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Illumination (Lighting)

Illumination (Lighting). Outline. Light Source Basic Illumination Models Shading Methods Ray-Tracing Methods Radiosity Lighting Model Shadow. Introduction. Realistic displays of a scene Perspective projections of objects Applying lighting effects. Introduction (Contd.).

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Illumination (Lighting)

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  1. Illumination (Lighting)

  2. Outline • Light Source • Basic Illumination Models • Shading Methods • Ray-Tracing Methods • Radiosity Lighting Model • Shadow

  3. Introduction • Realistic displays of a scene • Perspective projections of objects • Applying lighting effects

  4. Introduction (Contd.) • Illumination model • Lighting model or Shading model • Calculate the intensity of light for a given point on the surface of an object • Surface-Rendering algorithm • Use the intensity of a given point to determine the light intensity for all projected pixel position in a polygon

  5. (a) (b) Light Sources • Types of light source • light source (direct) • light reflector (indirect) • Two light emitter models • Point light source, see (b) • Distributed (Area) light source, see (a)

  6. - Position - Orientation - Material - Light source - Viewer L N V Illumination Models • Concerning methods for calculating light intensity • Also called Lighting Models • An approximation for physical optical laws

  7. Types of illumination models • Local Illumination Models • Only considering the interchanges of the light sources • Global Illumination Models • Concerning the interchange of light between all surfaces • Ray-Tracing, Light as a particle • Radiosity, Light as a energy

  8. D L T.L. S Phong Model • Phong in 1975 • The standard model that compromises between acceptable results and processing cost • Light source • A point light source • Light interactions with a solid: • incident light at a surface= light reflected +light scattered+ light absorbed+ light transmitted • Models reflected light as reflected light = ambient+ diffuse+ specular

  9. Ambient Light • Equally on all surfaces from all directions. • Results from multiple reflections of light from the many surfaces in the environment • Our illumination equation becomes I = IaKa Ia : intensity of the ambient light( to be constant) • Ka :ambient-reflection coefficient ( 0 ~ 1) • depends on object’s material • be empirical convenience

  10. N V L Diffuse Reflection • Dull and matte surfaces exhibit diffuse reflection • Equally bright from all viewing angles • The intensity on a given surface depends on the angle  between the light's directionL and surface's normalN I= IaKa+ IpKdcos • Ip : Intensity of point light source • Kd : diffuse-reflection coefficient (0 ~ 1) • cos : max( cos , 0)

  11. Total Diffuse Reflection • Assuming that N and L have been normalized, • I= IaKa+ IpKd(N dotL) Lis a constant if a point light source is at infinite (Called directional light source)

  12. Kd, with Ka= 0 0.0 0.2 0.4 0.6 0.8 1.0 Visual Effects of Different Values of Kd

  13. Visual effects of different values of Kd and Ka

  14. , where Diffuse Reflection - Further Discussions • Light-source attenuation • Colored lights and surfaces • Similar for IG and IB.

  15. N N L L R      V R Specular Reflection • Specular reflection can be observed on any shiny surface • The highlight presents the incident light n vary from 1 to ?00, it depends on surface material being simulated

  16. H N  L R   V  Specular Reflection • In Phong model does a good job of modeling specular reflection from plastic surfaces • Reduce the calculation of the reflection vector • where H= (L+ V)/| L+ V|

  17. Visual Effects of Specular Reflections Ks 1.0 0.5 8 16 32 64 128 Shinness

  18. Visual Effects of Combined Reflections See Figure 10-23

  19. L  x cones flaps Improving the Point-Light-Source Model • Models the directionality of the lights • Warn 83 • Multiple light sources

  20. Summary of the Phong Model • Light sources are assumed to be point sources • Light sources and viewer are located at infinity • Only the normal vector of a surface needs to be computed • The diffuse and specular terms are modeled as localcomponents • The color of the specularreflection is assumed to be that of the light source • ks is set to be a constant value independent of the surface color • The globalterm(ambient) is modeled as a constant • Drawback: • Gives the impression of colored plasticsurface

  21. Illumination Models and Surface-Rendering Methods • Light Source • Basic Illumination Models • Shading Methods • Ray-Tracing Methods • Radiosity Lighting Model • Shadow

  22. Normal Light View Shading Models for Polygons • Constant Shading

  23. Constant-Intensity Shading • Also called flat shading • A single intensity is calculated for each polygon • Advantage • Quickly displayed with same intensity value • If a polyhedron is not an approximation of an object with a curved surface, flat shading provides an accurate rendering • Disadvantage • Not accurate rendering for an object

  24. y I1 Ia Ib Ip I2 I3 Gouraud Shading • Also called Intensity-interpolation shading • Introduce Mach bands

  25. Phong Shading • Normal-vectorInterpolation Shading • The rendered image is more acceptable • Increase the cost of shading

  26. Gouraud Shaded Polygons with Diffuse Reflection

  27. Gouraud Shaded Polygons with Diffuse and Specular Reflection

  28. Phong Shaded Polygons with specular Reflection

  29. Curved Surfaces with Specular Reflection

  30. Viewing transformation Modeling transformation Modeling Rasterization (including shading) Clipping Display The Rendering Pipeline • For Z-buffer and Phong shading

  31. Demo and Trace Demo Program • DiffuseDemo • Diffuse.fx -> DiffuseVS() • AmbientDiffuseDemo • AmbientDiffuse.fx -> AmbientDiffuseVS() • AmbientDiffuseSpecularDemo • ambientdiffusespec.fx -> AmbientDiffuseSpecVS()

  32. A B B D A C D C Problems with Interpolated Shading • Polygonal silhouette • Improve by breaking the surface into a greater number of smaller polygons • Orientation dependence • Solved by decomposing polygons • Problems at shared vertices • Unrepresentative vertex normals A C B

  33. Normal Vectors • To get the normal vector for a vertex • For a differentiable surface, use calculus to find • For not differentiable, use vertex normal averaging Averaging the four polygon face normals

  34. R View N L Local R View T N L Global Global Illumination

  35. Global Illumination Models • Recursive Ray-Tracing • Light regards as particle • Sharpening renderer, and view dependent • Radiosity • Light regards as energy • Smoothy renderer, and view independent

  36. Ray-tracing

  37. Purple Hall

  38. Room with Metal and Glass Sphere

  39. Subdivisions on Radiosity Computation

  40. Figure of Radiosity

  41. Figure of Radiosity (Contd.)

  42. Figure of Radiosity(Contd.)

  43. Illumination Models and Surface-Rendering Methods • Light Source • Basic Illumination Models • Shading Methods • Ray-Tracing Methods • Radiosity Lighting Model • Shadow

  44. Shadows • Shadow algorithms determine which surfaces can be seen from light source. • In multiple light sources, we have problems. • umbra and penumbra. • Illumination equation involving shadows

  45. Hard Shadows

  46. Soft Shadows

  47. Remarks • Light Source • Basic Illumination Models • Shading Methods • Ray-Tracing Methods • Radiosity Lighting Model • Shadow

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