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Multiscale Retinex Technique with Color Correction Based on the Retinex Theory. Retinex Image Enhancement. Edward Land’86 There exists a discrepancy between the human vision system and the recorded color images. - PowerPoint PPT Presentation
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• Edward Land’86
• There exists a discrepancy between the human vision system and the recorded color images.
• Dynamic range difference results in the loss of the essentials features from the recorded images.
• Improved fidelity of color images to human observation can be obtained by
(a) Computation that combines dynamic range
compression, color constancy and color rendition
(b) Color restoration.
Log
*
Gain/offset
CR
*
*
Log
Log Σ
I(x,y)
F1(x,y)
F2(x,y)
F3(x,y)
Σ
W1
W2
α
W3
+
+
+
+
-
MSRCR
||||||||||||||||
MSR
CRF
• First proposed design of Surround function by E.Land’86 was inverse square spatial surround
F(x,y) = 1/ [1+(r2 + c2)]
• The surround function was later modified in Gaussian form
by Hurlbert’89
F(x,y) = exp(-r2 / c2 )
Where
r- √ x2 + y2 and
c- Surround Space Constant
The Single Scale retinex is given by
Ri (x,y)=log Ii (x,y) – log [F(x,y) * Ii (x,y) ]
Where
F(x,y) = K exp(-r2 / c2 )--- Surround Function c- Scalar value and selection of K is thatr- √ x2 + y2
∫∫ F(x,y) dx dy =1
The multi-scale retinex is represented by
Ri (x,y)= Σ Wn { log Ii (x,y) - log[ F(x,y) * Ii (x,y) ]}
Where
n -- Scaling Factor
Wn – Weights (1/3 for each color channel of RGB)
N
n=1
http://dragon.larc.nasa.gov/pub/papers/multsclrtx.pdf
Limitations of the MSR:
• The Selection of the value of ‘c’ in equ(1) is critical.• The DRC results in the violation of Gray world algorithm• The region of constant color bleaches out as a result of DRC.
Gray World Assumptions:
Gray World Assumption states is that, given an image with sufficient amount of color variations, the average value of the RED, GREEN, and BLUE components of the image should average out to a common gray value.
The color restoration is calculated using the expression
Ci(x,y) = β{log[α Ii (x,y)] – log[ Σ Ii(x,y)]}Where
β- Gain Constantα- Controls the strength of non-linearity
The Final representation of MSRCR is represented as
R MSRCRi (x, y) = G [Ci (x, y) * RMSRi (x, y) + b]
Where G- Gain Constant and
b- Gain Offset value
s
i=1
Wn - 1/3 - Weight used in Multiscale Retinex
N – Number of Scale =3
C1, C2, C3 - Surround Constant – 15, 80,250 respectively
G - Final Gain – 192
b - Offset Value – 30
α – Strength of non-linearity – 125
β – Control gain constant - 46
Gaussian Surround Function
F(r)
Image Co-ordinate
Input
Output
Space Constant c=80
Inputs
Outputs
Space Constant c=80
Inputs
Outputs
c=15 c=80 c=215
Output of MSRCR:
Inputs
MSROutput
MSRCROutput
Tak at IRIS Laboratory:
Inputs
MSROutput
MSRCROutput
Tak at McGhee Tyson Airport:
Inputs
MSROutput
MSRCROutput
Few Examples:
Input Software My results
Tak at IRIS and McGhee Tyson Airport:
Input Software My results
The following image was presented as an example in the paper, the same
image is used as input to both the software available and my
implementation.
Test Image
http://dragon.larc.nasa.gov/pub/papers/multsclrtx.pdf
My Implementation
From Available Software