Better algorithm for bug 248472 based on the HSL color system.

r=mvl

calendar/resources/content/calendar.js

@@ -208,16 +208,22 @@ function calendarInit()
        gCalendarStyleSheet.insertRule("." + containerName + " { background-color:" + calendarColor + "!important;}", 1);
 
        var calcColor = calendarColor.replace(/#/g, "");
-       var red = calcColor.substring(0, 2);
-       var green = calcColor.substring(2, 4);
-       var blue = calcColor.substring(4, 6);
+       var red = parseInt(calcColor.substring(0, 2), 16);
+       var green = parseInt(calcColor.substring(2, 4), 16);
+       var blue = parseInt(calcColor.substring(4, 6), 16);
 
-       // sum is between 3 * 255 = 765 and 0.
-       var sum = parseInt(red, 16) + parseInt(green, 16) + parseInt(blue, 16);
+       // Calculate the L(ightness) value of the HSL color system.
+       // L = (max(R, G, B) + min(R, G, B)) / 2
+       var max = Math.max(Math.max(red, green), blue);
+       var min = Math.min(Math.min(red, green), blue);
+       var lightness = (max + min) / 2;
 
-       // Consider all colors with a sum lower than 256 as dark backgrounds
+       // Consider all colors with less than 50% Lightness as dark colors
        // and use white as the foreground color.
-       if (sum < 256)
+       // Actually we use a treshold a bit below 50%, so colors like
+       // #FF0000, #00FF00 and #0000FF still get black text which looked
+       // better when we tested this.
+       if (lightness < 120)
          gCalendarStyleSheet.insertRule("." + containerName + " { color:" + " white" + "!important;}", 1);
 
        var calListItem = calListItems[i+1];

calendar/resources/content/calendarManager.js

@@ -1001,15 +1001,22 @@ function calendarColorStyleRuleUpdate( ThisCalendarObject )
 	   tempStyleSheets[j].insertRule("." + containerName + " { background-color:" +ThisCalendarObject.color + " !important;}",1);
 
            var calcColor = ThisCalendarObject.color.replace( /#/g, "" );
-           var red = calcColor.substring( 0, 2 );
-           var green = calcColor.substring( 2, 4 );
-           var blue = calcColor.substring( 4, 6 );
+           var red = parseInt( calcColor.substring( 0, 2 ), 16 );
+           var green = parseInt( calcColor.substring( 2, 4 ), 16 );
+           var blue = parseInt( calcColor.substring( 4, 6 ), 16 );
 
-           // sum is between 3 * 255 = 765 and 0.
-           var sum = parseInt( red, 16 ) + parseInt( green, 16 ) + parseInt( blue, 16 );
-           // Consider all colors with a sum lower than 256 as dark backgrounds
+           // Calculate the L(ightness) value of the HSL color system.
+           // L = ( max( R, G, B ) + min( R, G, B ) ) / 2
+           var max = Math.max( Math.max( red, green ), blue );
+           var min = Math.min( Math.min( red, green ), blue );
+           var lightness = ( max + min ) / 2;
+
+           // Consider all colors with less than 50% Lightness as dark colors
            // and use white as the foreground color.
-           if ( sum < 256 )
+           // Actually we use a treshold a bit below 50%, so colors like
+           // #FF0000, #00FF00 and #0000FF still get black text which looked
+           // better when we tested this.
+           if ( lightness < 120 )
               tempStyleSheets[j].insertRule( "." + containerName + " { color:" + " white" + "!important;}", 1 );
 	}
 }