RGB to Value | Python Fiddle

import colorsys

### See also: Detecting thresholds in HSV color space (from RGB) using Python / PIL
### http://stackoverflow.com/questions/4890373

img = [[(255, 255, 255),(  0,   0,   0),(127, 127, 127),(255, 127, 255)],
       [(255, 127, 127),(255, 255, 255),(255, 255, 255),(255, 255, 255)],
       [(255, 255, 255),(255, 255, 255),(255, 255, 255),(255, 255, 255)],
       [(255, 255, 255),(255, 255, 255),(255, 255, 255),(255, 255, 255)]]

def join2dArr(arr, d, func):
    return d.join([d.join([func(col) for col in row]) for row in img])

def toValue(rgb):
    #return str(colorsys.rgb_to_hsv(*rgb)[2])
    return str(int(max(rgb[0], rgb[1], rgb[2])))

def toLightness(rgb):
    #return '{0:d}'.format(int(colorsys.rgb_to_hls(*rgb)[1]))
    return str(int((max(rgb[0], rgb[1], rgb[2])+min(rgb[0], rgb[1], rgb[2]))/2))

## See: python — measuring pixel brightness
## http://stackoverflow.com/questions/6442118
def toLuminance(rgb):
    return '{0:d}'.format(int((0.2126 * rgb[0]) + (0.7152 * rgb[1]) + (0.0722 * rgb[2])))

print 'Value:         ' + join2dArr(img, '\t', toValue)
print 'Lightness:   ' + join2dArr(img, '\t', toLightness)
print 'Luminance: ' + join2dArr(img, '\t', toLuminance)

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