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Advanced Lighting and Shading

Advanced Lighting and Shading. 2002. 10. 14. Radiometry and Photometry : Definition. Radiometry Radiometry deal with the measurement of radiation throughout the electromagnetic spectrum

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Advanced Lighting and Shading

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  1. 컴퓨터 그래픽스 2002-2 Advanced Lighting and Shading 2002. 10. 14

  2. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry : Definition • Radiometry • Radiometry deal with the measurement of radiation throughout the electromagnetic spectrum • This range includes the infrared(적외선), visible(가시선), and ultraviolet(자외선) regions of the electromagnetic spectrum • wavelength from 1000 to 0.01 micrometer (=10-6 meter =10-3 millimeter) • Photometry • Photometry is like radiometry except that it weights everything by the sensitivity of the human eye • deals with only the visible spectrum (=visible band) • a wavelength range of about 380 to 780 nanometer (=10-9 meter) • do not deal with the perception of color itself, but rather the perceived strength of various wavelengths

  3. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry: Difference • Conversion & difference • The result of radiometric computations are converted to photometric units by multiplying by the CIE photometirc curve • The conversion curve and the units of measurement are the only difference between the theory of photometry and the theory of radiometry (Figure 6.1)

  4. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry: Radiometry (1/6) • Radiometry • radiant energy(Q) • basic unit of energy, measured joules(J) • the number of photon per joule : • radiant flux P or (= radiant power) of a light source • the number of joules per second emitted (=watt(W))

  5. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry: Radiometry (2/6) • radiant flux density • After photons leave a light source, the next step is to measure how they arrive at a surface • the radiant flux per unit area on a surface (=watts per square meter) • irradiance E (= radiant flux density) • when flux arrives at a surface • radiant exitance M (= radiosity B) • the amount of flux leaving a surface

  6. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry: Radiometry (3/6) • Solid angle (Figure 6.2) • the concept of a two-dimensional angle extended to three dimensions • measured in steradians (sr) • 4 steradians would cover the whole area of the unit sphere • radiance L • the most important radiometric unit for computer graphics • radiance is what we store in a pixel • The amount of radiant flux coming from that direction and hitting the surface at a given point

  7. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry: Radiometry (4/6) • incoming radiance at a surface • defined as the amount of power (watt) per unit area, per unit solid angle • surface independent form of the radiance equation

  8. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry: Radiometry (5/6) • radiance distribution • The radiance in an environment can be thought of as a function of five {six} variables • a location (three), direction (two) { + wavelength } • an environment map of everything in the scene represents the incoming radiance for all directions • Image based rendering • lightfield, plenoptic function

  9. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry: Radiometry (6/6) • An object's radiance value is not affected by distance • a surface will have the same luminance regardless of its distance from viewer • this seems in contradiction to the basic law that a light's intensity drops off with the square of the distance. • only number of pixels that changes in relationship to the distance from the light • radiance remains constant

  10. 컴퓨터 그래픽스 2002-2 Radiometry and Photometry: Photometry • Photometry • luminous energy (talbots)  radiant energy (joules) • lumen (lm)  the watt (W) (radiant flux의 측정단위) • Illumination (the luminous flux density)  irradiance • luminance  radiance • candela (cd) a measure of luminous power per solid angle • lux (lx) lumens(=luminous power) per square meter • candelas per square meter (nit)

  11. 컴퓨터 그래픽스 2002-2 Colorimetry: Definition (1/2) • Colorimetry • Light is perceived in the visible band • from 380 to 780 nm • distribution of wavelengths (light's spectrum) • Human  distinguish 10 million different colors • three different types of cone receptors in the retina • Standard condition for measuring color (CIE) Figure 6.4

  12. 컴퓨터 그래픽스 2002-2 Colorimetry: Definition (2/2)

  13. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (1/12) • Color Matching (Color Models) • RGB Color Model (Figure 6.5) • Primary colors: RED, GREEN, BLUE. • Secondary colors: YELLOW = red + green, CYAN = green + blue, MAGENTA = blue + red. • WHITE = red + green + blue. • BLACK = no light. • Disadv • cannot directly represent all visible colors (negative weights)

