Archived posting to the Leica Users Group, 2006/02/25
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Of course it does! :-) If you have generated a new original by expanded the jpeg then re-compress it there may or may not be spurious data in the modified file which is not discarded by the compression algorithm when you re-compress it, resulting in a different file size to the original. If you don't understand try it a few times and see if you notice any difference. If you don't it will be OK for you. Frank On 25 Feb, 2006, at 19:19, Raimo K wrote: > Sure - but this does not answer the question. > All the best! > Raimo K > Personal photography homepage at: > http://www.uusikaupunki.fi/~raikorho > > > ----- Original Message ----- From: "Frank Dernie" > <Frank.Dernie@btinternet.com> > To: "Leica Users Group" <lug@leica-users.org> > Sent: Saturday, February 25, 2006 7:41 PM > Subject: Re: [Leica] Daily Dose of Spring XI > > >> 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 >> _______________________________________________ >> 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