Basic Principles of Surface Reflectance Thanks to Shree Nayar Ravi Ramamoorthi Pat Hanrahan  Surface Appearance Image intensities   normal surface reflectance illumination  Surface Reflection depends
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Basic Principles of Surface Reflectance Thanks to Shree Nayar Ravi Ramamoorthi Pat Hanrahan Surface Appearance Image intensities normal surface reflectance illumination Surface Reflection depends

source sensor surface element normal brPage 3br BRDF Bidirectional Reflectance Distribution Function T I source viewing direction surface element normal incident direction I T I T surface I T surface I T Irradiance at Surface in direction I T Radian

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Basic Principles of Surface Reflectance Thanks to Shree Nayar Ravi Ramamoorthi Pat Hanrahan Surface Appearance Image intensities normal surface reflectance illumination Surface Reflection depends




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Presentation on theme: "Basic Principles of Surface Reflectance Thanks to Shree Nayar Ravi Ramamoorthi Pat Hanrahan Surface Appearance Image intensities normal surface reflectance illumination Surface Reflection depends"— Presentation transcript:


Page 1
Basic Principles of Surface Reflectance Thanks to Shree Nayar, Ravi Ramamoorthi, Pat Hanrahan
Page 2
Surface Appearance Image intensities = ( normal, surface reflectance, illumination ) Surface Reflection depends on both the viewing and illumination direction. source sensor surface element normal
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BRDF: Bidirectional Reflectance Distribution Function T I source viewing direction surface element normal incident direction I T I T surface I T surface I T Irradiance at Surface in direction I T Radiance of Surface in direction I T BRDF : surface surface I

T I T I T I T
Page 4
Important Properties of BRDFs T I source viewing direction surface element normal incident direction I T I T BRDF is only a function of 3 variables : I T I T I T I T Rotational Symmetry: Appearance does not change when surface is rotated about the normal. I I T T Helmholtz Reciprocity: (follows from 2 nd Law of Thermodynamics) Appearance does not change when source and viewing directions are swapped.
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Mechanisms of Surface Reflection source surface reflection surface incident direction body reflection Body Reflection: Diffuse Reflection Matte

Appearance Non Homogeneous Medium Clay, paper, etc Surface Reflection: Specular Reflection Glossy Appearance Highlights Dominant for Metals Image Intensity = Body Reflection + Surface Reflection
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Mechanisms of Surface Reflection Body Reflection: Diffuse Reflection Matte Appearance Non Homogeneous Medium Clay, paper, etc Surface Reflection: Specular Reflection Glossy Appearance Highlights Dominant for Metals Many materials exhibit both Reflections:
Page 7
Diffuse Reflection and Lambertian BRDF
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Diffuse Reflection and Lambertian BRDF viewing direction

surface element normal incident direction T S U I T I T Lambertian BRDF is simply a constant : albedo Surface appears equally bright from ALL directions! (independent of ) Surface Radiance : Commonly used in Vision and Graphics! cos S U T S U source intensity source intensity
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Rendered Sphere with Lambertian BRDF Edges are dark (N.S = 0) when lit head on See shading effects clearly.
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White out Conditions from an Overcast Sky CAN perceive shape in regions lit by the street lamp!! WHY?
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Specular Reflection and Mirror BRDF source intensity viewing

direction surface element normal incident direction specular/mirror direction I T I T I T Mirror BRDF is simply a double delta function : Very smooth surface. All incident light energy reflected in a SINGLE direction. (only when = ) Surface Radiance : I S I G T T G U I S I G T T G U I T I T specular albedo
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Delta Function too harsh a BRDF model (valid only for highly polished mirrors and metals). Many glossy surfaces show broader highlights in addition to mirror reflection. Surfaces are not perfectly smooth they show micro surface geometry (roughness). Example Models : Phong

model Torrance Sparrow model Glossy Surfaces
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Blurred Highlights and Surface Roughness Roughness
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Phong Model: An Empirical Approximation How to model the angular falloff of highlights: Phong Model Blinn Phong Model Sort of works, easy to compute But not physically based (no energy conservation and reciprocity). Very commonly used in computer graphics. shiny U shiny U
Page 15
Phong Examples These spheres illustrate the Phong model as lighting direction and shiny are varied:
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Those Were the Days synthetic images, we do not expect to

be able to display the object exactly as it would appear in reality, with texture, overcast shadows, etc. We hope only to display an image that approximates the real object closely enough to Bui Tuong Phong, 1975
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All components of Surface Reflection
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Reflections on water surfaces Glittering
Page 21
Split off specular Reflections in Woven Surfaces
Page 22
Why does the Full Moon have a flat appearance? The moon appears matte (or diffuse) But still, edges of the moon look bright (not close to zero) when illuminated by
Page 23
Why

does the Full Moon have a flat appearance? Lambertian Spheres and Moon Photos illuminated similarly
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Surface Roughness Causes Flat Appearance Actual Vase Lambertian Vase
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Surface Roughness Causes Flat Appearance Increasing surface roughness Lambertian model Valid for only SMOOTH MATTE surfaces. Bad for ROUGH MATTE surfaces.
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Oren Nayar Model Main Points Physically Based Model for Diffuse Reflection. Based on Geometric Optics. Explains view dependent appearance in Matte Surfaces Take into account partial interreflections. Roughness represented

like in Torrance Sparrow Model Lambertian model is simply an extreme case with roughness equal to zero.
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A Simple Reflection Model Dichromatic Reflection Observed Image Color = a x Body Color + b x Specular Reflection Color Klinker Shafer Kanade 1988 Color of Source (Specular reflection) Color of Surface (Diffuse/Body Reflection) Does not specify any specific model for Diffuse/specular reflection