Unexpected behaviour by imread and imwrite

workingDir = 'frames';
filename = [sprintf('%d',1) '.jpg'];
fullname = fullfile(workingDir,'imagesV',filename);
a2=imread(fullname);
a2(1,1)=1;
imwrite(a2,fullname);
a1=imread(fullname);
if a1==a2
    disp(23)
else
    disp(45)
end

The code above displays 45 instead of 23. What is the reason for this?

Answer 1

[Rik]

JPEG is not a lossless compression algorithm. This means that re-writing a loaded image might (and generally will) result in a different image. Another reason in your case would be that you changed a pixel value.

(side note: technically there are versions of JPEG that do support lossless compression, but the support is very limited)

[Walter Roberson]

Yup.

You should only use the original JPEG for display purposes, not for data archive, because JPEG lossy mode will blur all straight lines, similar in effect to anti-aliasing.

[Walter Roberson]

Avoid JPEG, JPEG2000, and (I suppose) GIF. The other formats do not lose information.