Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!zaphod.mps.ohio-state.edu!uwm.edu!bionet!arisia!tow From: tow@arisia.Xerox.COM (Rob Tow) Newsgroups: comp.sys.ibm.pc.misc Subject: Re: XGA video standard Summary: 24 bits is not True Color Keywords: color 24 bit vga Message-ID: <13129@arisia.Xerox.COM> Date: 12 Oct 90 21:10:48 GMT References: <3015@aecom.yu.edu> <35010020@hpfinote.HP.COM> <245@srchtec.UUCP> <339@aupair.cs.athabascau.ca> <4591@optilink.UUCP> Reply-To: tow@arisia.UUCP (Rob Tow) Distribution: na Organization: Xerox Palo Alto Research Center Lines: 81 In article <4591@optilink.UUCP> cramer@optilink.UUCP (Clayton Cramer) writes: >In article <339@aupair.cs.athabascau.ca>, kevinc@cs.athabascau.ca (Kevin Crocker) writes: >> In article <245@srchtec.UUCP> mra@srchtec.UUCP (Michael Almond) writes: >> >>> According to PC Week, 32,000 colors. They also said it's 16-bit >> >>> color. I don't really understand this. >> >> If we are talking about ~32000 hues on screen then is there any >> evidence to show that the human eye can discern this level of sublety? >> >> Kevin "auric" Crocker Athabasca University > >I read somewhere, many years ago, where I can doubtless not find >it, that the human eye is capable of distinguishing about 10 million >shades of color. (Presumably measured by taking two light sources >of known frequency some number of Angstroms apart, and seeing how >many people can distinguish them). > >This means that when you get to 24 bits of color information, there >is no point in going any further. > >-- >Clayton E. Cramer {pyramid,pixar,tekbspa}!optilink!cramer >If "right of the people" in the Second Amendment doesn't refer to an >individual right, what does it refer to in the First & Fourth Amendments? >You must be kidding! No company would hold opinions like mine! Actually, this is not true. We're talking my field here; I design color scanners, color correction systems, and work on the monitor to print problem, for Xerox. 24 bits is insufficient for more than one reason. The first thing to consider is that the dynamic range of the eye is not a simple function, and is not the same for each of the three (actually, four; did you know that under the right circumstances people can be tetrachromats? This is called *rod intrusion*) input channels (which may be equated with red, green, and blue sensation). Under good conditions, with a calibrated monitor, you can see banding in an 8 bit gray ramp. Also, the range of intensity that is expressed in that ramp does not cover the full range of intensity the eye can percieve; i.e., there are "whiter whites" that you can respond to. The situation gets even more fun with the addition of colors, as opposed to grays. Another fun complication is that the primaries used in video, print, etc., do not cover the full *gamut* of color vision. If you plot the gamut given by, say, the SMPTE phosphours on a CIE color chart you will get a triangle inside the full range of human vision. There are colors you can see perfectly well that no monitor can display. This gives rise to a whole host of interesting tradeoffs - do you clip colors to the hull of the gamut, or somehow map one gamut to another? We do *both* here, depending on the image and the device - for example, last year's SIGGRAPH proceedings cover had a gamut mapping applied to it from the video image to map it into the printer's gamut. Consider also that when you do this that you are squashing levels and will lose discernable differences. An important consideration from an artists view is perserving discernable differences even if there are hue shifts! ...these days I get somewhat exercised by the widespread use of the phrase "True Color" in reference to 24 bit displays; they are anything *but* true color. The sad truth is that very few people have much sophistication for color; it's rather like typography before 1984. --- Rob Tow Member Research Staff Electronic Document Lab Xerox PARC 3333 Coyote Hill Drive Palo Alto, CA 94304 (415)-494-4807