Why is a resaved JPG file different from the original?

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Osama AlThahab
Osama AlThahab 2013-8-20
when i read any image and then write it without any modification then read it again i found that the mattrix of them not equal there is a difference between them.
f=imread('lena.jpg');
imwrite(f,'lena1.jpg');
f1=imread('lena1.jpg');

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Walter Roberson
Walter Roberson 2013-8-20
JPEG is a lossy image compression format, and the amount of loss can be adjusted. The loss settings that were used to create the image might not be the same loss settings that are used by default when you imwrite() a JPEG image, so the imwrite() / imread() combination might be throwing away more information than was thrown away to create the original lena.jpg file.
Also notice that your code has you reading the same file twice :)
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Image Analyst
Image Analyst 2013-8-21
The answer is the same. You're saving to a lossy format so information is lost. Thus you won't read in the same data as before the save. Save to lossless PNG format instead and you'll be okay.

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