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color_histogram.rb.html
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<!DOCTYPE public PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN""http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
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<title>RMagick example: color_histogram.rb</title>
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<body>
<h1>color_histogram.rb</h1>
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<div class="bodyfloat">
<pre class="language-ruby">
# frozen_string_literal: true
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 do |pixel|
gc.stroke(pixel.to_color)
gc.line(start, 0, start, hist[pixel])
start = start.succ
end
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") do |options|
options.pointsize = 10
options.gravity = Magick::NorthGravity
options.stroke = 'transparent'
end
canvas.border!(1, 1, 'white')
canvas.border!(1, 1, 'black')
canvas.border!(3, 3, 'white')
canvas.write('color_histogram.gif')
exit
</pre>
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