[Digikam-users] re JPEG lossiness, PNG

[Simon Oosthoek]

On 14/01/12 17:15, Jean-François Rabasse wrote:
>
> JPEG can compress in a lossless way, until the algorithm bumps against
> some limits. GIF does so, PNG does so. But JPEG can boost compression
> efficiency if the user accept some losses.
> That's what Marie-Noëlle Augendre said, on this thread :
> "I guess that to produce something smaller, you'll have to loose
> something."
> Definitely right, there's no magic at all, and Santa Claus doesn't
> exist:-)

Maybe I've missed a part of the discussion, but the main concern with
Jpeg is, AFAIK, that jpeg is 8-bits, so always loses something when
using RAW as the reference, since RAW formats usually have 10-16 bits
per colour available as bandwidth and most SLR sensors have the ability
to provide that dynamic range to a certain extend, converting from RAW
to JPEG will at least cost you the difference in expressibility of
colour and brightness (e.g. 12 bits in RAW to JPEG: 4096 to 256 shades).
Even if no loss was caused by the JPEG algorithm, JPEG loses something.

This is why PNG and JPG2000 are popular choices; they allow 16-bit
values to be preserved and allow lossless compression (meaning it is
reversible to "RAW" in theory)

/Simon