#!/usr/local/bin/ruby19 -w

require 'RMagick'

NUM_COLORS = 256
HIST_HEIGHT = 200

img = Magick::Image.read('images/Hot_Air_Balloons_H.jpg').first
img = img.quantize(NUM_COLORS)

hist = img.color_histogram

# sort pixels by increasing count
pixels = hist.keys.sort_by {|pixel| hist[pixel] }

scale = HIST_HEIGHT / (hist.values.max*1.025)   # put 2.5% air at the top

gc = Magick::Draw.new
gc.stroke_width(1)
gc.affine(1, 0, 0, -scale, 0, HIST_HEIGHT)

# handle images with fewer than NUM_COLORS colors
start = NUM_COLORS - img.number_colors

pixels.each { |pixel|
    gc.stroke(pixel.to_color)
    gc.line(start, 0, start, hist[pixel])
    start = start.succ
}

hatch = Magick::HatchFill.new("white", "gray95")
canvas = Magick::Image.new(NUM_COLORS, HIST_HEIGHT, hatch)
gc.draw(canvas)

text = Magick::Draw.new
text.annotate(canvas, 0, 0, 0, 20, "Color Frequency\nHistogram") {
    self.pointsize = 10
    self.gravity = Magick::NorthGravity
    self.stroke = 'transparent'
    }

canvas.border!(1, 1, "white")
canvas.border!(1, 1, "black")
canvas.border!(3, 3, "white")
canvas.write("color_histogram.gif")

exit

