[digiKam-users] JPEG grows bigger. Why?

[Ramnarayan.K Digikam]

I also learnt a good deal.

Thank you

**
Top posting because not replying to content, just a general response.






On Sun 8 Aug, 2021, 18:34 Dmitri Popov, <dmpop at tokyoma.de> wrote:

> Hi Christian,
>
> Thank you for the detailed and technical explanation. Very interesting and
> useful stuff.
>
> Kind regards,
> Dmitri
> ---
> Tōkyō Made - https://tokyoma.de/
>
> August 8, 2021 2:51 PM, "CD.Graesser" <cd.graesser at gmail.com> wrote:
>
> > Some additions to the good explanation from Andrew.
> >
> > Jpeg makes a frequency analys of the image and then it cuts away high
> image content frequencies
> > where just a little change is in luminance or color. Example: The
> details (=high frequencies) of
> > small branches of a tree infront of a grey sky are well maintained in
> Jpeg75% compression, but the
> > detailes of allmost same green leaves of a bush are smeared out in Jpeg
> 75%, while they are almost
> > perfect at compression above 90%. I use 95% compression in jpeg after
> editing in Digikam.
> >
> > The coefficients for the frequency analyis may differ from jpeg vendor.
> Thus recompression gives a
> > different representation of the image.
> >
> > Jpeg works on blocks of 8x8 pix.
> >
> > Besides the smearing of details, 8x8 pixel blocks appear to be visible
> with a different luminance
> > or color than the adjacent 8x8 block.
> >
> > The default of jpeg compression 75% in digikam is way too low and thus
> it makes especially to
> > detailrich complex images visible quality issues.
> >
> > I use 95% when storing in jpeg in DK which is in my opinion similar to
> superfine setting of the
> > cameras.
> >
> > One reason for me to use sometimes RAW are the jpeg compression and
> noise reduction artefacts from
> > the camera. Both reduce the content of details and thus result in
> smaller filesize.
> > I shoot most times in the camera Jpeg superfine with low noise
> reduction, then I do in DK
> > whitebalance tool some brightness, gamma, color saturation and
> whitebalance correction and store it
> > again in jpeg 95% in DK.
> >
> > Happy dk-ing!
> >
> > Christian Graesser
> >
> > Andrew Goodbody <ajg02 at elfringham.co.uk> schrieb am Sa., 7. Aug. 2021,
> 23:31:
> >
> >> On 07/08/2021 20:33, Dmitri Popov wrote:
> >>> Hi Andrew,
> >>>
> >>> Thank you very much for your detailed reply.
> >>>
> >>>> But I suspect the most likely answer is that the original was saved
> at less than 100% quality.
> >>>
> >>> Just out of curiosity, how is that possible? I mean, if the original
> (let's call it IMAGE A) was
> >> saved with less than 100% quality, then some information would be
> inevitably discarded. Logically,
> >> saving IMAGE A at 100% shouldn't result in IMAGE B that is larger than
> IMAGE A. Because you can't
> >> have something more out of nothing. Am I making any sense?
> >>
> >> But yes, you do get something more out of nothing. Except that it is not
> >> out of nothing, it is the result of expanding information saved in a
> >> lossy compression into a representation of the original image. Not
> >> everything in that original image was accurately described so the
> >> expanded image is a reconstruction that is close but not identical to
> >> the original image. But it has the same dimensions as the original.
> >>
> >> Image A saved at 75% quality (some information lost, but still basically
> >> looks OK) but then expanded to be worked on gives you a working image
> >> that is the same dimensions as the original but the missing information
> >> is filled in with a best guess. This is how lossy compression works.
> >> Some information is lost but is replaced with a best guess based on the
> >> surrounding information (this is over simplified but good enough for
> here).
> >> You now save that as image B at 100%, this will now include a
> >> representation of the whole image including the parts that were filled
> >> in by best guess. So the added size is describing information that was
> >> not in image A. But when you reopen image B it will be a more exact
> >> representation of the image when saved.
> >>
> >> You cannot edit jpeg data directly. You can only edit the expanded
> >> image. So when the expanded image is saved it is compressed again. You
> >> are not saving an edited version of the original compressed data, you
> >> are saving a compressed version of the edited image. Saving at a higher
> >> quality will take more data.
> >>
> >> Andrew
>
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