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Colors

Colors. Color Systems. In computer graphics, we use RGB colors. But… Can it represent all colors? Is it linear? For example, (1.0, 1.0, 1.0) is white (1.0, 0.0, 0.0) is red Is (1.0, 0.5, 0.5) half white and half red?

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Colors

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  1. Colors

  2. Color Systems • In computer graphics, we use RGB colors. But… • Can it represent all colors? • Is it linear? For example, • (1.0, 1.0, 1.0) is white • (1.0, 0.0, 0.0) is red • Is (1.0, 0.5, 0.5) half white and half red? • Does the color (r, g, b)*0.5 look like the color (r, g, b) in half intensity?

  3. What is a Color, After All? • We may define a color by its wavelength. • However, most colors have energy spread in every wavelength. Figure 5.1 in Pharr’s book: • Fluorescent light • Lemon skin

  4. What is a Color (II) • What is more interesting is that different energy distributions may be perceived as the same color!

  5. Additive vs. Subtractive Color

  6. The CIE Color Matching

  7. CIE XYZ Space • To get rid of the negative values, CIE defined 3 new hypothetical light sources .

  8. Color and Spectrum in PBRT • XYZ color: • Spectrum class in PBRT: Class COREDLL spectrum { public: private: float c[COLOR_SAMPLES]; }

  9. Linearity • Unfortunately equal steps in the XYZ space does not produce perceptually equal steps in the color.

  10. CIE L*u*v Space • Designed to be perceptually uniform.

  11. Other Color Space • HSV: hue, saturation, value. • HSL: hue, saturation, lightness. From: http://en.wikipedia.org/wiki/File:Hsl-hsv_models.svg

  12. Gamma Correction • For a monitor, the light intensity follows an exponential curve such as:

  13. How to Determine the Gamma? • How to detect the gamma of your monitor? Compare it with dithering: (Hint: how do you produce a square with 50% gray on the screen?)

  14. Basic Radiometry

  15. What Do You Mean By Light Intensity? • What does I in rendering equation mean? • The power of light source • E.g., wattage of a light bulb. • Flux (Φ) measured in watts (W) or joules/second • Does it change with distance? • Another radiometric quantity needed here.

  16. Radiance and Irradiance • See Pharr’s 5.4.1 (2nd Ed.) • Irradiance E • Area density of flux. • Measured in W/m2 • E = Φ / 4πr2 From Watt’s p.278

  17. Radiance and Irradiance • Intensity and solid angle • Radiance L • Light energy density • Measured in W/(sr-m2) • Remains constant along a ray From Watt’s p.278

  18. Analogy to River Flow • How do you compare the water flow of two rivers? • How do you measure the intake of a irrigation canal attached to the river?

  19. Exercise (Sanity Check) • Q1: What do you mean when you say object A is “brighter” than object B? Does it refer to intensity, radiance, or irradiance?

  20. High Dynamic Range (HDR) Images

  21. What is Dynamic Range? • What does the brightness (or darkness) mean in a photograph?

  22. An Example

  23. Recovering High Dynamic Range Radiance Maps from Photographs • By Paul Debevec (SIGGRAPH 1997) • Step 1: Recovering the film response curve. • Step 2: Recovering the radiance map given the response curve.

  24. Recovering the Response Curve

  25. File Format • RGBE (.hdr) format that is used in Greg Ward’s RADIANCE. • OpenEXR: also used in Pharr’s PBRT • For more information, see: • http://ict.debevec.org/~debevec/Research/HDR/ • http://en.wikipedia.org/wiki/High-dynamic-range_imaging

  26. Tone Mapping • Converting HDR intensity to displayable range • In general: [0, infinity) to [0, 1) • Global – spatially uniform • Local – spatially varying in each pixel according to the local features of the image.

  27. Creative CommonsAttribution-Share Alike 3.0 Unported license. Source: http://en.wikipedia.org/wiki/File:Dundus_Square.jpg

  28. Creative CommonsAttribution-Share Alike 3.0 Unported license. Source: http://upload.wikimedia.org/wikipedia/commons/b/b3/HDRI-Example.jpg

  29. Tone Mapping In Digital Photography Creative CommonsAttribution 2.0 Generic license. Source: http://en.wikipedia.org/wiki/File:Dundus_Square.jpg

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