forest_coverage_by_map.rb

#!/usr/bin/env ruby
#
# @see https://www.eea.europa.eu/data-and-maps/figures/forest-map-of-europe-1
# @note uses imagemagick histogram to count unique colors
# 

COUNTRY_IMAGE=ARGV[0]

COLOR_WEIGHTS={
  '#FFFFFF' => 0..1,
  '#FFFFBE' => 2..10,
  '#D3FFBE' => 11..25,
  '#4CE600' => 26..50,
  '#38A800' => 51..75,
  '#267300' => 76..100,
}
COLORS = COLOR_WEIGHTS.keys

HIST=`convert #{COUNTRY_IMAGE} -define histogram:unique-colors=true -format %c histogram:info:-`

PIXEL_COUNTS = Hash.new(0).merge Hash[
  HIST.scan(/^\s+(\d+):.*\s(#[0-9A-F]+)\s.*$/).map do |count, color|
    [color, count.to_i]
  end
]

TOTAL_PIXELS = COLORS.reduce(0){|s, k| s + PIXEL_COUNTS[k]}

def calc_total_weighted(&method)
  COLORS.reduce(0) do |s, k|
    r = COLOR_WEIGHTS[k]
    m = method.call(r)
    w = m / 100.0
    s + PIXEL_COUNTS[k] * w
  end
end

def calc_total_percent(&method)
  (calc_total_weighted(&method) / TOTAL_PIXELS * 100).round
end

min = calc_total_percent{|r| r.min}
max = calc_total_percent{|r| r.max}
avg = calc_total_percent{|r| (r.max + r.min) / 2}
fin0 = calc_total_percent{|r| r.max * 0.925}

puts "total forested: min: #{min} || avg: #{avg} || fin0: #{fin0} || max: #{max}"