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Chapter 6. Color & Shading

Chapter 6. Color & Shading. Perception of objects. Perception of objects. The spectrum (energy) of light source . The spectral reflectance of the object surface . The spectral sensitivity of the sensor. How do we see an object?. Light. Eyes. Object. Luminance  Lightness

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Chapter 6. Color & Shading

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  1. Chapter 6. Color & Shading

  2. Perception of objects

  3. Perception of objects • The spectrum (energy) of light source. • The spectral reflectance of the object surface. • The spectral sensitivity of the sensor.

  4. How do we see an object? Light Eyes Object • Luminance  Lightness • Chrominance  Color Human eye is more sensitive to luminance than to chrominance

  5. Light Spectrum

  6. Chromaticity Diagram

  7. RGB Model

  8. RGB signals from a video camera

  9. RGB Colors • Colors specify: • A mixture of red, green, and blue light • Values between 0.0 (none) and 1.0 (lots) • Color • Red Green Blue • White 1.0 1.0 1.0 • Black 0.0 0.0 0.0 • Yellow 1.0 1.0 0.0 • Magenta 1.0 0.0 1.0 • Cyan 1.0 1.0 0.0

  10. Normalized RGB r+g+b=1

  11. HSI Model

  12. Light vs. Pigment

  13. CMY Model

  14. YIQ Model • TV transmission  digital space  YCBCR •  analog space  YIQ (NTSC) •  YUV (PAL)

  15. YUV & YCBCR Model

  16. TV Broadcast

  17. Color Histogram • Color Histogram are relatively invariant to • Translation • Rotation • Scaling • Simple methods for color histogram construction • Concatenate the higher order two bits of each RGB color code.  64 bins • Compute three separate RGB histograms (4 bits each) and just concatenate them into one.  48 bins

  18. Similarity measure for histogram matching • It is common to smoothing the histogram before matching •  to adapt minor shifts of the reflectance spectrum.

  19. Color Segmentation A plot of pixels (r,g) taken from different images containing faces. (r,g) : normalized red and green values

  20. Face Detection • Face region classification. (R>G>B) • Connected Component Labeling. • Select the largest component as face object assuming there is only one face in the image. • Discard remaining components or merges them with the face object. • Computing the location of eyes and nose.

  21. Shading

  22. Three types of Material Reflection • Diffuse Color of reflected light from diffuse reflection (light scattered randomly) • Ambient Amount of background light the surface reflects • Specular Color of reflected light from specular reflection (light reflected in a regular manner)

  23. Diffuse Reflection

  24. Specular Reflection

  25. Darkening with Distance

  26. Complications • The above models of illumination and reflection are simplified. • Some objects reflect light as well as emit light. For example: light bulbs. • In uncontrolled scenes, such as outdoor scenes, it is much more difficult to account for the different phenomena.

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