[Digikam-users] Re: SRBG, Adobe RGB, What to use ?

[Remco Viëtor]

on Monday 27 June 2011, David Vincent-Jones wrote: 
> There can be substantial losses between the 16 bit data of a raw file
> and an 8 bit version. Compare the histograms of a converted 16 bit file
> and the 8 bit version, they will frequently show 'gaps' in the data post
> conversion, especially if any data stretching has been done. Those gaps
> will invariably show as banding on a finished, decent sized, print.

Fiirst, a correction: to my knowledge, RAW formats are NEVER 16-bit...
Most commonly, the data is 12 bit, and some high-end cameras use 14 bit.

Next, if you convert a 16-bit format to an 8-bit format, of course there's 
information loss. However, a histogram that shows no gaps for a 16-bit image, 
will NOT show gaps for the corresponding image in 8-bit format. Gaps could 
possibly occur if there's another conversion done while converting to 8-bit 
e.g. a colour space conversion after the conversion to 8-bit format. 

Of course, if you start editing the converted 8-bit image, all bets are off...

> Add to this the irretrievable data losses incurred in creating the 8 bit
> jpg format. One cannot, for instance, reopen a jpg file and tweek the
> image later without possible serious information loss. IMHO the whole
> jpg scene is one to avoid. jpg is a nice compact format for snapshots
> but one that I would advise others to avoid at any cost for serious
> photography.

Here also I disagree:
properly encoded JPEG* is a good format for _finished_ images (note the 
"finished"!), which by definition don't need additional editing or 
conversions. For such images it is the most compact way of storing the 
information, with no visible artifacts. It's also the format most commonly 
accepted for printing images (the alternative would be TIFF, with all the 
problems that format has due to its different dialects).

To be complete, I agree that you should never use jpeg for images that need 
further tweaking. Editing jpeg files can lead to very visible artifacts. For 
any image that might need further work (including changing size) I use either 
PNG (16-bit or 8-bit), or the gimp's native format.

And for what it's worth, I know of some photographers that produce outstanding 
work with 8-bit formats all along their workflow, using the GIMP for their 
'detailed' processing, and JPEG as their final format for display or printing.

Remco

*i.e. picking the optimal quality setting for the purpose of the file, and 
NEVER re-encode a jpeg.