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Illumination Model

Illumination Model. 고려대학교 컴퓨터 그래픽스 연구실. Illumination. How do We Compute Radiance for a Sample Ray? Must derive computer models for ... Emission at light sources Scattering at surfaces Reception at the camera. Wireframe. Without Illumination. Direct Illumination. Overview.

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Illumination Model

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  1. Illumination Model 고려대학교 컴퓨터 그래픽스 연구실 cgvr.korea.ac.kr

  2. Illumination • How do We Compute Radiance for a Sample Ray? • Must derive computer models for ... • Emission at light sources • Scattering at surfaces • Reception at the camera Wireframe Without Illumination Direct Illumination cgvr.korea.ac.kr

  3. Overview • Direct Illumination • Emission at light sources • Scattering at surfaces • Global Illumination • Shadows • Refractions • Inter-object reflections Direct Illumination cgvr.korea.ac.kr

  4. Overview • Direct Illumination • Emission at light sources • Scattering at surfaces • Global Illumination • Shadows • Refractions • Inter-object reflections Direct Illumination cgvr.korea.ac.kr

  5. Modeling Light Source • IL(x,y,z,q,f,l) • Describes the intensity of energy, • Leaving a light source • Arriving at location(x,y,z) • From direction (q,f) • With wavelength l cgvr.korea.ac.kr

  6. Empirical Model • Ideally Measure Irradiant Energy for “All” Situations • Too much storage • Difficult in practice cgvr.korea.ac.kr

  7. Light Source Model • Simple Mathematical Models: • Point light • Directional light • Spot light cgvr.korea.ac.kr

  8. Point Light Source • Models Omni-Directional Point Source (E.g., Bulb) • Intensity (I0) • Position (px, py, pz) • Factors (kc, kl, kq) for attenuation with distance (d) cgvr.korea.ac.kr

  9. Directional Light Source • Models Point Light Source at Infinity (E.g., Sun) • Intensity (I0) • Direction (dx,dy,dz) No attenuation with distance cgvr.korea.ac.kr

  10. Spot Light Source • Models Point Light Source with Direction (E.g., Luxo) • Intensity (I0), • Position (px, py, pz) • Direction (dx, dy, dz) • Attenuation cgvr.korea.ac.kr

  11. Overview • Direct Illumination • Emission at light sources • Scattering at surfaces • Global Illumination • Shadows • Refractions • Inter-object reflections Direct Illumination cgvr.korea.ac.kr

  12. Modeling Surface Reflection • Rs(q,f,g,y,l) • Describes the amount of incident energy • Arriving from direction (q,f) • Leaving in direction (g,y) • With wavelength l cgvr.korea.ac.kr

  13. Empirical Model • Ideally Measure Radiant Energy for “All” Combinations of Incident Angles • Too much storage • Difficult in practice cgvr.korea.ac.kr

  14. Reflectance Model • Simple Analytic Model: • Diffuse reflection + • Specular reflection + • Emission + • “Ambient” Based on model proposed by Phong cgvr.korea.ac.kr

  15. Reflectance Model • Simple Analytic Model: • Diffuse reflection + • Specular reflection + • Emission + • “Ambient” Based on model proposed by Phong cgvr.korea.ac.kr

  16. Diffuse Reflection • Assume Surface Reflects Equally in All Directions • Examples: chalk, clay cgvr.korea.ac.kr

  17. Diffuse Reflection • How Much Light is Reflected? • Depends on angle of incident light • dL = dAcos Q cgvr.korea.ac.kr

  18. Diffuse Reflection • Lambertian Model • Cosine law (dot product) cgvr.korea.ac.kr

  19. Reflectance Model • Simple Analytic Model: • Diffuse reflection + • Specular reflection + • Emission + • “Ambient” Based on model proposed by Phong cgvr.korea.ac.kr

  20. Specular Reflection • Reflection is Strongest Near Mirror Angle • Examples: mirrors, metals cgvr.korea.ac.kr

  21. Specular Reflection • How Much Light is Seen? • Depends on angle of incident light and angle to viewer cgvr.korea.ac.kr

