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  1. From: Mathew Brennesholtz <msb@philabs.research.philips.com>
  2. Subject: Re: contrast of color
  3. Date: 01 Oct 1999 00:00:00 GMT
  4. Message-ID: <37F4DD62.3AABA5D2@philabs.research.philips.com>
  5. Content-Transfer-Encoding: 7bit
  6. References: <37F3395E.484C921A@netvision.net.il>
  7. Content-Type: text/plain; charset=us-ascii
  8. X-Complaints-To: usenet@philabs.research.philips.com
  9. X-Trace: news.philabs.research.philips.com 938783269 3770 130.140.53.137 (1 Oct 1999 13:07:49 GMT)
  10. Organization: Philips Research, Briarcliff Manor, NY
  11. Mime-Version: 1.0
  12. NNTP-Posting-Date: 1 Oct 1999 13:07:49 GMT
  13. Newsgroups: sci.engr.color
  14. One intention of the L*a*b* formulation is that when you calculate:
  15. delta-E = sqrt((delta-L*)^2 + (delta-a*)^2 + (delta-b*)^2),
  16. a delta-E = 1 should be just barely percievable to some viewers. If you
  17. want most people to be able to distinguish between two colors most of the
  18. time, a delta-E = 3 is needed. Delta-E = 3 is called a MPCD (minimum
  19. perceptible color difference) or a JND (just noticable difference) by
  20. some authors. Keep in mind that the L*a*b* space is not a perfectly
  21. uniform space, so the delta-E value that is percievable depends on
  22. location in x-y space and the direction between the two colors.
  23. Using a delta-E = 1 threshold, M. R. Pointer (The Number of Discernible
  24. Colours, Color Research and Application, 23:1 February 1998) calculated
  25. that there are 2.28 million discernable color/luminance combinations.
  26. Other studies (see references in Pointer) show that printing inks give
  27. 1.6 million colors and EBU phosphors give 1.35 million colors. These are
  28. far less than the 16 million colors claimed by 8 bit/color video cards,
  29. never mind 10 or 12 bit video systems. Obviously, some of these
  30. color/luminance combinations are indistiguishable from each other.
  31. Hope that helps.
  32. IPLAB wrote:
  33. > Hallow !!
  34. > I understand that to measure contrast between two objects in color
  35. > picture, I can calculate the vector distance on L a b space (between
  36. > the two objects).
  37. > Does anyone can tell me what is the minimum of the Vector distance
  38. > ,that eye can see ??
  39. >
  40. > Thanks
  41. >
  42. > from sally
  43. http://www.compuphase.com/cmetric.htm:
  44. typedef struct {
  45. unsigned char r, g, b;
  46. } RGB;
  47. long ColourDistance(RGB e1, RGB e2)
  48. {
  49. long r,g,b;
  50. long rmean;
  51. rmean = ( (int)e1.r + (int)e2.r ) / 2;
  52. r = (int)e1.r - (int)e2.r;
  53. g = (int)e1.g - (int)e2.g;
  54. b = (int)e1.b - (int)e2.b;
  55. return (((512+rmean)*r*r)>>8) + 4*g*g + (((767-rmean)*b*b)>>8);
  56. }