Archived posting to the Leica Users Group, 2006/02/25
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]If I have understood things correctly if you make a modification to an expanded jpeg then re-store it at as a new jpeg it is re- compressed into a completely new lossily compressed file. Since jpeg compression is not lossless and relies on masking to make a not too hideous job of throwing data away each time it is re-stored as a jpeg, after any changes have been made a new different set of data is discarded by the algorithm such that each re-compression results in further degradation. If you make no changes the algorithm may still not follow exactly the same compression on the expanded file than was done on the original unadulterated data, since some of it has been discarded and reconstructed to some extent inaccurately. Best not to do it, Frank On 25 Feb, 2006, at 15:35, Raimo K wrote: > So why it is not compressed - smaller, that is - the new file can > even be larger than the original. I know that no new information > cannot be created but does not compressing make files smaller - > that is the very idea. > All the best! > Raimo K > Personal photography homepage at: > http://www.uusikaupunki.fi/~raikorho > > > ----- Original Message ----- From: "Eric" <ericm@pobox.com> > To: "Leica Users Group" <lug@leica-users.org> > Sent: Saturday, February 25, 2006 2:25 PM > Subject: Re: [Leica] Daily Dose of Spring XI > > >> Raimo: >> >>> If you select to save JPGs using original quality it will not be >>> compressed again - at least the programs I use do it this way. >> >> I would be careful. All JPG compression is lossy. That is, when >> you save >> the jpeg file, it throws away some information. So the next time >> when you >> open the file, it doesn't have 100%. Which is fine for saving >> once and >> viewing. But if you save again, then you're tossing out more >> information. >> Viewing jpegs is fine. And there are some programs that can >> rotate jpegs >> without losing information. But if you save again, you will lose >> information. If you save using the original quality, I'm guessing >> that >> means use the same level of compression that was first used. If >> you change >> an image and then save the jpeg again, it *will* be compressed again. >> That's the way jpeg works. >> >> >> >> -- >> Eric >> http://canid.com/ >> >> _______________________________________________ >> Leica Users Group. >> See http://leica-users.org/mailman/listinfo/lug for more information > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information