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From: Colin Rafferty <colin@xemacs.org>
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Subject: Re: HTML foreground and background colors being the same... poll time!
Date: 03 Dec 1998 09:54:32 -0500
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William M Perry writes:

> I'd vote for (c), but I have no clue on how to go about finding or writing
> a good contrast-finding algorithm.  Anyone out there know of one?

I vote for (c), but I know that finding contrasting colors is hard to
do.  However, since all you need is a single constrasting color, you
could choose between black and white.

;; Take the "average" level, and go opposite

(defun infer-contrasting-color (r g b)
  (if (and (<= r 255)
	   (<= g 255)
	   (<= b 255))
      (if (< (+ r g b) 384) "white" "black")   ; (/ (* 256 3) 2) => 384
    (if (< (+ r g b) 98304) "white" "black"))) ; (/ (* 65536 3) 2) =>  => 382

;; Colin