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Re: filmscanners: File format

On Fri, 30 Mar 2001, Henk de Jong wrote:

> Laurie Solemon wrote:
> > Out of curiosity, how many timed did you do this and what sorts of
> > changes did you see?
> I took a picture, saved, closed and re-opened it ten times.
> After every step (save, close and re-open) I compared the new image with the
> original and found small differences, we all know are called artifacts. In
> the next step (save again, close and re-open) I also found the artifacts
> were different from the step before.
> In theory I think you are producing extra artifacts from artifacts...
> Tony Sleep wrote:
> >If you merely open and save a JPEG at the same compression,
> >without editing, you lose nothing
> With my small and simple test I found other results :-)

Come on guys, what happens when you save with jpeg or any other
compression that loses data? You save the file, the compression algorithm
desides what information can be thrown away, and then saves it. Now what
happens when you open that image? The image data (the pixels) are
recreated from the incomplete information that was saved in to the file.
But you end up with a file that has all the pixels. If you used a lossy
compression, then at least some of the pixels are wrong or different from
the original image. This you should all know.

Now, what happens if you save this image again? The exact same thing -
with the crusial exception that now your original image is the one with
the changed or wrong pixels. The compression will now be done to this new
image. And again, you will loose something in the process. 

You lose information every time - just as Henk de Jong's attached images
showed. You can all try it. Just try it with a heavy compression, then
you'll be able to see the effect easier. Of course, the same applies to
the less heavy compression, in which case you could do this a couple of
times and not notice any degration of the image. (Yes, we might over do
all this, but his is how it works...)

It doesn't matter if you changed anything in the image or not. Well,
generally. You might have some programs that let you chage the heared
portions of the image file. If you do not change any value of any pixels
in the image, why should you then save the image? Just close it. Then you
will not lose anything. Or, if you use lossless compression (like tiff),
then your recreation of the image is perfect, and that's why you don't
loose anything with opening the saved image.

It's a different thing if you are working on an image in PS and saving it
between every step. As long as you don't open the image (or do revert?)
you will not lose information in this way.

Best regards,

**   Hugo Gvert                                          **
**   hugo@cc.hut.fi             http://www.hut.fi/~hugo   **
**   Sometimes I think the surest sign that intelligent   **
**   life exists elsewhere in the universe is that none   **
**   of it has tried to contact us.   -- Calvin.          **


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