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RE: [OM] Scanning Res for Web

Subject: RE: [OM] Scanning Res for Web
From: Garth Wood <garth@xxxxxxxxxxxxxxx>
Date: Wed, 01 Dec 1999 07:06:53 -0700
At 10:06 AM 12/1/99 +0000, Ian A. Nichols wrote:
> 
>
>On Tue, 30 Nov 1999, Garth Wood wrote:
>
>> Depending on the luck of the draw, some images can withstand a
>> huge amount of compression with little if any noticeable effect on the
>> perceived quality of the image.  Other images start to noticeably
>> degrade almost immediately (that is, with very little lossy
>> compression applied).  Still others will compress very little even
>> when cranking up the compression to the maximum, and there will also
>> be virtually no perceived loss of!
>
>Indeed, it tends to be those images with low contrast or large areas
>of similar colour that compress well (with lossy or lossless
>compression).  Those with a stark subject set against a plain background
>also compress well but tend to show up artifacts around the edges of the
>subject at relatively low (JPEG) compression.

"Compress well" is relative, I guess.  Yeah, lossy compression works well on a 
severe clear blue sky, but artifacts galore show up, and the smooth 
transitional nature of the blue expanse is shot all to Hell.  Instead, you get 
"banding."

[snip]

>> That's why it's a crapshoot doing JPEG compression.  The best general
>> advice I was ever given was "Save as lossless, open, rename file, and
>> start cranking up the lossy compression, viewing the results at each
>> step.  At some point, image degradation will become unacceptable *to
>> you*.  This is when you stop." 
>
>I used to try that but it got a bit too time consuming.  Nowadays I tend
>to settle for a Q of 95 for archiving them on CD, 85 (and scaled so
>that the longest side is 768 pixels) for a web site and only change it
>if the file size is absurdly large or there are unacceptable artifacts.
>This usually gives me about 1:10 compression ratio.

The advice is best for tyros.  After a while, your eye gets trained to "see" 
when an image could be a problem, even if you can't quite articulate why.  
Then, instead of going for your usual compression factor, you start to home in 
on one that seems to work better.  One of these days, I'm gonna post some 
discussion of my own surrounding compressing images, with (of course) the 
images themselves as guides.  Especially the "surprising" ones!

>Going back to lossless compression again: the best compression ratios
>I've had with photographs used PNG (I've no idea what the algorithm used
>for these is or how it works).  The downside was that they took *ages*
>to compress & decompress.

Never used PNG myself.  Until recently, wasn't all that motivated.

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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