[digiKam-users] JPEG grows bigger. Why?

CD.Graesser cd.graesser at gmail.com
Sun Aug 8 13:51:50 BST 2021

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,

> 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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