Given a background color, black or white text?

I'm trying to find a method for determining whether to use black or white text, given a background color (as a hex value). Has anyone dealt with this before? Is there an effective way to do this?

In my case, I would be using PHP to implement the logic (though any experience with this in other languages is welcome).

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Take a look at this page: Calculating Color Contrast with PHP

Keep in mind that if black and white are your only choices you're bound to have cases where neither of them works particularly great.

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I think the best way is the Luminosity Contrast algorithm :

ADVISE: The next functions works well most of the time, But sometimes there are colors in which it does not work properly.

public function getContrastColor($hexColor) {

        //////////// hexColor RGB
        $R1 = hexdec(substr($hexColor, 0, 2));
        $G1 = hexdec(substr($hexColor, 2, 2));
        $B1 = hexdec(substr($hexColor, 4, 2));

        //////////// Black RGB
        $blackColor = "#000000";
        $R2BlackColor = hexdec(substr($blackColor, 0, 2));
        $G2BlackColor = hexdec(substr($blackColor, 2, 2));
        $B2BlackColor = hexdec(substr($blackColor, 4, 2));

         //////////// Calc contrast ratio
         $L1 = 0.2126 * pow($R1 / 255, 2.2) +
               0.7152 * pow($G1 / 255, 2.2) +
               0.0722 * pow($B1 / 255, 2.2);

        $L2 = 0.2126 * pow($R2BlackColor / 255, 2.2) +
              0.7152 * pow($G2BlackColor / 255, 2.2) +
              0.0722 * pow($B2BlackColor / 255, 2.2);

        $contrastRatio = 0;
        if ($L1 > $L2) {
            $contrastRatio = (int)(($L1 + 0.05) / ($L2 + 0.05));
        } else {
            $contrastRatio = (int)(($L2 + 0.05) / ($L1 + 0.05));
        }

        //////////// If contrast is more than 5, return black color
        if ($contrastRatio > 5) {
            return 'black';
        } else { //////////// if not, return white color.
            return 'white';
        }
}

Some results:

NOTE: The font color is determined by the previous function. The number in brackets is the contrast ratio.

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Another simpliest and less precise way called YIQ (because it converts the RGB color space into YIQ):

public function getContrastColor($hexcolor) {               
    $r = hexdec(substr($hexcolor, 0, 2));
    $g = hexdec(substr($hexcolor, 2, 2));
    $b = hexdec(substr($hexcolor, 4, 2));
    $yiq = (($r * 299) + ($g * 587) + ($b * 114)) / 1000;
    return ($yiq >= 128) ? 'black' : 'white';
}                   

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  function getTextColour($hex){
    list($red, $green, $blue) = sscanf($hex, "#%02x%02x%02x");
    $luma = ($red + $green + $blue)/3;

    if ($luma < 128){
      $textcolour = "white";
    }else{
      $textcolour = "black";
    }
    return $textcolour;
  }

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