now a lot can be done to synthetically "fix" them
information IS LOST in the jpg compression
yes it went into a black hole BUT the info was NOT conserved on the EH nor passed to a different dimension to be averaged out in the multiverse
-- it's gone --
A test image " Kodak's lenna " --- a industry standard used to calibrate equipment in labs
Click to view attachment
the eyes and nose are good for showing results ( "g" rated ones )
( the below images are jpg's but , oh well . )
the orig. close up
a jpg artifact added ( two saves at 65% 2x2x2x2 )
over the years i have found a few things to do this NOTHING works very very well . However GREYCstoration now Gmic works well , within reason.
a gimp add on and a bash command line tool .
the gimp tool ONLY works on 8 bit / layer ( 24 bit rgb )
the command line tool works on 8,16 singed & unsinged ,and 32 bit floating point .
warning this only artificially fixes them
for now the gimp tool
i like the "edge preservation flow" pde
and the close up
the artifacts are gone BUT the image is way too smooth
add some noise
a small amount 4 will do
and the close up AND the orig. image
the terminal command
gmic 4.2.04b.jpg -gimp_edgepreserving_smoothing 0.8,0.5,0.6,2.5,50,10,0,0 -gimp_noise 4,0,0,0,0 -o clean.png
where "4.2.04b.jpg" is the input
and "clean.png" is the output
example from photojournal
left orig./ right "fix"