Why is a resaved JPG file different from the original?

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');

[Osama AlThahab]

f not equal f1

Answer 1

[Walter Roberson]

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 :)

[Osama AlThahab]

iam sorry i write the rong code and this is the correct one f=imread('lena.jpg'); imwrite(f,'lena1.jpg'); f1=imread('lena1.jpg');

[Image Analyst]

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.

[Osama AlThahab]

thanks alot