On Sunday, March 08, 2009 18:13, Frank van Lindert wrote:
> I am curious to know what the subsequent file sizes of all your
> 'generations' were, Chuck. After all jpeg is only intended for
> compressing the original file without to much quality loss for your
> specific goal. You can always revert to raw, when quality is
> important.
> If the files stop getting smaller a further jpeg save would make no
> sense, or am I wrong here?
>
> Frank van Lindert
> Utrecht NL.
>
>
Chuck posted a note copied from the PWP forum and it got my curiosity up about
the subject.
I did some experimenting and posted some of it to the web space that comes
with my ISP.
http://members.localnet.com/~doug9345/JPEG_Compare.html
I left the pictures fairly large and generally not highly compressed. I didn't
want to and any other sources of changes than the ones I was experimenting
with. It loads fairly slow on dial-up. If it gives enough people loading
issues I'll reduce the size of the pictures. It will take me about 3 more
passes to get rid of the typos, spelling and grammer mistakes.
To answer your question repeated saving isn't to reduce file size but a by
product of working on a photo, saving it, and then coming back, working on it
some more and saving it again.
How the file size varied from one save to the next depended on the quality of
the save. For my image of corn at 1000 x 800 pixels the tif is 6.7MB,and the
png with maximum compresion 2.3 MB. the best quality jpeg is 694.7KB. After
9 more saves it is 622.4KB. On the other had with the quality set to 50 on a
scale of 100, the first save after the highest quality one is 78.2KB. It the
climbs to 78.5 on the final save. Using bzip2 to compress the tiff gave me a
file size of 1.6MB. The png, compressed the same way, was 1.5MB. The high
quality jpeg increased in size 694.7KB to 695.8KB when compressed. This is
normal behavior for already compressed files. I hope this answers your
question.
-Doug
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