Hi all, I'm trying to measure overall contrast in an image.  Currently, I have the Michelson method working:

1.   float getContrast() {
   
2.     float maxLum = 0;
   
3.     float minLum = 10000;
   
4.     for (int i=0; i<img.pixels.length; i++) {
   
5.       float r = img.pixels[i] >> 16 & 0xFF;
   
6.       float g = img.pixels[i] >> 8 & 0xFF;
   
7.       float b = img.pixels[i] & 0xFF;
   
8.       float brightness = (0.2126 * r) + (0.7152 * g) + (0.0722 * b);
   
9.       if (brightness > maxLum) maxLum = brightness;
   
10.       if (brightness < minLum) minLum = brightness;
    
11.     }
    
12.     return (maxLum - minLum)/(maxLum + minLum);
    
13.   }

This works, but (as far as I can tell) measures the difference between the darkest and lightest values - not a very good overall idea of contrast.  An all-gray, all-white, or all-black image turns out 0 contrast, but a super-contrasty image and a not-so-contrasty one but with a few very dark and light areas both return high contrast.

Looking at the Root Mean Square (RMS) method ( http://en.wikipedia.org/wiki/Contrast_%28vision%29#RMS_contrast), this seems a better bet.  However, I can't figure out the formula!  Any help or other suggestions would be fantastic!