At 11:25 AM +0200 18/4/11, Žorvaršur Davķšsson wrote:
>On 07.04.2011, at 03:42, Geoffrey Heard wrote:
>
>> It is desirable to avoid editing in iPhoto as
>>it applies rather savage lossy compression each
>>time you edit and save and you have no control
>>over it, so it is best to edit in an external
>>program and save without loss back into iPhoto.)
>
>This may be the right time to ask about
>something I always wanted to know about but was
>afraid to ask.
>
>Every time I open a jpg file in Finder with a
>double-click it is decompressed when it opens.
>When I close the file it is compressed again. If
>I understand this process correctly then
>repeatedly compressing and decompressing a jpg
>file will cause it to progressively lose
>quality. Therefore I'm beginning to ask myself
>whether it is not best to convert ALL valuable
>Photos (such as family photos) to PNG (which is
>a lossless format) with GraphicConverter
>*before* I import them into iPhoto.
>
>Most of my photos have been downloaded from the
>Internet and they are all in jpg format. Should
>I also convert them to PNG? Or do I have to open
>and close a jpg file several thousand times
>before I begin to notice any quality loss?
>
>Pictures are really not my strength, therefore I
>would be grateful for any recommendation
>regarding this.
hello Žorvaršur
When you double click and open the JPEG to look
at it, it doesn't get compressed again when you
close it, it stays the same. JPEG compression is
not like packaging something in a bag and
squashing it, it is making a permanent change to
the contents so they are smaller. Basically,
differences are removed. In an uncompressed file,
you might have a bunch of 64 pixels (number
simply pulled out of the air for illustration)
that are all a little different and therefore all
are described separately so the file is big;
after JPEG compression, if the differences are
small within the limits of compression applied,
then all those 64 pixels will be considered the
same, so they will all have the same description,
and that one description will simply be applied
to all 64 pixels. But where you had a fine
gradation beforehand, now you have a single tone.
So while the file has been made much smaller, it
has lost detail and that lost is irrevocable..
So in the end the file is smaller because there
is less information in it! It's not like
packaging up a bunch of code in a .zip or .bin
file which expands when you open it. With JPEG
compression of graphics, you actually have
discarded information which cannot be retrieved.
This happens only when you save the file, with
compression applied. Not when you simply open it
to look at it. However, if you open it then save
it again with more compression applied, you lose
further detail because the existing file, whether
it has been compressed or not, is regarded as the
100% original, so a 70% quality level saving is
70% compared with the file's 100%, even though
that file is already at 70% quality compared with
the original. So it groups out 64 pixels with
other neighboring pixels, which weren't grouped
with it in the first saving and the area of a
single tone is increased again ...
PNG format? That's a web format. It is pretty
stripped out but superior to JPEG in retaining
transparency (JPEG makes transparent areas
white). I'm not sure how iPhoto handles PNG. I
would store important photos in TIFF format. It's
big but it works. There is non-lossy compression
available.
The way around all this is to specify in the
iPhoto prefs that you will use an external
editor. I am using PhotoLine, you will prefer
Graphic Converter, Erick (and one of two others
around the world) use Photoshop. Once you have
selected that in the prefs, then when you double
click on a thumbnail in iPhoto, it launches the
external converter with the photo open ready for
editing. When you save back, it saves in whatever
format you specify in the external program. I'm
currently saving back in TIFF or 100% JPEG (no
compression in PhotoLine).
iPhoto Buddy came in before iPhoto allowed
multiple libraries. it remains a handy way to
manage multiple libraries; handy for me at any
rate. And since it is donation-ware, it doesn't
cost much.
Cheers, geoff
Geoffrey Heard
Business & Environment Writer, Editor, Publisher
The Worsley Press
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