Question

Contrast Stretching: Write your own program to implement a piecewise linear transformation for contrast adjustment using MATLAB/C/C++/Java/Python. The piecewise linear transformation function is illustrated in Fig. 1. Apply the transformation to an 8-bit gray-level image (a low-contrast image of your choice) to adjust its visual appearance. Experiment the transformation with three different sets of parameters: (ri, si) & (r2, s2). on to an8-itgrayovel imaont tne transformation with three (1) Show the original image and its intensity transformed images; (2) Describe specific change(s) (in terms of visual appearance) in the intensity adjusted image after each transformation (of different set of parameters) is applied; (3) Turn in your implementation codes in a separate file.

Answer 1

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(image.png)

We will apply contrast stretching on above image.

3) SOLUTION: (contrast_stretch.m)

clear all;
close all;
clc;

%% Reading an image
a=imread('image.png');
a=double(a);
s=size(a);

%% Defining points and calculating equation parameters
p1=[0,0];
p2=[75,5];
p3=[140,250];
p4=[255,255];
m1=(p1(1,2)-p2(1,2))/(p1(1,1)-p2(1,1));
m2=(p2(1,2)-p3(1,2))/(p2(1,1)-p3(1,1));
m3=(p3(1,2)-p4(1,2))/(p3(1,1)-p4(1,1));

c1=p1(1,2)-m1*p1(1,1);
c2=p2(1,2)-m2*p2(1,1);
c3=p3(1,2)-m3*p3(1,1);

%% Transformation function
t=[];
for x=0:255
if(x<=p2(1,1))
t=[t (m1*x+c1)];
end
if(x>p2(1,1) && x<=p3(1,1))
t=[t (m2*x+c2)];
end
if(x>p3(1,1) && x<=p4(1,1))
t=[t (m3*x+c3)];
end
end

%% Getting output image
for n=1:s(1,1)
for m=1:s(1,2)
ot(n,m)=t(a(n,m)+1);
end
end

plot(t)
grid on;
xlabel('Intensity in input image');
ylabel('Intensity in output image')
title('Piece-wise linear transformation : Contrast stretching function')

figure()
subplot(1,2,1)
imshow(a/255)
title('Original image')
subplot(1,2,2)
imshow(ot./255)   
title('Contrast stretching')

1) & 2) OUTPUT:

Figure 1 Figure 2 g z+ Z- -f* Insert Text Axes Grid Autoscale z+ z, t. Insert Text Axes Grid Autoscale Piece-wise linear transformation: Contrast stretching function 300 250 Original image Contrast stretching 200 150 100 5아 50 100 300 150 Intensity in input image 200 250

We have stretched the range from 75-140 to 5-250. This makes it clearer and the differences more visible to naked eyes.