[Digikam-users] re JPEG lossiness, PNG

[Jean-François Rabasse]

On Sat, 14 Jan 2012, Thorsten Schnebeck wrote:

>> If one selects the best, 100% quality, there won't be data loss but the
>> final file size won't be that small, comparable to the size of the same
>> image saved in PNG format.

> Nope, 100% jpeg quality still creates a minor loss of information.
> http://www.jpeg.org/faq.phtml?action=show_answer&question_id=q404fa5e29aeb6
>
> Bye
>
>  Thorsten

Hi Thorsten,

Hem, yes and no. It's not inherent to the standard but implementation 
dependent. Since 1993, and during the 2000s, many enhancements have been
proposed and developed for the compression scheme.

See, e.g. http://www.jpeg.pro/articles/132599/Lossless-Further-Compression

But only some JPEG generation software implement them because in most cases
they are useless. As the article you mention states clearly, concerning the
final result, "certainly as far as the human eye can detect".

Probably the ambiguity comes from the definition of what is lossy or 
lossless. Or, what exactly is lost ?
I agree the word "data" is not accurate and often used as a "joker" word.
Do we speak about "binary data" loss or "quality" loss ?
Binary data is expected to be somewhat modified, because all heuristic
schemes have different solutions to represent the same image.
But the important thing for us, photographers, is the visual image quality,
colours rendering, brightness gradients, etc.
And this can be achieved without modifications and with the guarantee that
your image will stay as you have it, as you shot it.

But ok for my lexical unaccuracy, I'd replace data by quality.

Regards,
Jean-François