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159.235 Graphics & Graphical Programming

159.235 Graphics & Graphical Programming. Lecture 30 - Colour, Physics and Light - Part 2. Radiometry : Radiance. Radiometry is the science of light energy measurement Definition: The radiance (luminanc e) is the power per unit area per unit solid angle. Properties:

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159.235 Graphics & Graphical Programming

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  1. 159.235 Graphics & Graphical Programming Lecture 30 - Colour, Physics and Light - Part 2 Graphics

  2. Radiometry : Radiance Radiometry is the science of light energy measurement Definition: The radiance (luminance) is the power per unit area per unit solid angle. Properties: 1. Fundamental quantity 2. Stays constant along a ray 3. Response of a sensor proportional to radiance Graphics

  3. Radiometry: Irradiance and Radiosity Definition: The irradiance (illuminance) is the power per unit area incident on a surface. Definition: The radiosity (luminosity) is the power per unit area leaving a surface. Graphics

  4. Irradiance: Distant Source Graphics

  5. Irradiance: Point Source • Inverse square law fall off • Still has cosine dependency. Graphics

  6. What does Irradiance look like? Graphics

  7. The Reflection Equation • Linear response • 2. Bidirectional reflectance distribution function (BRDF) defines outgoing radiance for a given incoming irradiance – characteristic property of surface. Graphics

  8. Approximating the BRDF • All illumination models in graphics are approximations to the BRDF for surfaces. • Frequently chosen for their visual effect, and ease of implementation, rather than on physical principles. • BRDF is approximated by reflection functions. • Usually a total hack ! Graphics

  9. Types of Reflection Functions • Ambient. • Ideal Specular • Mirror • Reflection Law • Ideal Diffuse • Matte • Lambert’s Law • Specular • Glossiness and Highlights • Phong and Blinn Models Graphics

  10. Ambient Reflection. • Simplest illumination model. • There is assumed to be global ambient illumination in the scene, Ia • Amount of ambient light reflected from a surface defined by ambient reflection coefficient, ka. • Ambient term is I = Ia.ka • No physical basis whatsoever ! Graphics

  11. Mirror: Ideal Specular Surface Calculation of the reflection vector involves mirroring L about N. Law of Reflection i r r=i Graphics

  12. Matte: Ideal Diffuse Reflection • Dull surfaces such as chalk exhibit diffuse or Lambertian reflection. • Reflect light with equal intensity in all directions. • For a given surface, brightness depends only on the angle between the surface normal and the light source. Graphics

  13. Matte: Ideal Diffuse Reflection • 2 effects to consider : • The amount of light reaching the surface. • Beam intercepts an area dA/ cos  • cos  dependence. • The amount of light seen by the viewer. • Also cos  dependence per unit surface area • BUT amount of surface seen by viewer also has cos  dependence. Ip   dA Graphics

  14. Matte: Ideal Diffuse Reflection Ip The diffuse lighting equation is :   dA Graphics

  15. Matte: Ideal Diffuse Reflection • Diffuse coefficient defined for each surface. • Diffusely lit objects often look harshly lit • Ambient light often added. • Poor physical basis for diffuse reflection. • Internal reflections inside the material etc… Graphics

  16. Specular reflection • Can be observed on a shiny surface, e.g nice red apple lit with white light. • Observe highlights on surface. • Highlight appears as the colour of the light, rather than of the surface. • Highlight appears in the direction of ideal reflection. Now view direction important. • Materials such as waxy apples, shiny plastics have transparent reflective surface. Graphics

  17. The Phong model • Assume specular highlight is at a maximum when  = 0 , and falls off rapidly with larger values of  • Fall-off depends on cosn . • n referred as specular exponent. • For perfect reflector, n is infinite.    Graphics

  18. The Phong model • An alternative formulation uses halfway vector, H • It’s direction is halfway between viewer and light source. • If the surface normal was oriented at H, viewer would see brightest highlights. • Note    , both formulations are approximations.     Graphics

  19. Rough Surface : Microfacet distribution Physical justification for Phong model is that the surface is rough and consists of microfacets which are perfect specular reflectors. Distribution of microfacets determines specular exponent. Graphics

  20. Material Selection Ambient 0.52 Diffuse 0.00 Specular 0.82 Shininess 0.10 Light intensity 0.31 Ambient 0.39 Diffuse 0.46 Specular 0.82 Shininess 0.75 Light intensity 0.52 Graphics

  21. Choosing “shininess” n 0 is dull, infinity is perfect reflector Graphics

  22. Colour, Physics & Light - Summary • Surface reflection specified by BRDF. • BRDF approximated by ambient, diffuse and specular reflection. • Lambertian reflection. • Phong Lighting model • Acknowledgments - thanks to Eric McKenzie, Edinburgh, from whose Graphics Course some of these slides were adapted. Graphics

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