From: axman@acadia.Kodak.COM (Mike Axman)
Newsgroups: rec.photo
Subject: Re: Photo-CD technical data
Date: 15 Oct 92 21:13:56 GMT
Organization: Eastman Kodak Co., Rochester, NY

In article <1992Oct15.145742.26242@tdb.uu.se>, bredell@tdb.uu.se (Mats Bredell) writes:
>
>Hi!
>
>I've heard about the Photo-CD for a year now, and when it's finally released
>I went to my local photo dealer to get some info. The first dealer I tried
>only said "You press this button, and the picture shows on this screen". He
>didn't know anything more about it. Finally, I found someone who said he'd
>been to some internal Kodak education about the Photo-CD. This are some of
>the facts he told me:
>

Either the internal Kodak education instructor was on drugs, or your 
friend didn't pay attention.  Most of the info reported here is filled 
with factual errors.

>The resolution is always 2048x3072. There are four more, lower resolutions,
>but the image is always 2048x3072 on the disc. The player can deliver a lower
>resolution if you want to, but in that case it's derived from the higher
>resolution on the disc.
>
This is essentially correct for the consumer Photo CDs being currently 
written.  The format is a hierarchical pyramid of resolution which facilitates 
quick access to components at 5 resolutions: 128x192, 256x384, 512x768, 
1024x1536, and 2048x3072.  It's important to note that the file format supports
resolutions and other features beyond this, but the consumer transfer system 
does not yet support them.

>The compression is lossless. It's very simple: you take a 2x2-dot square, and
>calculate the average value, and this is stored on the disc.

This is garbage.  The compression is NOT numerically lossless in the baseline
consumer system.  The Photo CD format allows for lossless encoding for markets
which demand it.  Consumer photography is not one of those markets, although 
nothing precludes Kodak, Fuji, Agfa, or others from introducing a lossless 
option in a consumer system.  We have an affectionate term for the compression
quality; we call it "photographically lossless", but it is not numerically 
lossless.  This 2x2-dot square sounds like someone's desciption of a simple
average down method (ie average 2048 pixels down to 1024 pixels, etc...)
Photo CD does not utilize this method, as it produces "phase" errors, extensive
aliasing, and other problems.  If you don't know what these terms are, you 
don't really want to know, they're just digital artifacts.

>
>You always get exactly 100 pictures on the disc.

The compression is variable depending on many factors, and therefore the 
exact number of images is not fixed.  If you shoot fine grain film of 
low "information/chaos/entropy/etc..." scenes, you might fit many more than
100.

>
>Ok, this sounds weird to me. I thought the disc itself supported 5 different
>resolutions. The compression also sounds strange. How can it be lossless if
>it works like that? I thought the JPEG algorithm was developed for use in
>the Photo-CD technology. I also thought that the number of pictures on a
>disc varied, depending on the type of pictures (amount of contrast and so
>on).

JPEG was developed by a consortium of companies, universities, etc... of 
which Kodak was a participant.  Dr. Rabbani of Kodak (and his group) had been 
working with the Discrete Cosine Transform for image compression long before 
the JPEG organization decided to utilize it.  He and others have determined 
that JPEG is overkill for very high quality applicatons.  The DCT is very 
useful in applications requiring large compression ratios with good image 
quality, when computation time is not an issue.   However, as the desired
quality approaches lossless and if file size is not as big of an issue, the 
DCT (hence JPEG) is not an optimal solution.  It is extremely computational 
(compared to other options) and when it "fails", it's artifacts are not very 
"photographically acceptable"; they stand out like "sore thumbs".
These issues, and the fact that Kodak had no way of knowing _when_ JPEG would 
be commercially available, led us to develop our own method.

JPEG is good when you need alot of compression, and you know exactly what 
the output size/etc... is for your application.  Peiter van der Meule 
recently posted a JPEG'd 4BASE Photo CD image and stated that the quality
was indistinguishable from the original.  This may be true at a particular
output size, but what about if you want to take that data and then zoom it
2x, or make a poster size print?  You will find that the quality differences 
become very apparent.  It's like saying that you can't see any difference in 
Ektar 25 or Gold 100 on a wallet sized print.  Try making an 8x10 or a 20x30 
and then tell me what you see.  Is this clear; it's a very important concept 
in image quality comparisons.

You are correct in assuming that the number of pictures depends on the 
image characteristics.  Just stick a Photo CD in a CD-ROM and do a listing
by size of the files; each one is different due to the variable rate 
compression algorithm.

>
>So, what's the truth here?

There is no truth; only television :-)

On another note, I would be very interested in hearing about your needs in 
the area of "medical systems", as indicated in your .signature .
Please e-mail me with any features/requirements/etc... you would envision.

eastmanly,
mike

>
>/Mats
>-- 
>Mats Bredell                                   Mats.Bredell@udac.uu.se
>Uppsala University Computing Center (UDAC)     Ph:  +46 18 187817
>Department of medical systems                  Fax: +46 18 187825
>Sweden                                         Think straight - be gay!

-- 
Mike Axman  
CD-Imaging Advanced Development
Eastman Kodak Company
axman@kodak.com


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