[ILUG] SDMI initiative

[fergal daly]

One factor that may make a difference is that you probably won't know
the quality of the initial SDMI compression. What happens if you use
different qualities at each generation?

Colm Buckley wrote:
> 
> Niall O Broin writes:
> 
> > However, JPEG uses a similar kind of compression (psycho-visual
> > model ?), discarding information that the eye doesn't take much
> > account of, but you will surely see the effects of repeated
> > compression/decompression with JPEG.
> 
> No, you don't!
> 
> Look:
> 
> ogma$ ls -l gen0.jpg
> -rw-r--r--    1 colm     users      581874 Sep 28 08:20 gen0.jpg
> ogma$ for a in $(seq 0 9)
> do
>     cjpeg -quality 80 -optimize < picture.ppm > gen$a.jpg
>     djpeg < gen$a.jpg > picture.ppm
> done
> ogma$ ls -l gen*
> -rw-r--r--    1 colm     users      188828 Sep 28 08:30 gen0.jpg
> -rw-r--r--    1 colm     users      188881 Sep 28 08:30 gen1.jpg
> -rw-r--r--    1 colm     users      188877 Sep 28 08:30 gen2.jpg
> -rw-r--r--    1 colm     users      188877 Sep 28 08:30 gen3.jpg
> -rw-r--r--    1 colm     users      188877 Sep 28 08:30 gen4.jpg
> -rw-r--r--    1 colm     users      188877 Sep 28 08:30 gen5.jpg
> -rw-r--r--    1 colm     users      188877 Sep 28 08:30 gen6.jpg
> -rw-r--r--    1 colm     users      188877 Sep 28 08:30 gen7.jpg
> -rw-r--r--    1 colm     users      188877 Sep 28 08:30 gen8.jpg
> -rw-r--r--    1 colm     users      188877 Sep 28 08:30 gen9.jpg
> ogma$ cmp gen2.jpg gen9.jpg
> ogma$
> 
> So, there is what looks like a tiny amount of lossage with the first
> two generations, and after that the file remains identical through
> multiple compression/decompression cycles.  I believe the JFIF encoder
> is pretty standard these days, but if there were different JPEG
> encoders being used at each generation, you might get more loss for
> more generations, but eventually the file would either stabilise and
> be unchanged by a given decompression/compression cycle, or enter a
> set of stable states and alternate between them.  Pretty much the same
> goes for PAM-based audio compression, using ATRAC or MP3 or whatever.