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Dynamic Range Dynamic Range CompressionCompression& Color Constancy& Color Constancy
Dynamic Range Dynamic Range CompressionCompression& Color Constancy& Color Constancy
Democritus University of Thrace Democritus University of Thrace 2006 2006
Democritus University of Thrace Democritus University of Thrace 2006 2006
Dynamic Range: The ratio between the maximum and the minimum tonal values in an image (cd/m2)
Commercial cameras have only a dynamic range of 256:1 (the maximum value is 256 times greater than the minimum value that they can capture)
Scenes with grater dynamic range than 256:1 are not captured correctly (intensities are clipped)
Dynamic RangeDynamic Range
The dynamic range of natural scenes is a lot more than 256:1(object in frond of a backlight)
The camera can capture correctly only the bright or the dark area, but never both of them (underexposure, overexposure)
The Dynamic Range ProblemThe Dynamic Range Problem
Underexposured
(no visible details)
Normaly
exposuredOverexposured
(no visible details)
Normaly
exposured
Images with dynamic range problemImages with dynamic range problem
Our approach –center surroundOur approach –center surround
surround
center
CenterSurround
Every pixel (center) is compared with its neighborhood (surround) and is assigned a new value, in order to maximize the contrast in the dark regions of the image
Surround is the average intensity value of the neighborhood (0-255)
Center is the intensity of the pixel (0-255)
Center-surround transfer functionCenter-surround transfer function
CenterSurround
In a dark image region (surround is small)
When the center is dark
23
85Before:
Surround 18
Center 23
After:
Surround 18
Center 85
18
Center-surround transfer functionCenter-surround transfer function
CenterSurround
In a dark image region (surround is small)
When the center is bright
241
245
Before:
Surround 18
Center 241
After:
Surround 18
Center 245
18
Center-surround transfer functionCenter-surround transfer function
CenterSurround
In a bright image region (surround is high)
When the center is dark
36
36
Before:
Surround 240
Center 36
After:
Surround 240
Center 36240
Center-surround transfer functionCenter-surround transfer function
CenterSurround
In a bright image region (surround is high)
When the center is bright
244
244
Before:
Surround 240
Center 244
After:
Surround 240
Center 244240
ConclusionConclusion
CenterSurround
In a dark image region (shadows or underexposured areas) the value of the pixel is increased relatively its neighborhood, increasing the local contrast
In a bright image region (normally exposured areas) the value of the pixel is unchangeable
ResultsResults
ResultsResults
ResultsResults
ResultsResults
ResultsResults
The unknown illuminant problemThe unknown illuminant problem
whitewhite
whitewhite
whitewhite
The HVS has a degree of color constancy
Images under color illuminantImages under color illuminant
Incandescent lights
Green water Fluorescent light
The specularity problemThe specularity problemSpecularities and direct light sources have a greater intensity than the response to pure white
Which intensity is the response to pure white?
Our approach: estimate the white
response
Our approach: estimate the white
response
ResultsResults
ResultsResults
ResultsResults
ResultsResults
