Contrast (CON) (this is also called "sum of squares variance"):
Contrast equation
Explanation:
When i and j are equal, the cell is on the diagonal and (i-j)=0. These values represent pixels entirely similar to their neighbour, so they are given a weight of 0.
If i and j differ by 1, there is a small contrast, and the weight is 1.
If i and j differ by 2, contrast is increasing and the weight is 4.
The weights continue to increase exponentially as (i-j) increases.
Calculation example: for the horizontal GLCM, Contrast equals
Contrast weights: X horizontal GLCM = multiplication result
0 1 4 9
0.166 0.083 0.042 0
0 0.083 .168 0 1 0 1 4
0.083 0.166 0 0
0.083 0 0 0 4 1 0 1
0.042 0 .249 0.042
.168 0 0 .042 9 4 1 0
0 0 0.042 0.083
0 0 .042 0
Sum of all elements in the multiplication result table = 0.586
In detail:
.166*(0-0)2 + .083*(0-1)2 + .042*(0-2)2 + 0*(0-3)2 +
.083*(1-0)2 + .166*(1-1)2 + 0*(1-2)2 + 0*(1-3)2 +
.042*(2-0)2 + 0*(2-1)2 + .250*(2-2)2 + .042*(2-3)2 +
0*(3-0)2 + 0*(3-1)2 + .042*(3-2)2 + .083*(3-3)2
= .166(0) + .083(1) + .042(4) + .083(1) + .166(0) + .042(4) + .25(0) + .042(1) + .042(1) + .083(0)
= .083 + .168 + .083 + .168 + .042 + .042
= .586
Important practical matter: Since Contrast can evidently be <1, it must be recorded in an image channel equipped to handle real numbers. If put into an 8-bit or 16 bit integer channel, the value would become 0.
Self test: a. What is the degree of this measure? b. What does a Contrast of 0 mean?
Exercise: Calculate the Contrast for the vertical GLCM and compare it with the Contrast for the horizontal GLCM