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JPEG lossless re-sizing?

 
 
Bob & Anni
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      12-07-2003
I know it's possible to do JPEG lossless rotation and cropping but is it possible to do lossless re-sizing as well?
Starting with large images (pixel count) I use jpegcrop to crop to the required composition but then I want to resize the resulting
image to fit my web pages.
--
Bob


 
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Bob & Anni
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      12-07-2003
Thinking about this, it's probably only possible to do a lossless resize to 1/2 1/4 1/8 1/16th etc
Just realised that it wouldn't help me anyway as I need to add a logo and sometimes adjust the contrast or colour, so that would
probably mean recompressing anyway
Interesting subject never the less
--
Bob


 
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Andy Hewitt
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      12-07-2003
Bob & Anni <(E-Mail Removed)> wrote:

> Thinking about this, it's probably only possible to do a lossless resize
> to 1/2 1/4 1/8 1/16th etc Just realised that it wouldn't help me anyway as
> I need to add a logo and sometimes adjust the contrast or colour, so that
> would probably mean recompressing anyway Interesting subject never the
> less


Why not convert the image to a non compressed file format, such as TIFF
or PICT, even the editor's own format (Photoshop has it's own file
format for example). Ensure you start off the with minimal compression
JPG file though.

You can then edit and resize with minimal, or no, loss. Save as a JPG at
the final stage.

If you're making images for web pages, you need to keep size below
640x480 anyway, and preferably keep the 4/3 ratio as well.

--
Andy Hewitt ** FAF#1, OSOS#5 - BMW K100RS 8v, Honda Concerto 16v
Windows free zone (Mac G3)
<http://homepage.ntlworld.com/ahewitt/index.htm> (last update 12.02)
 
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Trev
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      12-07-2003

"Bob & Anni" <(E-Mail Removed)> wrote in message
news:TaDAb.18061$(E-Mail Removed)9.net...
> Thinking about this, it's probably only possible to do a lossless

resize to 1/2 1/4 1/8 1/16th etc
> Just realised that it wouldn't help me anyway as I need to add a logo

and sometimes adjust the contrast or colour, so that would
> probably mean recompressing anyway
> Interesting subject never the less
> --

I think you mean without too much of a loss. It is impossible to remove
75% of the pixels with out a loss of 75%.
On the other hand the jpeg compression of a 600 x 400 does not need to
be high enough to cause bad artefacts when getting the file size down


 
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Roland Karlsson
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      12-07-2003
"Bob & Anni" <(E-Mail Removed)> wrote in
news:q3DAb.18057$(E-Mail Removed)9.net:

> I know it's possible to do JPEG lossless rotation and cropping but is it
> possible to do lossless re-sizing as well? Starting with large images
> (pixel count) I use jpegcrop to crop to the required composition but
> then I want to resize the resulting image to fit my web pages.


Lossless downsizing is never possible, JPEG or not.

Lossless upsizing is not possible with JPEG. JPEG is
coded in 16x16 squares. Each such square is transformed
from the spatial to the frequency plane and then the
resulting frequency representation is compressed lossy.


Roland
 
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Andrew Koenig
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      12-07-2003
> I know it's possible to do JPEG lossless rotation and cropping but is it
possible to do lossless re-sizing as well?

What does "lossless re-sizing" mean? Reducing the number of pixels while
retaining the same number of pixels?


 
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Toke Eskildsen
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      12-07-2003
Andrew Koenig wrote:

>> I know it's possible to do JPEG lossless rotation and cropping
>> but is it possible to do lossless re-sizing as well?

>
> What does "lossless re-sizing" mean? Reducing the number of
> pixels while retaining the same number of pixels?


Given that lossless cropping of JPEGs is also an oxymoron, but that
most people understand the meaning (avoiding artefact introducing
recompression), a bet would be that Bob & Anni wants to do the same
for resized images.


I wouldn't be too worried about the errors introduced by a complete
recompression, if we're talking about resizing to a smaller image. The
old JPEG artefacts would likely be rendered invisible by the resizing,
so only new artefacts would be visible. Besides, for webuse I'd
normally compress harder than for other use so a recompression would
be worth considering even if it was possible to do without.
 
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Bob & Anni
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      12-07-2003
At last, someone who understood what I meant by lossless - you pedantic lot
I naturally thought everyone would understand that reducing the size in pixels would constitute an OBVIOUS "loss" but I was
referring to artefacts and the overall effect this has on the overall quality.
I'll try and be more precise next time, seems so many people are quick to jump in with smart comments.

Thanks Andrew, yes as I said in my second post, it's probably not possible to do an arbitrary resizing without recompressing.
I've always taken the jpegs from the camera (it only offers jpegs before anyone jumps in with a comment) and converted them to tiff
before doing any work on them. Converting back to jpeg just prior to posting to the web.
--
Bob


"Toke Eskildsen" <(E-Mail Removed)> wrote in message news:Xns944AADDE9DE88tokeeskildsen@130.133.1.4...
> Andrew Koenig wrote:
>
> >> I know it's possible to do JPEG lossless rotation and cropping
> >> but is it possible to do lossless re-sizing as well?

> >
> > What does "lossless re-sizing" mean? Reducing the number of
> > pixels while retaining the same number of pixels?

>
> Given that lossless cropping of JPEGs is also an oxymoron, but that
> most people understand the meaning (avoiding artefact introducing
> recompression), a bet would be that Bob & Anni wants to do the same
> for resized images.
>
>
> I wouldn't be too worried about the errors introduced by a complete
> recompression, if we're talking about resizing to a smaller image. The
> old JPEG artefacts would likely be rendered invisible by the resizing,
> so only new artefacts would be visible. Besides, for webuse I'd
> normally compress harder than for other use so a recompression would
> be worth considering even if it was possible to do without.



 
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Samuel Paik
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      12-07-2003
Roland Karlsson <(E-Mail Removed)> wrote:
> Lossless upsizing is not possible with JPEG. JPEG is
> coded in 16x16 squares. Each such square is transformed
> from the spatial to the frequency plane and then the
> resulting frequency representation is compressed lossy.


I'll have to think about the details, but power of two
magnifications ought to be doable.

Sam
 
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Robert E. Williams
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      12-07-2003


Bob & Anni wrote:

> At last, someone who understood what I meant by lossless - you pedantic lot
> I naturally thought everyone would understand that reducing the size in pixels would constitute an OBVIOUS "loss" but I was
> referring to artefacts and the overall effect this has on the overall quality.
> I'll try and be more precise next time, seems so many people are quick to jump in with smart comments.
>
> Thanks Andrew, yes as I said in my second post, it's probably not possible to do an arbitrary resizing without recompressing.
> I've always taken the jpegs from the camera (it only offers jpegs before anyone jumps in with a comment) and converted them to tiff
> before doing any work on them. Converting back to jpeg just prior to posting to the web.
> --
> Bob


Converting from a jpeg to tiff BEFORE working on an image buys you nothing.
You can't INCREASE on the information content of the original jpeg.
When you open a jpeg image in Photoshop, PS uncompresses the image to .psd format in much the same way that tiff would do. So you are
always working on an uncompressed image file. Only when you SAVE the image does it make sense to convert to .tiff rather than .jpeg.
SAVING in jpeg will apply another layer of compression on top of the original .jpeg degrading it somewhat . Having said that, however,
I might add that you can SAVE in highest quality .jpeg compression quite a few times before the difference in quality is noticeable.
Bob Williams

 
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