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# Chapter 6. Color & Shading - PowerPoint PPT Presentation

Chapter 6. Color &amp; 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
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
• Chrominance  Color

Human eye is more sensitive to luminance than to chrominance

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
YIQ Model
• TV transmission  digital space  YCBCR
•  analog space  YIQ (NTSC)
•  YUV (PAL)
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
Similarity measure for histogram matching
• It is common to smoothing the histogram before matching
•  to adapt minor shifts of the reflectance spectrum.
Color Segmentation

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

(r,g) : normalized red and green values

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.
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)
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.