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...
The succeeding images differs hardly from each other, but the original
question was not if you can see it with your eyes, but if the picture
degrades more and more after each step.
My opinion is Yes, the image when saved as .jpg will degrade more and more
after each time you save, close and re-open the image.
> Have you tried the same experiment using another image editing
> program to eliminate the possibility that it might be more a by-product of
> what PSP is doing than what is generic to JPEG compressions?
No, I only did the test with PSP. But why should PSP use a non-generic JPEG
compression algorithm with the risc of compatibility problems when opening
the .jpg file in an other application? I am not shure, but I think the basic
compression algorithm for JPEG images is the same for all applications.
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 :-)
I know it is against the rules to send attachments to the list, but the
images are very small.
Aap.tif is the original.
Aap1.tif is the difference between the original and the .jpg saved image
after the first step.
Aap2.tif is the difference between the original and the .jpg saved image
after the tenth step.
I enhanced the images Aap1.tif and Aap2.tif by giving them the same amount
of gamma correction to make the differences visible.
Every extra step shows slightly more degrading.
With kind regards,
Henk de Jong
Homepage Nepal - Trekking Around Annapurna - Photo Gallery: