Wow. I think this is called "too much free time".
Fun to play with... I played around with Imagemagick's command line converter, convert.exe. Two things govern the degradation: the number of conversions, and the compression (quality) setting. I found that with 80% quality even 1000 cycles didn't do much; on the other hand if I chose 1% (very severe compression) quality, the changes happen more quickly. There comes a point where the image settles down and doesn't degrade much more as you can see...
Original
System specs Shuttle SN78SH7, 4 GB DDR2 RAM, 3.0 GHz AMD Phenom II 945 (quad core), Windows 7 Pro 64 bit, images stored on a USB 2.0 external hard drive, converted with ImageMagick 6.6.2-2, image size 401x433, 24 bit colour.
Converted 10 times at 1% quality setting (0.61 seconds)
Converted 100 times at 1% quality setting (6.4 seconds)
Converted 5000 times at 1% quality setting (240.38 seconds)
the script I used (a quick hack, I know)
jpegloop.bat
@echo off
setlocal enabledelayedexpansion
set limit=%2
set quality=%3
set /a stop=%limit%-1
del "%~n1-converted-*.jpg"
copy "%~n1.jpg" "%~n1-converted-0.jpg"
set stime=%time%
for /l %%N in (0,1,%stop%) do (
set num1=%%N
set /a num2=!num1!+1
echo !num2!.jpg
convert %~n1-converted-!num1!.jpg -quality %quality%%% %~n1-converted-!num2!.jpg
if %%N LSS %stop% del %~n1-converted-!num1!.jpg
)
del %~n1-converted-%num1%.jpg
set etime=%time%
echo start %stime%
echo end %etime%
command line example
jpegloop.bat face.jpg 5000 1
