dan@mind.UUCP (Dan Kimberg) (04/27/88)
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)hedley@cbmvax.UUCP (Hedley Davis) (04/28/88)
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
dan@mind.UUCP (Dan Kimberg) (04/29/88)
I'm posting my response in hope of someone seeing this and getting a sudden
inspiration along the lines of a solution to my problem. Also because I
didn't explain very well what I'm doing and why I'm doing it.
The experiments I want to perform have to do with binocular vision,
specifically stereopsis at isoluminance. Anyone who has every looked through
an old "stereopticon" at two pictures, nearly identical but taken from a few
inches apart, should have a good idea what stereopsis is - it's the depth
effect you get from the disparity between what your eyes see.
In order to test this, I wanted to generate isoluminant random dot
stereograms on the amiga - stereo pictures which are just bunches of dots
until you look at them in stereo. I did, and got the results I
hoped for, but there's a strong feeling that it's a poor test if I was at
anything short of isoluminance. Oh, isoluminance means the dots are of
uniform luminance, i.e. distinguishable only by color contrast information.
So the problem is I need to be able to generate these stereograms with
individually distinguishable dots of two different colors, such that at least
one (ideally both) of the colors can be varied continuously. It doesn't much
matter what the levels are, so long as I can show that for the subject there
is no point along the continuum at which they are unable to resolve the
disparity.
More specific responses:
In article <3707@cbmvax.UUCP> hedley@cbmvax.UUCP (Hedley Davis) writes:
>of charecters ) look pretty bad. Pots would also tend to not be
>very repeatable, and therefore might not be suitable for serious
>experimentation.
Repeatability isn't a big problem as long as I can get them to be monotonic -
i.e. always get brighter when i turn them one way, darker the other.
Fuzziness would be bad, though. I think the display is already fuzzy enough.
>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.
Hmm. That's actually a nice suggestion. Although I need to have at least
two colors, I could leave the third color connected normally, right?
Ideally I'd like to be able to use all three colors that way, though.
>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.
Unfortunately subjects will be about ten inches away.
>Whatcha doin anyway ?
>Hedley
Hope I walked the middle ground between concise and explanatory up there.
Thanks a lot for the help. I really have no feel for how the hardware
behaves, so I can use all the help I can get. I think I still want to find
something which will give me full control from a black box, though
if it's at all possible. Thanks again,
-Dan (dan@mind.princeton.edu)