  14. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (2/12) • Greyscale • BLACK = 0% brightness, 100% grey. • WHITE = 100% brightness, 0% grey. • NTSC phosphors (older) • Y=0.30R+0.59G+0.11B • CRT and HDTV phosphors (modern) • Y=0.2125R+0.7154G+0.0721B

  15. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (3/12) • CIE XYZ Color Model (Figure 6.6)

  16. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (4/12) • chromaticity diagram • curved line  color of the spectrum • purple line  line connecting the ends of the spectrum • white point  x=y=z=1/3 • Saturation  The relative distance of the color point compared to the distance to the edge of the region • Hue  the point on the region edge

  17. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (5/12) • gamut

  18. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (6/12) • Disadvantage • the 2D diagram failed to give a uniformly-spaced visual representation of what is actually a three-dimensional color space

  19. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (7/12) • CIE LUV CIE LUV CIE LU’V’

  20. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (8/12) • CIE LAB • retinal color stimuli are translated into distinctions • between light and dark • between red and green • between blue and yellow. • CIELAB indicates these values with three axes: L*, a*, and b*.

  21. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (9/12) • HSV (=HSB) • Hue, Saturation, Value (=Brightness) • HUE is the actual color. • measured in angular degrees around the cone • red = 0 or 360 (so yellow = 60, green = 120, etc.). • SATURATION is the purity of the color • measured in percent from the center of the cone (0) to the surface (100). • At 0% saturation, hue is meaningless. • BRIGHTNESS • measured in percent from black (0) to white (100). • At 0% brightness, both hue and saturation are meaningless.

  22. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (10/12) • HLS • Hue, Lightness, Saturation • is similar to the HSV cone • but with the primary colors located at L = 0.5 and with the colors of black and white acting as ends of the cones.

  23. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (11/12) • CMYK • Primary colors: CYAN, MAGENTA, and YELLOW. • Secondary colors: BLUE = cyan + magenta, RED = magenta + yellow, GREEN = yellow + cyan. • BLACK = cyan + magenta + yellow (in theory). • BLACK (K) INK is used in addition to C,M,Y to produce solid black. • WHITE = no color (on white paper, of course). • Standard Color Printer

  24. 컴퓨터 그래픽스 2002-2 Colorimetry: Color Model (12/12) • YIQ • Used by US commercial color television broadcasting (Used by NTSC standard) • Y: encodes luminance • I, Q: encode color (chromaticity) • For black and white TV, only the Y channel is used • People are more sensitive to the illuminance difference • We can use more bits (bandwidth) to encode Y and less bits to encode I and Q

  25. 컴퓨터 그래픽스 2002-2 BRDF Theory: Definition (1/2) • BRDF • Bidirectional Reflectance Distribution Function • Describe how lights reflected from a surface (Material properties) • Input • incoming and outgoing azimuth and elevation angles, wavelength of incoming light (Hue and saturation remain constant) • The relative amount of energy reflected in the outgoing direction, given the incoming direction

  26. 컴퓨터 그래픽스 2002-2 BRDF Theory: Definition (2/2) • Helmholtz reciprocity • I/O angles can be switched and the function alue will be same • Normalization • Total amount of out going energy must always be less than or equal to incoming energy • It is an approximation of BSSRDF • Bidirectional Surface Scattering Reflectance Distribution Function • Include the scattering of light within the surface • position change • Adding incoming and outgoing locations as inputs • Travel along the incoming direction • From one point to the other of the surface • Along the outgoing direction

  27. 컴퓨터 그래픽스 2002-2 BRDF Theory: Reflectance equation (1/2) • Reflectance equation • Given a BRDF and an incoming radiance distribution • The outgoing radiance for a given viewing direction • Integrating the incoming radiance from all directions on the hemisphere above surface • Determine the incoming radiance • Multiply it by BRDF for this direction and the outgoing direction • Scale by the incoming angle to the surface • integrate

  28. 컴퓨터 그래픽스 2002-2 BRDF Theory: Reflectance equation (1/2) • Single Point light source • Simplify the notation • Replace the azimuth and elevation • The cosine term for light using the surface normal n • More than one light, computed and summed together • For diffuse surface, The BRDF is trivial

