Another John <lal...@hotmail.com
> A (fairly) naive question about JPGs:
> I'm not a "photographer", but I am a very keen "snapper": I've taken
> photographs since I had my first Kodak Cresta in the 50s.
> I also spent my working life in computing (as a documenter and educator,
> not a coder), so I do know a fair amount.
> I have the ingrained habit (since the days, when disk space was really
> expensive) of editing my pictures down to a smaller size. (I never take
> RAWs, only JPGs).
> Mainly, I scale them (maybe 50%). (I have used the sainted
> GraphicConverter since its Version2.)
I waste of time, these days, with HDs costing cents/GB.
> BUT (the question, finally): if I simply Save the JPG without doing
> anything to it, it will often go down in size very considerably (e.g.,
> say 5.5MB to 3.2MB).
There is some default setting for the compression, in that case.
> What is the data that is being lost?
Fine detail. However, it is the wrong question,
because there is data and data.
'Data' doesn't distinguish between real data, that is your image,
and just noise.
In the sense of Shannon information gets lost in compression.
In a real sense what gets lost in going from RAW to JPEG
is mostly sensor noise.
Remember that in the sense of Shannon
a picture that is nothing but noise
has the highest information content of all.
> I do know all about "JPG loss of quality", and have done since I started
> using GC. I use GC's "90% quality" when saving - I can detect
> virtually no difference in the pixels, even when enlarged to a great
> scale, so I'm curious as to what has gone from the original JPG, for it
> to have lost around 30% of the size.
You should go to great scale, then the look at the fine detail.
> My scaled down pictures, btw, always appear great on a computer screen,
> on our digital display frame (nowadays the most-used medium), and even
> when printed out at the local Max Spielmann.
This 'loss of information in compression' matra
is no more than a red herring that survives from the days
when pictures were measure in kilobytes,