsA caustic is the envelope of light rays reflected or refracted by a curved surface or object, or the projection of that envelope of rays on another surface.

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Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 9

Texture mappingThe art (and science) of applying realistic textures to geometric imagesImportant issues:

ParameterizationBump mapping(As usual) – Antialiasing Multiresolution3D textures

(from “Beowulf”)

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Theory of LightThe physics of light treated by these models:

Ray Optics – Geometric rays traveling in y p y goptic media.Electromagnetic optics – polarization, dispersion.Wave optics – interference and diffraction.Photon optics – Interaction of light andPhoton optics – Interaction of light and matter.

In computer graphics, we mostly apply ray optics.

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Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 10

TerminologyRadiometry – the description of physical light (most of our work).Photometry – Including the description of light perception. (Tone mapping, HDR lighting).

From “Half-life 2”19

Light ScatteringWhen light encounters an obstacle, it is either absorbed or scattered.The local illumination part of lighting a scene.Common models:

BRSSDF – Light enters the material and scatters around before leaving the surface in another point.BRDF – All light is reflected at the same location at which it hits the surface.

M t d i k l th i l BRDFMost rendering works apply the simpler BRDF.

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Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 11

Light Transport (Heckbert Notation)In order to generalize the reference to the path the light goes, we use a regular expression defined thus:

L – Light source(s)E – The eye(s)S – A specular reflectionD – A diffuse reflection(n)+ - one or more events of n(n)* - zero or more events of n(n)? – zero or one n event(n | m) - either m or n event

For instance – L (S|D)+ D E means a light from a light source, with one or more specular or diffuse reflections, finally reflected as a diffuse into the eye.A full global illumination solution – L (S|D)* E

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Simple Illumination Model

Light Object Eye (L (S|D) E)Diffuse

Specular/Glossy cosn(θ)

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Specular/Glossy - cosn(θ)

Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 12

Ray Tracing Illumination Model

Light Object Specular Specular … Eye (LD?S*E)

S l fl tiSpecular reflection

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Specular refraction

Radiosity Illumination Model

Light Diffuse Diffuse Diffuse … Eye (L D+ E)

C l bl diColor bleeding

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Image Synthesis Global Illumination

M. Ben-Chen, A. VaxmanComputer Science Dept. Technion Page 13

Caustics

Light Specular … Specular Diffuse Eye (L S+ D E)

R l ldReal world

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Rendered