Although generally JPEG is lossy, there are some implementations where it is
lossless.
This old FAQ's is very out of date, but has great discussion of issues.
http://www.faqs.org/faqs/jpeg-faq/part1/
>>
Lossless JPEG has never been popular --- in fact, no common applications
support it --- and it is
now largely obsolete. (For example, the new PNG standard outcompresses
lossless JPEG on most
images.) Recognizing this,the ISO JPEG committee recently finished an all-new
lossless
compression standard called JPEG-LS (you may have also heard of it under the
name LOCO).
JPEG-LS gives better compression than original lossless JPEG, but still nowhere
near what you can
get with a lossy method. It's anybody's guess whether this new standard will
achieve any
popularity.
It's worth repeating that cranking a regular JPEG implementation up to its
maximum quality setting
*does not* get you lossless storage; even at the highest possible quality
setting, baseline JPEG
is lossy because it is subject to roundoff errors in various calculations.
Roundoff errors alone
are nearly always too small to be seen, but they will accumulate if you put the
image through
multiple cycles of compression (see section 10).
Many implementations won't even let you get to the maximum possible
setting,because it's such an
inefficient way to use regular JPEG. With the IJG JPEG software, for example,
you have to not
only select "quality 100" but also turn off chroma downsampling to minimize
loss of information.
The resulting files are far larger and of only fractionally better quality than
files generated at
more reasonable settings. And they're still slightly lossy!
If you really need lossless storage, don't try to approximate it with regular
JPEG.
<<
AND:
>>
Strictly speaking, JPEG refers only to a family of compression algorithms; it
does *not* refer to
a specific image file format. The JPEG committee was prevented from defining a
file format by
turf wars within the international standards organizations.
Since we can't actually exchange images with anyone else unless we agree on a
common file format,
this leaves us with a problem. In the absence of official standards, a number
of JPEG program
writers have just gone off to "do their own thing", and as a result their
programs aren't
compatible with anyone else's.
The closest thing we have to a standard JPEG format is some work that's been
coordinated by people
at C-Cube Microsystems. They have defined two JPEG-based file formats:
* JFIF (JPEG File Interchange Format), a "low-end" format that transports
pixels and not much else.
* TIFF/JPEG, aka TIFF 6.0, an extension of the Aldus TIFF format. TIFF is
a "high-end" format that will let you record just about everything you
ever wanted to know about an image, and a lot more besides :-).
JFIF has emerged as the de-facto standard on Internet, and is what is most
commonly meant by "a
JPEG file". Most JFIF readers are also capable of handling some
not-quite-JFIF-legal variant
formats.
<<
For those interested in the math involved in the original MPEG method:
http://www.faqs.org/faqs/compression-faq/part2/section-6.html
When you realise that to do high compression they are allowed to use decimation
of the chroma
information by factors of 2 or 4 , you know this is terrible for non-human
things like ocr or
automatic inspection software. Often default websettings for quality are to set
2:1 "chroma
subsampling" (decimatiom) on all images. PS apparently changes from 1:1 (none)
to 2:1 somewhwere
along the % qaulity setting, but it depends on version. Worse it does not alert
you when it is
changing.
Here is an interesting site showing that the jpeg files themselves, are
losslessly compressible a
bit further. Normally general purpose compression pgms like pkzip, can't
compress jpegs at all.
http://compression.ca/act/act-jpeg.html
Here is a site with some practical examples illustrating some of the issues:
http://www.photo.net/learn/jpeg/
I like the color information decimation issues illustrated there, under
"chroma-subsampling"
jpeg2000 algorithms, support completely lossless and 16 bit compression, but
there is not much
support for jpeg2000!
Regards,
Tim Hughes
--- Brian Swale <bj@xxxxxxxxxxxxxxx> wrote:
>
> Hi all,
>
> > Tim Randles wrote:
> >> what is wrong with JPEG format?
>
> Other replies did not point out that, recognising that jpg is a lossy format
> (ie,
> if you keep editing the same jpeg image it gets screwed up horribly), there
> are some image editing programs that always convert the jpeg image to
> something else (a not lossy format) while the editing is being done.
>
> Programs that behave this was include
>
> Embellish
> Adobe Photo Deluxe
> Gimp ( I think)
>
> I suspect all the Adobe Photoshop versions do, but I don't run any because
> they are too big. Not forgetting the cost, which is also too big.
>
> I am not at all sure if Irfanview does.
>
> Brian
>
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