  29. 컴퓨터 그래픽스 2002-2 BRDF Theory: Theoretical models of BRDF (1/3) • Theoretical models of BRDF • How surfaces behave • Microfacets • Tiny, flat mirror on the surface, with random size and angle • Gaussian distribution of sizes and angles • Specular reflection • Direct reflections from some micro facets • Diffuse reflection • Interreflection off several facets, scattering with in the surface material itself (shadow, mask) • Height correlation • Microfacets have size near the wavelength of the light • Diffraction can be simulated

  30. 컴퓨터 그래픽스 2002-2 BRDF Theory: Theoretical models of BRDF (2/3) • Fresnel reflectance (Figure 6.11) • Importance for non-conductive or dielectric, matrials such as plastic, glass, and water • All materials become fully reflective at the shallowest grazing angle • Describes the reflectance of a given surface at various angles conductive dielectric

  31. 컴퓨터 그래픽스 2002-2 BRDF Theory: Theoretical models of BRDF (3/3) • Limitation of BRDF theoretical model • Do not account for anisotropy • If the viewer and the light source do not move and a flat sample of the material changes its appearance when it is rotated about its normal • Brushed metal, vanished wood, woven cloth, fur, hair • Anisotropic BRDF • Both fiand fo are needed to evaluate the BRDF (four angles) • Isotropic BRDF • Relative angle f = fi -fo (three angles) • Not necessarily useful for representing some given material sample

  32. 컴퓨터 그래픽스 2002-2 BRDF Theory: Another approach (1/3) • Another approach to represent BRDF • Acquire BRDF data from the actual surface • With basis summation techniques • Capture the BRDF’s surface as the weighted sum of a set of functions • Phong/Blinn lighting model represent BRDF by just two functions • A diffuse component and a specular lobe

  33. 컴퓨터 그래픽스 2002-2 Implementing BRDFs • Implementing BRDFs • To save on computation • To store data for a large number of elevations and azimuths • Memory intensive • Care needs to be taken, as measured data is usually noisy and have gaps in the set • Compact representation • Advantages • Avoid the evaluation costs for precise theoretical models • Avoid storage requirements and noisiness of acquired datasets • Two approaches • Factorization • Environment map filtering

  34. 컴퓨터 그래픽스 2002-2 Implementing BRDFs: Factorization (1/2) • Factorization • Convert a BRDF into a set of pairs of 2D textures • One texture is accessed by the incoming direction • The other by outgoing (Figure 6.13)

  35. 컴퓨터 그래픽스 2002-2 Implementing BRDFs: Factorization (2/2) • Forming the texture pairs • The incoming and outgoing direction vectors are evaluated at each vertex of the model • Two pairs of texture coordinates are then used generated using the same reparameterization • These texture coordinates are then used to access the textures on the surface • Multiply the two resulting pixel colors together • Successive pairs of textures are multiplied in the same fashion and added to the final pixel color. • Limitation • At least two texture accesses are needed for every light source in the scene • Only point and directional light sources can be used

  36. 컴퓨터 그래픽스 2002-2 Implementing BRDFs: Environment Mapping Filtering (1/3) • Environment Mapping Filtering • Environment map • Render a perfectly shiny surface • Extended to glossy and diffuse surfaces (reflection map) • Fuzzy reflection • Uniformly blur the EM • Use the phong specular equation • Weighted contribution

  37. 컴퓨터 그래픽스 2002-2 Implementing BRDFs: Environment Mapping Filtering (2/3) • Irradiance environment map • Gives the diffuse and ambient lighting for that direction • Lumped into a single ambient term (Phong lighting model) • Quickly and accurately • Two ways to store and access lighting • Sphere map • Photograph a sphere painted with flat white paint • valid for only one eye direction • Cube map • Advantage • Eliminating per vertex lighting calculation from pipeline • Limitations • Lights and reflected object are distant • Do not change with location of objected viewed

  38. 컴퓨터 그래픽스 2002-2 Implementing BRDFs: Environment Mapping Filtering (3/3) • Problem with environment mapping • The dynamic range of the light captured is usually limited to 8 bits per color channel • Not enough to simultaneously capture the full range of incident illumination • Solution • High Dynamic Range Image (HDRI) • Reference HDR Shop

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