[Digikam-users] jpg to png conversion

[Andrew Goodbody]

On 13/01/12 13:56, Dr. Martin Senftleben wrote:
> Am 13.01.2012 10:25, schrieb Andrew Goodbody:
>> I have to ask why you converted jpg files to png?
>
> I'm not sure if that's a rhetorical question. Anyway, I was doing some
> changes on the images, and didn't want to lose more info due to the
> jpg-compression.

No, not a rhetorical question. If you have a choice and you want to edit 
photos then it is best to start with raw files. Otherwise a lossless 
format such as tiff or png is better than jpg to start from.

> I think of it this way (maybe I'm wrong): every time I change a jpg
> image, some info of the origional gets lost and the image cannot ever
> be put in the original state by just reverting the changes, because it
> drops the original info and adds in some new based on the changes.
> But (that's what I think) when I change a png image, I can get the
> original state back by reverting the changes.

Yes, you will lose more information every time you re-save a jpg file. 
However there is no way that I know of to revert edit changes made to a 
png image without keeping a copy of the original file. In both png and 
jpg any edits permanently affect the image data that is saved, it is 
just with png the saving process does not cause a loss of data but with 
jpg it does.

> I knew that png makes the files bigger, but I still do not understand
> how the jpg can become 10 times bigger by converting it to png. Even
> the raw images of the same picture are less than half the size of the
> png file.
> I'll probably never fully understand the mysteries behind this,
> because I can't even see the difference. I'll probably stick to jpg
> even though some information is not stored in the file. Why don't
> camera manufacturers use a non-lossy format that produces smaller
> images than the raw images? Or isn't that possible at all?

Sticking with jpg is probably just fine for you if you are not noticing 
any difference in quality. You may like to read this link
http://dptnt.com/2012/01/does-saving-a-jpeg-multiple-times-reduce-image-quality/
Raw files, like most formats, are compressed. Converting to another 
format, from raw, will require de-mosaicing which will actually increase 
the amount of data that needs to be stored. So getting a lossless format 
to be smaller than raw would be quite a challenge.
jpg is the fallback because it can be used as it is, everywhere.

>
> Regards
>
> Martin

Andrew