Find skin tones with ruby-vips.

skin.rb

#!/usr/bin/ruby

require 'rubygems'
require 'vips'

im = VIPS::Image.new(ARGV[0])

begin
    # import to CIELAB with lcms
    # if there's no profile there, we'll fall back to the thing below
    cielab = im.icc_import_embedded(:relative)
rescue
    # nope .. use the built-in converter instead
    cielab = im.srgb_to_xyz().xyz_to_lab()
end 

# convert to LCh
lch = cielab.lab_to_lch()

# get the h band
h = lch.extract_band(2)

# find all pixels with a hue in the range 60 - 90 (approximately skin tones)
mask = h.more(60).and(h.less(90))

mask.write(ARGV[1])