why the mse and psnr changing?

I use this coding to find mse and psnr, why everytime i ran the code, the image's psnr and mse changing??

clear all

close all

clc

imdata = imread ('/Users/hp/desktop/FYP IMAGE/blue/160blue.jpg')

figure

imshow (imdata);

title ('Original image');

ref = rgb2gray (imdata);

figure

imshow (ref);

title ('gray image');

N = imnoise (ref, 'salt & pepper', 0.02);

figure

imshow (N);

title ('Noise Image');

error = immse (N, ref);

peaksnr = psnr (N, ref);

subplot (311); imshow(imdata); title ('oroginal image');

subplot (312); imshow(ref); title ('gray image');

subplot (313); imshow(N); title ('Noise Image');

Answer 1

[Nagasai Bharat]

Hi,

The change in the value of peaksnr and error is due the probabilistic nature of the imnoise function to add noise into the image.

From the documentation of imnoise the algorithm is used is:

To add 'salt & pepper' noise with density d to an image, imnoise first assigns each pixel a random probability value from a standard uniform distribution on the open interval (0, 1).

• For pixels with probability value in the range (0, d/2), the pixel value is set to 0. The number of pixels that are set to 0 is approximately d*numel(I)/2.
• For pixels with probability value in the range [d/2, d), the pixel value is set to the maximum value of the image data type. The number of pixels that are set to the maximum value is approximately d*numel(I)/2.
• For pixels with probability value in the range [d, 1), the pixel value is unchanged.