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Jim Townsend wrote:
> it is possible to do 90-degree rotations and flips losslessly,
> if the image dimensions are a multiple of the file's block size
> (typically 16x16, 16x8, or 8x8 pixels for color JPEGs).

Oh Oh.
I always use the Irfanview lossless rotation plugin on Windows to
rotate photos of my kids at 90 degrees, 180 degrees, and 270 degrees.

QUESTION #1:
Does this mean the 180 and 270 degree rotations are actually lossy
(but not the 90 degree rotations)?

I also often crop my photos of the kids in many cases, to non-standard
sizes.
QUESTION #2:
Does cropping destroy the 8x8 pixel multiplication such that a
subsequent Irvanview lossless rotation suddenly becomes lossy even at
90 degrees?

Scared,
Susan Henderson




> Yes, it's lossless.. Because the operation doesn't open
> the file first, it doesn't have to re-compress it.. It's
> the compression that causes the loss.
>
> There's a good explanation on just about everything you
> wanted to know about JPEG here:
> ``
> http://makeashorterlink.com/?Z1BB6294B
>
> Search the article for rotation and you'll find:
>
>
> "There are a few specialized operations that can be done on a JPEG file
> without decompressing it, and thus without incurring the generational loss
> that you'd normally get from loading and re-saving the image in a regular
> image editor. In particular it is possible to do 90-degree rotations and
> flips losslessly, if the image dimensions are a multiple of the file's
> block size (typically 16x16, 16x8, or 8x8 pixels for color JPEGs). This
> fact used to be just an academic curiosity, but it has assumed practical
> importance recently because many users of digital cameras would like to be
> able to rotate their images from landscape to portrait format without
> incurring loss --- and practically all digicams that produce JPEG files
> produce images of the right dimensions for these operations to work. So
> software that can do lossless JPEG transforms has started to pop up. But
> you do need special software; rotating the image in a regular image editor
> won't be lossless".

On Sun, 19 Jun 2005 13:34:31 -0700, 0m (Paul Mitchum)
wrote:

->Ralf Schmode <> wrote:
->
->> Dimitrios Tzortzakakis wrote:
->>
->> > Lossless means identical to the uncompressed original.AFAIK jpeg
is a
->> > lossless compression format, in contrast to MPEG which is lossy.
->>
->> Hi, Dimitrios,
->>
->> sorry, that is not true. JPEG compression, as offered by consumer
image
->> editing software on the market, is lossy even at 100% quality
setting.
->
->This isn't strictly true any more. There's a newish format called
->JPEG2000 which has a lossless setting. Unfortunately, not much
software
->supports JPEG2000 at the moment, especially the software in your
camera.


Actually JPEG2000 isn't lossless either. But with the new compression
algorithms used it is much less lossy than JPEG or any other lossy
formats. It's advertised as lossless because it is visually lossless to
some pretty high level of compression. But if the developers are really
pushed on it they will admit that it is technically still a lossy
format. So for most general photographic purposes I guess it could be
said to be lossless. I suspect that there is still some number of
manipulations and resaves which would eventually render it visually
lossy as well.
MikeZ

MikeZ wrote:
[]
> Actually JPEG2000 isn't lossless either. But with the new compression
> algorithms used it is much less lossy than JPEG or any other lossy
> formats. It's advertised as lossless because it is visually lossless
> to some pretty high level of compression. But if the developers are
> really pushed on it they will admit that it is technically still a
> lossy format. So for most general photographic purposes I guess it
> could be said to be lossless. I suspect that there is still some
> number of manipulations and resaves which would eventually render it
> visually lossy as well.
> MikeZ

Both standard JPEG and JPEG 2000 do offer a truly lossless compression
option with zero data loss, but this option is rarely implemented in
consumer software, and (obviously) does not produce such small files.

David

thx for all the answers, anyway, lossy or not, I like my digital camera very
much, while I think that it will be a long time when I 'll replace my 8mm
camcorder.

--
Tzortzakakis Dimitrios
major in electrical engineering, freelance electrician
FH von Iraklion-Kreta, freiberuflicher Elektriker
dimtzort AT otenet DOT gr
Ï "David J Taylor"
<david-> Ýãñáøå
óôï ìÞíõìá news:KOtte.53131$ k...
> MikeZ wrote:
> []
> > Actually JPEG2000 isn't lossless either. But with the new compression
> > algorithms used it is much less lossy than JPEG or any other lossy
> > formats. It's advertised as lossless because it is visually lossless
> > to some pretty high level of compression. But if the developers are
> > really pushed on it they will admit that it is technically still a
> > lossy format. So for most general photographic purposes I guess it
> > could be said to be lossless. I suspect that there is still some
> > number of manipulations and resaves which would eventually render it
> > visually lossy as well.
> > MikeZ
>
> Both standard JPEG and JPEG 2000 do offer a truly lossless compression
> option with zero data loss, but this option is rarely implemented in
> consumer software, and (obviously) does not produce such small files.
>
> David
>
>

Susan (Graphic Artist) wrote:
> Jim Townsend wrote:
>
>>it is possible to do 90-degree rotations and flips losslessly,
>>if the image dimensions are a multiple of the file's block size
>>(typically 16x16, 16x8, or 8x8 pixels for color JPEGs).

> Oh Oh.
> I always use the Irfanview lossless rotation plugin on Windows to
> rotate photos of my kids at 90 degrees, 180 degrees, and 270 degrees.
>
> QUESTION #1:
> Does this mean the 180 and 270 degree rotations are actually lossy
> (but not the 90 degree rotations)?

No. Lossless rotation is possible with any multiple of 90 degrees.

> I also often crop my photos of the kids in many cases, to non-standard
> sizes.
> QUESTION #2:
> Does cropping destroy the 8x8 pixel multiplication such that a
> subsequent Irvanview lossless rotation suddenly becomes lossy even at
> 90 degrees?

Multiple questions here.

A crop that is not a multiple of 8 (or sometimes 16) will become a
multiple of 8 (or 16) when losslessly rotated. There is no loss of
quality, but you might lose a few pixels around the edges. This is
not what most people mean by "lossy".

Lossless cropping is possible, but it is constrained by the same
8 (or 16) pixel boundary issue as lossless rotation. A lossless
crop will always be a multiple of 8 (or 16) pixels in each dimension.

Cropping in an image editor is lossy if the software has to decompress
the JPEG in order to show it to you and then re-compress the result of
the crop. Irfanview doesn't run on my OS, so I don't know anything
about it. I use jpegtran for lossless cropping and rotation under
Linux.

Paul Allen

"Freedom55" <"joinertake this out"@ns.sympatico.ca> wrote in message
news:Y8ete.49313$...
> I have lossless rotation for digital photos in PSP8. Does lossless
> really mean lossless? I mean there has to be some loss in a jpeg when
> you rotate it.

Display a jpeg on your monitor, then pick up the monitor and physically
rotate it by 90 degrees. How much data have you lost by doing this?
Rotating a jpeg by 90 degrees is a rows-and-columns book-keeping issue which
does not require the existing data in the file to be re-compressed, only
rearranged. Some restrictions apply, as the travel ads say:

" Lossless Rotation: The rotation of an image without the loss of image
quality. This works best on images with dimensions that are a multiple of 8
or 16. ACDSee digital photo software includes a lossless rotation function.
"

--From the ACDSee glossary, obtained using the Google search expression '
define: "lossless rotation" '. Include the double quotes, but not the
singles.


But always remember that even data in a normal jpeg rotation are never truly
lost--they're just waiting for you in jpeg heaven. Or so it is written.