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Re: [OM] OT: JPEG lossiness

Subject: Re: [OM] OT: JPEG lossiness
From: Garth Wood <garth@xxxxxxxxxxxxxxx>
Date: Fri, 15 Jan 1999 06:59:13 -0700
At 11:17 AM 1/15/99 +0100, Omer Nezih GEREK wrote:

[snip]

>As for the second sentence, actually it is JPEGs weak point not 
>to keep the general trend of colour change accross the blocks. Therefore, 
>usually smooth images are unintentionally compressed too much and the 
>8x8 blocks become visible. I would like to suggest that you produce an 

>artificial very smooth "wave" image and a very high contrast "busy" 
>image, and code at 40etting. You will see that at the same setting, 
>the smooth image has a smaller JPEG but the JPEG image is unbearable. Low 
>pass (smooth) images and sharp transitions of smooth regions (graphics 
>art images) are bad for JPEG.

This has been my experience working with JPEGs of certain subjects with 
continuous shading, particularly photos with large areas of blue sky with 
gradual transitions from light to dark areas of blue.  Too much JPEG 
compression "blocks" the sky, producing weird stepping.  Thus, in the 
Unofficial Olympus Web Photo Gallery, Dr. Matthew J. Cordery's Australia and 
New Zealand submissions have areas of sky with this stepping artifact, which I 
find unpleasant.  Unfortunately, to try and keep the file sizes somewhat down, 
I had little choice.

Compare these to a picture by Branko Turk in the Gallery.  This is a photo of 
an older brick office building which I originally tried to save at quality 
level "5" in Photoshop.  Observing the result, I found that the image was 
indistinguishable from the original submission, and the file size had hardly 
changed at all.  So then I tried saving it at "1".  Surprise!  Almost no change 
in quality, and very little change in file size (it was still relatively large).

I've received complaints from people who say that some of the images in the 
Gallery are too large, and that I should compress them more.  Respectfully, 
I've had to disagree -- even the compression I've done on some of the photos 
has done harm to them, in my opinion.  Matthew Cordery's original scans were 
breathtaking -- the stuff I was forced to post in the Gallery, while nice, was 
nothing compared to the originals.  JPEG forced compromises between size of 
file and quality of image.

I hate compromises like that.

Garth


 
"A bad day doing photography is better
 than a good day doing just about 
 anything else."
 
The Unofficial Olympus Web Photo Gallery at:

   http://www.taiga.ca/~gallery/


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