Sayısal Görüntü İşleme Teknikleri
Doç. Dr. Mehmet Serdar Güzel
Slides are mainly adapted from the following course page:
http://www.comp.dit.ie/bmacnamee
&&
https://www.tutorialspoint.com/dip/histogram_equalization.htm
Lecturer
Instructor: Assoc. Prof Dr. Mehmet S Güzel
Office hours: Tuesday, 1:30-2:30pm
Open door policy – don’t hesitate to stop by!
Watch the course website
Assignments, lab tutorials, lecture notes
•slide 2
Contrast Stretching
It is easy to fix images that have poor contrast by relating a pretty simple contrast specification
The interesting part is how do we decide on this transformation function?
Histogram Equalisation
Spreading out the frequencies in an image (or equalising the image) is a simple way to improve dark or washed out images
The formula for histogram equalisation is given where
rk: input intensity
sk: processed intensity
k: the intensity range (e.g 0.0 – 1.0)
nj: the frequency of intensity j
n: the sum of all frequencies
) (
kk
T r
s
k
j
j
r r
p
1
) (
k
j
j
n n
1
Summary
We have looked at:
Different kinds of image enhancement
Histograms
Histogram equalisation
Next time we will start to look at point processing and some neighbourhood operations
Gray Level Transformation
There are three simple gray level transformation.
Linear
Logarithmic Power – law
Gray Level Transformation
The overall graph of these transitions has been shown below
Gray Level Transformation
Linear transformation
Linear transformation includes simple identity and negative transformation.
Negative transformation
The second linear transformation is negative
transformation, which is invert of identity transformation.
In negative transformation, each value of the input image is subtracted from the L-1 and mapped onto the output image.
Gray Level Transformation
Logarithmic transformation further contains two type of transformation. Log transformation and inverse log
transformation.
The log transformations can be defined by this formula:
s = c log(r + 1).
Logarithmic transformation
Gray Level Transformation
Power – Law transformations that include nth power and nth root transformation. These transformations can be given by the expression:
s=cr^γ
