Path: utzoo!utgpu!water!watmath!clyde!att!att-ih!ihnp4!cbmvax!hedley
From: hedley@cbmvax.UUCP (Hedley Davis)
Newsgroups: comp.sys.amiga.tech
Subject: Re: An Infinite Number of Colors
Message-ID: <3707@cbmvax.UUCP>
Date: 28 Apr 88 12:57:41 GMT
References: <2388@mind.UUCP>
Reply-To: hedley@cbmvax.UUCP (Hedley Davis)
Organization: Commodore Technology, West Chester, PA
Lines: 61

In article <2388@mind.UUCP> dan@mind.UUCP (Dan Kimberg) writes:
>I've heard from someone thirdhand that it is possible to set up a black box
>between the Amiga and the 1080 monitor with three potentiometers, each of which
>modulates the brightness of one of the colors.  Does anyone know of anyone who
>has done this, or that failing, can anyone tell me what is involved in designing
>something of that sort?  I need some sort of device to produce visual stimuli
>in red, green, and blue, at varying brightnesses, and it would be nice to use
>the amiga if its limitation to 15 degrees of brightness could be overcome.
>
>                                     -Dan (dan@mind.princeton.edu)

The problems with using potentiomenters is that the RGB lines want
to have a nice clean 75 ohm impedence to eliminate ringing and
overshoot in the signal. Using Pots would tend to yeild a much higher
impedence and this would make transitions in the video signal ( edges
of charecters ) look pretty bad. Pots would also tend to not be
very repeatable, and therefore might not be suitable for serious
experimentation.

In the B2000, there are two video slots, one right next to the other.
12 bits of raw digital video data is available at these slots. There
are four bits each of R, G & B information.

Seems to me that video-dac manufacturers make D-A converters with very
nice little 75 ohm outputs built right in. You could work out a scheme
where you connect the four bits from the RED gun to the higher 4 bits
on a videodac, and the four bits from the green gun to the lower four
bits. This would give you a 75 ohm 256 step output. You could then
switch this output to the R,G or B input on the monitor and presto,
lotsa shades of R,G, or B.

Another technique would be to use dithered patterns to simulate various
brightnesses. This would require the subject under test be sufficiently
far from the monitor that the overall color is visiable, but not the
underlying patterns.

There are a couple of pitfalls with this approach. If you try to
simulate a color 50% between two other colors by switching back and
forth on every pixel, the video may not yeild the correct value because
you are pushing the video outputs in the amiga very hard. Not all
pixels are of the same exact length. Some may only be 20 ns long
where they should be 35, some may be longer. A cleaner way to do this
is dither every other line so that the video coming from the monitor
is essentially a DC level for a long time ( entire line ).

Another box of problems is the monitor brightness settings, the contrast
settings and a big meany called gamma correction. ( Monitors are
notiriously non-linear when it comes to displayed brightness vs. the
input voltage. ). These items combined make it hard for you to 
calibrate your readings against a known standard. You most certainly
would need a good light meter to use to measuring what is actually
coming from the monitor for a given setup.

If you have a good light meter ( not a photographic model, but something
more along the lines of a good lab instrument ), and your experiment
does not involve rapid changes between colors, then maybe you could
simply use the amiga to approximate the color you want, and tune it in
via the brightness control on the monitor using the meter.

Whatcha doin anyway ?
Hedley