Does copying degrade jpg image quality?

[Mark Kaine]

im just curious because jpg apparently is not lossless,  so what happens if you copy a jpg photo from one hard drive to another for example? 

[heimdali]

Copying the file won't make it non-lossless.  The loss has already occurred before you can copy it.

 

What happens when you open an image in Gimp, make some changes and save the result as 100% jpg?  Will that create more losses?

 

[PDifolco]

3 minutes ago, heimdali said:

Copying the file won't make it non-lossless.  The loss has already occurred before you can copy it.

 

What happens when you open an image in Gimp, make some changes and save the result as 100% jpg?  Will that create more losses?

 

But just copying won't degrade the image further neither, file copy IS lossless  

[Mark Kaine]

5 minutes ago, heimdali said:

Will that create more losses?

 

most definitely! 

 

1 minute ago, PDifolco said:

But just copying won't degrade the image further neither, file copy IS lossless  

i see, that's what I thought so you only get degradation when editing  -> saving as jpg , but not by simply copying? 

 

that's why I use png whenever possible,  but my phone camera seems to only do jpg, so i was wondering...

 

 

[da na]

File copying is not lossy. All that happens is the file is moved, its contents are not altered.

Opening and re saving an image IS lossy, because it compresses the already compressed image further.

[LAwLz]

As others have said, copying a file is lossless, assuming nothing goes wrong of course and the file gets corrupted.

Copying a file from one hard drive to another does not alter the content of the file.

 

Lossy formats irreversibly destroys data when they are encoded. For example when a program like paint creates an image file. If you open an image in paint, alter a pixel in the left corner and then save it as a JPEG, chances are the entire image will get degraded, not just the left corner. Do it again and the image will be even more degraded.

 

 

This thread reminded me of rotational velocidensity.

[Mark Kaine]

I guess that answers that... and yes, i was just asking because sometimes when you edit a jpg file it looks just fine and other times it looks horrible... so i thought it may be the same for copying,  but i see... it shouldn't degrade doing that (unless corruption etc)

[heimdali]

51 minutes ago, Mark Kaine said:

most definitely!

 

How do you know?  It's not like it looks different?

 

I tried it and saved a jpeg image at 100% and the files are different.  But it could be other things that gimp puts into the file.  And it would make perfect sense to me that the copy or altered version of an image, when saved at 100% quality, doesn't loose any more than the jpeg it was created from already lost.  There's no reason that it should degrade any further, especially not when it's a copy.

 

[Blue4130]

On 11/21/2022 at 10:42 PM, heimdali said:

 

How do you know?  It's not like it looks different?

 

I tried it and saved a jpeg image at 100% and the files are different.  But it could be other things that gimp puts into the file.  And it would make perfect sense to me that the copy or altered version of an image, when saved at 100% quality, doesn't loose any more than the jpeg it was created from already lost.  There's no reason that it should degrade any further, especially not when it's a copy.

 

It absolutely degrades further with each generation of copy that you make. You are recompressing it. Each time you recompress, more data is lost. 

[Mark Kaine]

35 minutes ago, Blue4130 said:

It absolutely degrades further with each generation of copy that you make. You are recompressing it. Each time you recompress, more data is lost. 

^yup can definitely see it... it just varies from picture to picture...

[heimdali]

Well, it makes sense that there is only method for saving jpegs which always will create losses, and I'm too lazy to look at the source code of gimp to find out.  It would be very inefficient to put more effort into it when you can simply copy the file when you want a lossless copy.