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# Chapter 6. Color & Shading - PowerPoint PPT Presentation

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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Presentation Transcript

• The spectrum (energy) of light source.

• The spectral reflectance of the object surface.

• The spectral sensitivity of the sensor.

Light

Eyes

Object

• Luminance  Lightness

• Chrominance  Color

Human eye is more sensitive to luminance than to chrominance

• 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

r+g+b=1

• TV transmission  digital space  YCBCR

•  analog space  YIQ (NTSC)

•  YUV (PAL)

• 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

• It is common to smoothing the histogram before matching

•  to adapt minor shifts of the reflectance spectrum.

A plot of pixels (r,g) taken from different images containing faces.

(r,g) : normalized red and green values

• 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.

• 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)

• 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.