imwrite and imread are not giving the same matrix

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Khalid Ibne Masood
Khalid Ibne Masood 2020-11-17
评论： Khalid Ibne Masood 2020-11-18
Hello,
I want to show a matrix in an image (and save it for later use) and then retrieve the matrix from that image using imread. I followed the approach presented in https://www.mathworks.com/matlabcentral/answers/242747-how-can-i-convert-a-matrix-with-decimal-values-into-a-image-and-then-retrieve-the-same-matrix-back-f
However, while saving the image with the command imwrite, and then later read it with imread, I see two different matrices. Here is what I have done:
clear all, close all, clc
% A=rand(10); %random matrix(10x10) with values between 0 to 1.
% B=A*150;
B = [3 1; 2 5];
sz = size(B);
R = typecast(uint64(B(:) * 2^56),'uint8');
R1 = reshape(R, [8 sz]);
R2 = permute(flipud(R1), [2 3 1 4]);
R3 = reshape(R2, [sz(1), sz(2)*8, 1]);
imshow(R3);
imwrite(R3, 'TestFile.jpg')
close all
A = imread('TestFile.jpg');
sz = size(A);
U3 = reshape(A, [sz(1), sz(2)/8, 8, 1]);
U2 = flipdim(permute(U3, [3 1 2 4]),1);
UB = reshape(double(typecast(U2(:), 'uint64')) / 2^56, [sz(1), sz(2)/8, 1]);
Now my question is how to get the correct matrix from imread.
Thank you.
Khalid

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采纳的回答

Walter Roberson
Walter Roberson 2020-11-18
This is normal and expected for image formats that use lossy compression, such as the way most people use jpeg.
You can:
  • tell imwrite to use jpeg lossless mode; or
  • use an image file format such as png that is lossless; or
  • arrange the data ahead of time to be flawed in exactly the same way that jpeg throws away data so that what gets written out matches what gets read in; or
  • adjust your expectations so that you no longer expect them to match.
I recommend that you adjust your expectations, and because of that, you stop using jpg for scientific work (jpeg2000 is a different matter and worth considering.)

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