Newsgroups: comp.graphics
Path: utzoo!utgpu!watserv1!watcgl!imax!dave
From: dave@imax.com (Dave Martindale)
Subject: Re: Digital Film Recorders
Message-ID: <1990May4.142205.13258@imax.com>
Organization: Imax Systems Corporation, Toronto Canada
References: <20674@versatc.versatec.COM> <66.UUL1.3#5109@pantor.UUCP>
Date: Fri, 4 May 90 14:22:05 GMT

In article <66.UUL1.3#5109@pantor.UUCP> richard@pantor.UUCP (Richard Sargent) writes:
>
>However, claims of 3K x 4K, while true, are
>also misleading. Our experiences have been
>that addressability may be high, but resolvability
>is a lot lower. I have heard claims that
>low priced systems have a resolvability measure
>that is about half the claimed resolution. Usually,
>the spot size is considerably larger than 1/4000th
>of the screen inside the camera. Very high-priced
>cameras actually do have resolution and addressability
>match 1:1.

Actually, that's no longer true.  For example, the high-end Celco
recorder used to be called the CFR-4000, even though it had a vertical
addressability of 8192 lines, and horizontal addressability was
whatever you wanted (set by an analog trimpot).  A data buffer
limitation prevented you from having more than 4096 pixels or 2048 runs
per scanline, though.

The spot size of this recorder is specified as .0008 inch, with a 7
inch CRT of which about 6 inches is usable, so a 4:3 aspect-ratio image
would actually occupy an area of 4.8 by 3.6 inches on the CRT.
Dividing spot size into image dimensions gives an estimate of 6000 by
4500 pixels as the maximum possible resolution.

Then the Dunn "8000-line" digital recorder came out, which certainly
had no better resolution than the Celco (electrostatic focus vs.
magnetic focus CRT).  Celco responded by renaming their top-of-the-line
machine the CFR-8000, without changing anything.

Then Management Graphics came along with their "Solitaire 16", with an
addressability of 16384 pixels, although it uses a CRT with a spot size
rated at .001 inch - certainly no better than the Celco.  Still, lots
of people thought that 16000 sounded twice as good as 8000.  Celco
responded by making their data buffer and vertical DAC bigger, and now
they too have addressability of 16384.

But the maximum resolution of any of these recorders is still limited
by the CRT spot size and faceplate diameter - and is still no more than
6000 x 4500; probably worse in fact.  The Dicomed family has always had
15 or 16 bits of addressability in X and Y, while using a spot size
comparable to these other recorders and a 5 inch tube, so it's no
better either.

One way of looking at it is that, with a 16K-addressability recorder,
you can position lines or edges in the image with an accuracy of about
1/3 of the minimum possible line width.  I *suspect* it would allow you
to plot things like text and bar charts and so on with no anti-aliasing
whatsoever and still get excellent results.  So the extra
addressability is actually useful in some circumstances.

It just means that you have to be aware of the difference between
addressability and resolution, and the fact that there is often a
*large* difference between them.


On the other hand, this is not a criticism of vendors like Celco,
Dicomed, and Management Graphics - they produce film recorders that can
give excellent images, by CRT film recorder standards.  But they are
expensive.

And you do often get what you pay for.  I was once asked to look at a
low-end video-fed film recorder that didn't give sharp images.  We took
it apart, and found that:

    -   the CRT was an ordinary consumer-type small CRT with a rounded
	(bulging) face; there is no way that the centre and edge of the
	image would ever be in focus at the same time.

    -   the lens was an enlarger lens, which would not have been
	corrected for use at the reproduction ratio it was being used
	at, and was further degraded by use of a common "close-up
	filter" to keep the optical path short.

    -   it looked like the manufacturer just picked filters that were
	roughly red, green, and blue, rather than matching their
	characteristics to the phosphor and film.

In general, buying a high-priced film recorder gets you a CRT that was
designed for imaging, a lens that is at least roughly suited to the
task at hand, and reasonable-quality filters.

So, do lots of tests when comparing recorders!

	Dave Martindale