  22. Specular Reflection • Phong Model • {cos(a)}n cgvr.korea.ac.kr

  23. Reflectance Model • Simple Analytic Model: • Diffuse reflection + • Specular reflection + • Emission + • “Ambient” Based on model proposed by Phong cgvr.korea.ac.kr

  24. Emission • Represents Light Emitting Directly From Polygon Emission ≠ 0 cgvr.korea.ac.kr

  25. Reflectance Model • Simple Analytic Model: • Diffuse reflection + • Specular reflection + • Emission + • “Ambient” Based on model proposed by Phong cgvr.korea.ac.kr

  26. Ambient Term • Represents Reflection of All Indirect Illumination This is a total hack (avoids complexity of global illumination)! cgvr.korea.ac.kr

  27. Reflectance Model • Simple Analytic Model: • Diffuse reflection + • Specular reflection + • Emission + • “Ambient” cgvr.korea.ac.kr

  28. Reflectance Model • Simple Analytic Model: • Diffuse reflection + • Specular reflection + • Emission + • “Ambient” cgvr.korea.ac.kr

  29. Reflectance Model • Sum Diffuse, Specular, Emission, and Ambient cgvr.korea.ac.kr

  30. Surface Illumination Calculation • Single Light Source: cgvr.korea.ac.kr

  31. Surface Illumination Calculation • Multiple Light Sources: cgvr.korea.ac.kr

  32. Overview • Direct Illumination • Emission at light sources • Scattering at surfaces • Global Illumination • Shadows • Refractions • Inter-object reflections Global Illumination cgvr.korea.ac.kr

  33. Global Illumination cgvr.korea.ac.kr

  34. Shadows • Shadow Terms Tell Which Light Sources are Blocked • Cast ray towards each light source Li • Si = 0 if ray is blocked, Si = 1 otherwise Shadow Term cgvr.korea.ac.kr

  35. Ray Casting • Trace Primary Rays from Camera • Direct illumination from unblocked lights only cgvr.korea.ac.kr

  36. Recursive Ray Tracing • Also Trace Secondary Rays from Hit Surfaces • Global illumination from mirror reflection and transparency cgvr.korea.ac.kr

  37. Mirror Reflection • Trace Secondary Ray in Direction of Mirror Reflection • Evaluate radiance along secondary ray and include it into illumination model Radiance for mirror reflection ray cgvr.korea.ac.kr

  38. Transparency • Trace Secondary Ray in Direction of Refraction • Evaluate radiance along secondary ray and include it into illumination model Radiance for refraction ray cgvr.korea.ac.kr

  39. Transparency • Transparency coefficient is fraction transmitted • KT = 1 if object is translucent, KT = 0 if object is opaque • 0 < KT < 1 if object is semi-translucent Transparency Coefficient cgvr.korea.ac.kr

  40. Refractive Transparency • For Thin Surfaces, Can Ignore Change in Direction • Assume light travels straight through surface cgvr.korea.ac.kr

  41. Refractive Transparency • For Solid Objects, Apply Snell’s Law: • hrsinQr = hisin Qi cgvr.korea.ac.kr

  42. Summary • Direct Illumination • Ray casting • Usually use simple analytic approximations for light source emission and surface reflectance • Global illumination • Recursive ray tracing • Incorporate shadows, mirror reflections, and pure refractions cgvr.korea.ac.kr

  43. Illumination Terminology • Radiant power [flux] (Φ) • Rate at which light energy is transmitted (in Watts). • Radiant Intensity (I) • Power radiated onto a unit solid angle in direction( in Watt/sr) • e.g.: energy distribution of a light source (inverse square law) • Radiance (L) • Radiant intensity per unit projected surface area( in Watts/m2sr) • e.g.: light carried by a single ray (no inverse square law) • Irradianc (E) • Incident flux density on a locally planar area (in Watts/m2 ) • Radiosity (B) • Exitant flux density from a locally planar area ( in Watts/m2 ) cgvr.korea.ac.kr

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