[Digikam-users] re JPEG lossiness, PNG

[Jean-François Rabasse]

Hi Remco,

And thanks for that accurate post. You definitions are indeed far less
cloudy as were mine:-)

On Sun, 15 Jan 2012, Remco Viëtor wrote:

> Afaik, there's no ambiguity in the case of compression algorithms:
> lossless means that the binary data recovered after a
> compression-decompression cycle is exactly the same as the original
> data.

Definitely unambiguous. From a mathematical point of view, you mean the
function is reversible.

> And the problem is exactly there, when images are edited: one jpeg
> compression/decompression cycle might not induce _visible_ artifacts,
> but it seems we agree it can cause 'binary' artifacts. Those artifacts
> will increase with each compression cycle (edit session), leading
> finally to visible degradation of the image (and personally, I really
> dislike the kind of artifacts it causes).

Ok, I trust you. Never seen that but probably because I'd never "cycled"
enough times.

> I prefer using png for intermediate edits (with the space penalty that
> implies), also so i can easily see which image to edit (raw, png:
> editable, jpeg: final). Thus no need to mess with filenames etc.
> for non-final images (but that's not a quality issue).

Ok again. I'd skipped that edition cycles because - as for me - that was
outside a discussion about flat images formats. As I'm a GIMP user,
the natural edition process is to open raw files with GIMP and its UFRAW
plug-in, and keep editable versions under the GIMP xcf format.

It's probably the worst format regarding file size, but I don't know
other solution to keep edition stuff such as layers, masks et al.



I have a question (no longer quality issue), for you and other users of
course, about what could be considered as a "lifetime" for non final
images files ?

Said otherwise, after what time a no more modified final image should be
considered as definitive, and intermediate files, raw file, editable
file, could be discarded to recover storage space ? 
Six months, one year, two years ?

Or do users keep all, "just in case", and fill CD or DVD for the
future ? And if yes, how long do people keep CD before rewriting them to
be sure to save a readable copy, even 10 years later ?

Regards,

Jean-François