[digiKam-users] JPEG grows bigger. Why?

[Dmitri Popov]

Hi Andrew,

Thank you so much for your reply! I've learned something new today. :-)

Kind regards,
Dmitri
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Tōkyō Made - https://tokyoma.de/

August 7, 2021 11:31 PM, "Andrew Goodbody" <ajg02 at elfringham.co.uk> wrote:

> 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