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From: aduncan@rhea.trl.oz.au (Allan Duncan)
Newsgroups: comp.sys.amiga.tech
Subject: re: Nonlinear video info?
Message-ID: <923@trlluna.trl.oz>
Date: 19 Jan 90 00:22:43 GMT
Sender: root@trlluna.trl.oz
Lines: 52

Doug Merrit of Hunter Systems asks about the non-linearity of the
amiga's output, and the variability of KNOWN monitor non-linearity.

I have been working with video (on and off) for many years, mainly from
the originating end of things, and have been disturbed by the blind
ignorance in which most computer terminals have been designed(?).  This
has been particularly galling with the use of EHB.  I had intended to
post a tutorial to the world in the near future, but will pre-empt it
with a quick run-down.

1. All crt displays are non-linear from voltage in (the usual way of
driving them) to cathode current out.  The relation between cathode
current and the screen light amplitude is approximately linear.  Note
that the BRIGHTness is the square of the amplitude.  There is a degree
of uniformity in the overall characteristic, and a particular value is
chosen for broadcasting purposes.  This is described as gamma in the
equation
         intensity = const1xV^gamma + const2

In Australian broadcasting, gamma is taken as 2.8


2. If there is ambient light, the darker portions of the picture will
dissappear in the reflected glow, so some elevation of these brightness
levels is needed.

3. Computer generated output (amiga included) is LINEAR.  Now in TV, the
camera output is compessed at the bright end the match the display
gamma, different types of pick-up tubes have different gammas, and have
to be matched accordingly.  CCD camera sensors are just as linear as the
amiga, however the camera has a gamma corrector built in.  THIS is my
beef about EHB - it isn't HALF bright at all, it is only 14% bright!
The correct solution would have been to have the video output corrected,
but it is too late now.  Look at stuff that uses EHB for an entire
screen - it looks like twilight, and unless you wind up the brightness to
overcome incident light, half missing.

4. The only way you can fully utilise EHB is to add (3) gamma
correctors, and then paint accordingly.  I don't know what is in the
Video Toaster, but it really should have them, or a lot of the paint
output will be sub-optimum.  The trouble is, the artist produces the
picture looking at an uncorrected output, and chooses a pallete
accordingly.

5. In the end, since human vision perceives brightness logarithmically,
what you see on the screen is really what counts, and subjective tests
underly all of the TV industry constants and standards.

Allan Duncan	ACSnet	aduncan@rhea.trl.oz
		ARPA	aduncan%rhea.trl.oz.au@uunet.uu.net
		UUCP	{uunet,hplabs,ukc}!munnari!rhea.trl.oz.au!aduncan
Telecom Research Labs, PO Box 249, Clayton, Victoria, 3168, Australia.