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From: oziebel@gwdgv1
Subject: Re: Are 24-bit video adapters toys?
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References: <1994Oct13.121017.481@cs.joensuu.fi> <37m17b$mh6@kodak.rdcs.Kodak.COM> <usenet-1410941051060001@lowry.eche.ualberta.ca> <37uo35$2vb@kodak.rdcs.Kodak.COM> <usenet-1910941241490001@lowry.eche.ualberta.ca> <388gj0$k0c@kodak.rdcs.Kodak.COM>,<1994Oct25.113957.13989@cs.joensuu.fi>
Date: Tue, 25 Oct 1994 15:01:30 GMT
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In article <1994Oct25.113957.13989@cs.joensuu.fi>, jlavi@cs.joensuu.fi (Jarkko Lavinen) writes:
>bryant@neural.Kodak.COM (Steve Bryant) writes:
>
>>In fact, my colleague here says that he has read research articles stating
>>that the eye is not capable of resolving less than 3 parts in 256. Granted,
>>this is pure hear-say evidence and I dont have the reference to the article,
>
>Once again the problem is that this doesn't say anything about 
>the density difference. One should say the result rather in 
>ratios or percentages, something, something like that human 
>eye can't see a 1% difference in luminance or whatever. Ratios 
>can be easily converted to density difference by taking a 
>logarithm in base 10.
>
>Earlier Alan Roberts wrote:
>
>>to exceed 50:1. To view a picture with contrast of 1000:1 you will
>>need a very good (low flare) CRT and a completely blacked out room.
>>I have measured, in a lab, contrast on a CRT of over 3000:1, but that
>....
>>then you will be very lucky to get contrast of more than 35:1. This
>>results from flare in the glass of the CRT, and in the AGC action of
>...,
>>Basically, you might be able to store pictures with contrast of 1000:1
>>using 10-bit values, but you'll never be able to see them properly on
>>a CRT.
>
>One could perhaps have a flare model of the CRT and use that then during
>the viewing to compensate the loss of contrast. The viewing program
>would first estimate whether it can view the requested contrast or not
>and then try to make best out of the situation. This approach could
>also easily compensate dark corners of the CRT.
>
>To get more color accuracy, one cheap approach might be to simply 
>dither 30-bit images to 24-bit. When viewing photographic continous 
>tone images this dithering disappears in high resolution and would 
>mimic 30-bit Truecolor. Similar approaches are used to simulate 24-bit 
>Truecolor in non-24-bit adapters but here the effect would be much more
>subtle because 24 is already almost continous. When mimicing 24-bit in
>16-bit the effect is much more coarse.
>
>Neverthless this would requite that density reference to be coded 
>somehow in image. I think PhotoCD does this because luminance is encoded 
>in logarithmic steps. For my purposes PhotoCD takes too much space
>(7000 images would fit only as a thumbnail in CD) and I would rather 
>do with JPEG-like approach, but plain JPEG doesn't store density 
>information. Perhaps JPEG could be modified to support 10 bits and 
>contain density reference. Otherwise one doesn't know what gamma 
>correction the data contains. One may also ask that whats the use 
>of talking about lossy image compression with 30-bit accuracy... :-(
>
>Earlier Ron Natalie wrote:
>
>>I have some test images and we've never managed to get down to
>>being able to see 3 step differences across a wide range of
>>intensities.
>
>I made an experiment with using netpgm package and xv 3.0,
>with following command:
>	pgmramp -lr 1024 100 | xv -owncmap -
>
>The monitor I had was IBM 6091 19i, hooked to IBM RS/6000 with
>the low-end GT1 adapter on 1280x1024/256 mode. The room was as
>dark as it can be - it was night, windows were shaded, lights
>turned off etc.
>
>The pgmramp produces a continous linear scale 0..255 with given
>geometry. I rotated the scale vertical, then Maxpected, then
>zoomed into various parts to get the bars more visible. I then varied
>brightness with maximum contrast and tried to see boundaries
>of shades. Both in the dark and bright end there were areas
>where I was able to see fringes with difficulty but not beyond.
>In between these ends there were a scale where steps were
>visible without difficulty when zoomed enough.
>
>I list my results as 4 numbers: First one is the point where
>I can first see steps. Last one is where steps disappear.
>Second and third describe the scale where steps appear
>continuously and are seen relatively easily. I tried first
>with maximum contrast varying brightness and then with
>medium contrast. My results were:
>
>hi  con, low bri: 50, 60..180, 220
>hi  con, med bri: 30, 30..130, 190
>hi  con, low bri: 15, 20..130, 150
>med con, med bri: 50, 65..140, 190
>
>Needless to say that these numbers are very subjective. One 
>should actually draw a sensitometric curve like those used
>for photographic media. Also I should have done this in all
>three guns separately etc.
>
>It would be interesting to hear what kind results people get
>from high-end machines like Silicon Graphics with this
>setup I used. In a way these numbers are pretty meaningless
>without sensitometric control, but this is a quick and dirty
>experiment.
>
>According to the Gamma Measurement Image in Monitor Gamma page at:
>	http://www.cs.cmu.edu:8001/afs/cs/user/rwb/www/gamma.html
>the gamma on the 6091 was 2.2 (max contrast).
>
>	Jarkko Lavinen
>jlavi@cs.joensuu.fi, http://balrog.joensuu.fi/~jlavi


Just for the magnitude:
 With an ET400 VGA card I got with normal monitor settings the following
 brightness for gray-values (all 3 colors equal intensity)

 brightness = 0.048 x^2 -0.27 x + 0.563   Candela

 x are the 3 DAC register settings
 x-range is from 0 to 63
 with 24 bit I think you would just get finer steps 

					Olaf Ziebell

