Adding noise into an image manually instead of using imnoise

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Sufyan
Sufyan on 30 Aug 2012
Commented: AMIR KHAN on 29 Aug 2021
I am to trying to understand the algorithms behind matlab way of adding noise into an image,
The algorithm which Matlab use to add Gaussian noise is this,
b = a + sqrt(p4)*randn(sizeA) + p3;
When I tried to implement this algorithm manually it worked successfully however it doesn't work unless i changed the image class to double. Why is that so ? Why should i suppose to change the class to double when adding gaussian noise ?
Here is my code,
I = imread('2.jpg');
J = rgb2gray(I);
p3= 0;
p4 = 0.05;
J = im2double(J);
b = J + sqrt(p4)*randn(size(J)) + p3;
imshow(b)
Just like Gaussian Noise i tried adding the Salt n Pepper noise manually, here is the algorithm Matlab use to add Salt n Pepper noise,
b = a; <-- Assign b to the input image
x = rand(sizeA); <--- Generate random pixels from the image pixels
d = find(x < p3/2); <--- Find the pixels whose values are less than half of the mean value
b(d) = 0 <-- Implement minimum saturation to them
d = find(x >= p3/2 & x < p3) <--- Find the pixels whose values are greater than half of the mean value & less than mean value
b(d) = 1; <-- Implement maximum saturation to them
and I implemented it as below,
I = imread('2.jpg');
J = rgb2gray(I);
p3= 0.5;
x = rand(size(J));
d = find(x < p3/2);
J(d) = 0; % Minimum value
d = find(x >= p3/2 & x < p3);
J(d) = 1; % Maximum (saturated) value
imshow(J)
but the output Image doesn't show any Salt n pepper noise in the Image , I wonder where the final image is actually stored in my code?
  2 Comments
sravani honey
sravani honey on 20 Apr 2018
sir, i am doing my project on random impulse noise removal and i am adding random noise using noisyimage = (I + p*rand(size(I)))/(1+p) this function but it doesn't clear results on my project. can u please give any other command related to adding random impulse noise to the image?

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Accepted Answer

Walter Roberson
Walter Roberson on 30 Aug 2012
Images read by imread() are often data class uint8(). The uint8() property will be retained in the rgb2gray() step. Your saturation assignments of 0 and 1 are, however, based upon the notion that the minimum and maximum are 0 and 1 instead of 0 and 255.
Also, the minimum value that a uint8 variable can be is 0, so there is no need to explicitly set 0 as the minimum.
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More Answers (2)

Image Analyst
Image Analyst on 31 Aug 2012
Gaussian noise can have any magnitude. So how are you going to add 4.28435435 to an image that can have only integer values? If you could and then it was converted to an integer, you'd lose the fractional part of the number, which means it would no longer be Gaussian. So that's why you have to convert the image to floating point.
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Ameerah Omar
Ameerah Omar on 21 Nov 2015
how i can add noise in image it come from Cauchy distribution???
  1 Comment
Image Analyst
Image Analyst on 21 Nov 2015
Use the formula for the CDF and then use rand() to draw numbers from the distribution. Attached is an example where I do that for the Rayleigh distribution.

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