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RE: [OM] Scanning Res for Web

Subject: RE: [OM] Scanning Res for Web
From: "Ian A. Nichols" <I.A.Nichols@xxxxxxxxxxxxx>
Date: Wed, 1 Dec 1999 10:06:07 +0000 (GMT)
 

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.
 
>  quality (in other words, they're "lossy compression-resistant" --
> there's a picture from Branko Turk in the Unofficial Olympus Web Photo
> Gallery which is exactly like this, and was a source of real surprise
> to me when I first worked on it). 

I'm mildly surprised myself (just had a look at it).  Must be all those
dramatic dark/light transitions in the brickwork - my gut reaction would
be that the milder texture in the concrete and near monotone in the
shadows between the carpark levels would compress easily.

> 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.

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.

-- 
________________________________________________________________________
*             |                                                        |
|  /  | |/-\  |                      Ian A. Nichols                    |
| |   | |   | |                                                        |
|  \-/| |  /  |                  i.a.nichols@xxxxxxxxxx                |
|             *                    iann@xxxxxxxxxxxxx                  |
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