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69 lines
2.4 KiB
69 lines
2.4 KiB
15 years ago
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From: Mathew Brennesholtz <msb@philabs.research.philips.com>
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Subject: Re: contrast of color
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Date: 01 Oct 1999 00:00:00 GMT
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Message-ID: <37F4DD62.3AABA5D2@philabs.research.philips.com>
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Content-Transfer-Encoding: 7bit
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References: <37F3395E.484C921A@netvision.net.il>
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Content-Type: text/plain; charset=us-ascii
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X-Complaints-To: usenet@philabs.research.philips.com
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X-Trace: news.philabs.research.philips.com 938783269 3770 130.140.53.137 (1 Oct 1999 13:07:49 GMT)
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Organization: Philips Research, Briarcliff Manor, NY
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Mime-Version: 1.0
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NNTP-Posting-Date: 1 Oct 1999 13:07:49 GMT
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Newsgroups: sci.engr.color
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One intention of the L*a*b* formulation is that when you calculate:
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delta-E = sqrt((delta-L*)^2 + (delta-a*)^2 + (delta-b*)^2),
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a delta-E = 1 should be just barely percievable to some viewers. If you
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want most people to be able to distinguish between two colors most of the
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time, a delta-E = 3 is needed. Delta-E = 3 is called a MPCD (minimum
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perceptible color difference) or a JND (just noticable difference) by
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some authors. Keep in mind that the L*a*b* space is not a perfectly
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uniform space, so the delta-E value that is percievable depends on
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location in x-y space and the direction between the two colors.
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Using a delta-E = 1 threshold, M. R. Pointer (The Number of Discernible
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Colours, Color Research and Application, 23:1 February 1998) calculated
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that there are 2.28 million discernable color/luminance combinations.
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Other studies (see references in Pointer) show that printing inks give
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1.6 million colors and EBU phosphors give 1.35 million colors. These are
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far less than the 16 million colors claimed by 8 bit/color video cards,
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never mind 10 or 12 bit video systems. Obviously, some of these
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color/luminance combinations are indistiguishable from each other.
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Hope that helps.
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IPLAB wrote:
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> Hallow !!
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> I understand that to measure contrast between two objects in color
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> picture, I can calculate the vector distance on L a b space (between
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> the two objects).
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> Does anyone can tell me what is the minimum of the Vector distance
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> ,that eye can see ??
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>
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> Thanks
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>
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> from sally
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http://www.compuphase.com/cmetric.htm:
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typedef struct {
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unsigned char r, g, b;
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} RGB;
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long ColourDistance(RGB e1, RGB e2)
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{
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long r,g,b;
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long rmean;
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rmean = ( (int)e1.r + (int)e2.r ) / 2;
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r = (int)e1.r - (int)e2.r;
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g = (int)e1.g - (int)e2.g;
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b = (int)e1.b - (int)e2.b;
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return (((512+rmean)*r*r)>>8) + 4*g*g + (((767-rmean)*b*b)>>8);
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}
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