[KLUG Members] jpg deterioration

[bill]

Jpeg format is a lossy format.  That is, it's file size can be
compressed and it's compression scheme LOSES data.  That degrades
picture quality somewhat.  It's a sliding scale usually measured in
percent.  More compression = less quality.

To contrast, gif images are compressed but not lossy.

No program degrades images simply by opening and closing them. 
Compression only occurs when an image is saved.  Neither can an image
degrade by simply viewing it multiple times in a presentation program. 
Again, if you think about it, it would have to be saved differently
somehow to change at all.

Image editing programs do this compression stuff when they save the
image.  Usually, when you save a jpeg, there is an option to adjust the
compression/quality.

If you open a jpeg and then save it in such a program, it will be saved
at whatever the compression setting is.  So, it -is- possible to
open/save/compress, open/save/compress(further), etc.

Thus, graphic artists always save their original work (usually in a
non-lossy format).  Then, when editing needs to be done, they open the
original, do their edits, and then "save as" the result.

Simply opening a non-graphic file containing a jpeg and saving the
resulting file repeatedly will not make the jpeg get worse.  Such
programs do not save in a lossy manner.  

There are exceptions, pdf files come to mind, and I believe Powerpoint
can compress image files.   But even in those cases, the reduction in
quality is, I believe, a one time thing.  Once it's been saved, simply
opening the file and saving the file will not make the images inside to
grow worse.  This only applies, of course, if you are only editing the
presentation and not the images.

Do image editing in image editing programs, save a good copy, then put
them in your presentations, documents, etc.  That way you always have an
original to go back to.

Hope this helps.

kind regards,

bill


On Thu, 2006-08-31 at 19:56, Eric Beversluis wrote:
> I've read that jpg images deteriorate somewhat each time you open and
> close them. But I'm not clear about exactly what that means.
> Specifically, if I put a jpg image into a powerpoint or an Impress
> presentation, does it deteriorate each time I run the presentation?
> 
> Thanks.
> 
> EB
> 
> _______________________________________________
> Members mailing list
> Members at kalamazoolinux.org
> 
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