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Re: [OM] fwd: Dodgital Cameras etc

Subject: Re: [OM] fwd: Dodgital Cameras etc
From: Paul Wallich <pw@xxxxxxxxx>
Date: Fri, 7 Jul 2000 09:45:56 -0400
At 10:52 AM +1000 7/7/00, you wrote:


"Terry and Tracey" <foxcroft@xxxxxxxxxxxxxxxxxxxxxx> wrote:

Also, he said jpeg is a mathmatical representation of the image, not the
image itself. And it is the same quality as the original. So with no
compression selected, jpeg will be about 1/8 of a tiff file. Increase
compression, and the trade off with image quality happens quick. Resave, and
it recomputes and quality drops.


Crap. Get better friends. ;-)

All Jpeg images are compressed to a greater or lesser extent. Jpeg
compression is lossy meaning that image data is discarded in the process
of converting the image size. The trick with Jpeg is that human eye is
unlikely to notice the missing data. But data has gone and there is no
way to get it back.

Wrong, wrong, wrong (as a couple people have pointed out already). JPEG
compression can be either lossless or lossy depending on the ratio of size
between compressed and uncompressed images. There's only a finite amount of
information in the original pixels, so you can express it as a modified
fourier transform with no loss of information. (If the original image were
continuous-tone, i.e. brightness levels and pixel positions not restricted
to 8/12 bits and an NxM matrix, you might potentially need an infinite number
of terms in a fourier-type representation to get perfect representation, but
all digital images are already quantized.)

Lossless JPEG compression typically cuts file sizes by a factor of 2 or 3
(very similar to other compression methods). You can open, rotate, scale etc
without losing info. If you want greater compression (and most people do) you
will lose information, but it's not a necessary part of saving in JPEG form.

This is important to understand because, among other things, storing as TIFF
takes up several times as much space as lossless JPEG for the same information.
(For most digital cameras this is particularly ironic because most of the information in an image is made up anyway...)

paul

--
Paul Wallich                                            pw@xxxxxxxxx

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