[Leica] Re:JPEG compression

[red735i at earthlink.net (Frank Filippone)]

Repeated compression and decompression and then compression again, even at
the same compression ratios, destroys real data.  Do it enough times, and
you get perfect garbage ( although it is a lot of times.)  If you do not
believe this, go ahead and try it.

Now trying the same trick with making the file larger on re-compression,
will do the same thing, and I suspect, make it happen even faster.....

Remember, those pixels that were created are really not part of the actual
scene.  They are some form of interpolation.  So upon re-compression, you
have created even more and second generation artifacts.

One last comment... Jpeg compression algorithms work least well on images
that have rapid changes in image over relatively small areas of image.   ( I
mean to say that images like a bunch of bright lines or shapes on a
perfectly white background is a lot of high frequency stuff going on.  The
opposite image that is very easy on JPG is a picture of an egg.  Round,
small changes in pixel values from one to another pixel.)  Effectively this
states that the worst image you can take to compress is one of like a chess
board.  Sudden changes in image across a 1 pixel boundary.   SO if you try
my suggestion,, pick something with lots of quick change in image pixel
value.... don't try a landscape of gently rolling hills.

Frank Filippone
red735i@earthlink.net