There has been a lot of talk lately about improvements in the jpeg
compression ideas being used (Allume -- makers of StuffIt -- has some
new stuff out that is supposed to shrink jpeg files up to 30% without
any loss of quality) and a lot of speculation about how Apple is
somehow reading the scheme used to compress the original imported jpeg,
then using the exact same scheme to compress the newly saved image
(this is supposed to maintain quality longer, through more levels of
edits --- how Apple does this is also supposed to be a closely guarded
secret -- at least according to the tin-hat-wearing folks, a bit
doubtful since if Apple can get this info out of an arbitrary jpeg
image, so can anyone else.)
Not being a big iPhoto user (Go GIMP! Go PhotoShop!), I have only
followed those discussions out of idle curiosity.
But in terms of how iPhoto does do some of its wonders, you need to be
careful here. Apple cheats. (Reeeeaaalllllyyyy big toothy grin!!!!!)
If you import a jpeg photo, make changes, quit, come back make changes,
quit et.c etc. the saved photo looks really good. Okay so far. But
there is a menu option that says "Revert to Original". Choosing this
takes you all the way back to the original imported image. So what
Apple is doing is keeping a history of your editing for the photo and
applying that to the original and showing you that. (Also if you take a
gander through the iPhoto library you will see lots of data files
popping up for your albums, especially after you start editing).
On the other hand, if you edit a photo, then Export that, then import
it back in, make changes, export it and import it again, etc. then you
will begin to see image quality degradation within a few generations
(using the jpeg format) that do not show up for the same edits on the
one that stays inside of iPhoto the whole time.
One thing that is creeping into the lexicon is the idea of what
"lossless" means. With these new attempts to make jpeg last longer
(instead of switching to the better jpeg2000 -- for jpeg compression)
there is a lot of talk of thing being lossless in image quality, but
not being a bit-for-bit identical copy. Hmm, it does sound like the
words are going to be altered here soon.
Shoot RAW NEFs, it's just better (smile).
Jerry
On Mar 28, 2005, at 11:00 AM, John Robinson wrote:
> Lee,
>
> Thanks, as usual for a great explanation, but just for your info. my
> eyes glaze over on most anything you say as you speak so far above my
> capabilities, but boy do I ever learn from you!!
>
> John R.
>
>
>
>
> On Mar 28, 2005, at 10:54 AM, Lee Larson wrote:
>
>> On Mar 27, 2005, at 9:22 PM, Bill King puzzled:
>>
>>> I just read an interesting article posted on MacSurfer from the
>>> Syracuse Post-Standard newspaper. It concerned the ability of
>>> iPhoto 5 to repetitively save a picture using JPG compression and
>>> apparently losslessly.
>>>
>>> I would love to hear for any graphics compression experts about the
>>> authenticity of this technique...
>>
>> I'm not a graphics expert, but I think I can explain what's going on.
>>
>> The JPEG scheme is actually a whole collection of different
>> compression techniques all lumped into one standard. The most common
>> one chosen is what's called "discrete cosine transform" (DCT)
>> compression. Within the DCT algorithm, you can choose the amount of
>> information to be retained. (It's just the value of a constant in the
>> formula.) The more information that's retained, the larger is the
>> size of the compressed file. It's possible to choose lossless DCT
>> compression, at the expense of almost no compression for complicated
>> images.
>>
>> You can see this in programs like Canvas, which has a slider control
>> to select the quality of the output image. Sliding it over to 100%
>> results in a big file with no quality loss.
>>
>> The JPEG scheme also includes a lossless algorithm called entropy
>> encoding which I believe is less often used than the DCT.
>>
>> I don't know which scheme Apple uses, but It's probably one of those
>> two.
>>
>> As a postscript to this, let me note that I've had disagreements with
>> so-called experts about this. They claimed that JPEG is an inherently
>> lossy format while I countered that it need not be. Most "experts"
>> don't really understand the capabilities of JPEG because most
>> programs don't take advantage of JPEG's real capabilities. I bring up
>> the DCT and their eyes glaze over.
>
>
>
> | The next meeting of the Louisville Computer Society will
> | be March 22. The LCS Web page is <http://www.kymac.org>.
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>
>
-----------------------------------
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| The next meeting of the Louisville Computer Society will
| be March 22. The LCS Web page is <http://www.kymac.org>.
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