MATLAB Answers


Reading a double image

Asked by Andrew
on 6 Feb 2013

I have encoded and decoded a 256*256 image. At first i did it using the image in "*uint8" precision and everything works well. but when i change uint8 to "double", i get an error while finding the MSE of the image. the error says that the decoded image is of size [256 33]. please look at my code and advice if there is a mistake am making here.

% Read the contents back into an array
close all;
clear all;
fid = fopen('Lenna.out');
m5 = fread(fid, [256, 256], '*double');
imagesc(m5),axis off;
A = imread('..\Lenna.bmp');
M = 256;
N = 256;
 for i=1:256
     for j=1:256
   PSNR = 10*log10(double((255^2)/MSE))

PS: Lenna.out is the decoded image



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1 Answer

Answer by Jan Simon
on 6 Feb 2013
Edited by Jan Simon
on 6 Feb 2013

You can read a file in binary format only in the format it has been written in. When you have written Lena.out as UINT8 values, you cannot read it in double precision. So please show us how the file has been created.

It is surprinsing that the imported data have the size [256 x 33]. When it would be [256 x 32] I'd assume, that you have simply read the file, which contains 256*256 bytes (UINT8 values) as doubles, which need 8 bytes per element: 8*32=256. But where could the column come from? How did you encounter the size of the matrix?

I guess you want:

m5 = fread(fid, [256, 256], 'uint8');  % Now m5 has the type double!
m5 = m5 / 255;

And finally without an expensive loop:

MSE = sum((A(:)-m5(:)) .^ 2) / (M*N);


on 6 Feb 2013

this is how i create a file Lenna.dat

A = imread('Lenna.bmp');
fid = fopen('Lenna.dat','wt');
count = fwrite(fid,A,'*double');
fid = fopen('Lenna.dat');
m5 = fread(fid, [256, 256], '*double');
I then perform image compression and decompression on Lenna.dat to get Lenna.out. i then use the code in the question to view the image
Jan Simon
on 6 Feb 2013

It is still not clear how you have written "Lena.out" and "Lena.dat" does not occur in the code you have posted in your question. Did you confuse the .dat with the .out file?

Please copy the error message and mention the line, which causes the error. Check the size of m5 after reading: disp(size(m5)) and explain how Lena.out has been written.

You're writing out uint8s as doubles but didn't convert them. Try this:

count = fwrite(fid,double(A),'*double');

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