histogram.rb

# This routine needs the color_histogram method
# from either ImageMagick 6.0.0 or GraphicsMagick 1.1
# Specify an image filename as an argument.

require 'RMagick'

class PixelColumn < Array
    def initialize(size)
        super
        fill {Magick::Pixel.new}
    end
    
    def reset(bg)
        each {|pixel| pixel.reset(bg)}
    end
end

module Magick
    
    class Pixel
        def reset(bg)
            self.red = bg.red
            self.green = bg.green
            self.blue = bg.blue
            self.opacity = bg.opacity
        end
    end
    
    class Image

      private
        HISTOGRAM_COLS = 256
        HISTOGRAM_ROWS = 200
        MAX_QUANTUM = 255
        AIR_FACTOR = 1.025

        # The alpha frequencies are shown as white dots.
        def alpha_hist(freqs, scale, fg, bg)
            histogram = Image.new(HISTOGRAM_COLS, HISTOGRAM_ROWS) {
                self.background_color = bg
                self.border_color = fg
            }

            gc = Draw.new
            gc.affine(1, 0, 0, -scale, 0, HISTOGRAM_ROWS)
            gc.fill('white')

            HISTOGRAM_COLS.times do |x|
                gc.point(x, freqs[x])
            end

            gc.draw(histogram)
            histogram['Label'] = 'Alpha'

            return histogram
        end

        def channel_histograms(red, green, blue, int, scale, fg, bg)
            rgb_histogram = Image.new(HISTOGRAM_COLS, HISTOGRAM_ROWS) {
                self.background_color = bg
                self.border_color = fg
            }
            rgb_histogram['Label'] = 'RGB'
            red_histogram   = rgb_histogram.copy
            red_histogram['Label'] = 'Red'
            green_histogram = rgb_histogram.copy
            green_histogram['Label'] = 'Green'
            blue_histogram  = rgb_histogram.copy
            blue_histogram['Label'] = 'Blue'
            int_histogram = rgb_histogram.copy
            int_histogram['Label'] = 'Intensity'
            int_histogram.matte = true

            rgb_column   = PixelColumn.new(HISTOGRAM_ROWS)
            red_column   = PixelColumn.new(HISTOGRAM_ROWS)
            green_column = PixelColumn.new(HISTOGRAM_ROWS)
            blue_column  = PixelColumn.new(HISTOGRAM_ROWS)
            int_column   = PixelColumn.new(HISTOGRAM_ROWS)

            HISTOGRAM_COLS.times do |x|
                HISTOGRAM_ROWS.times do |y|

                    yf = Float(y)
                    if yf >= HISTOGRAM_ROWS - (red[x] * scale)
                        red_column[y].red = MaxRGB
                        red_column[y].green = 0
                        red_column[y].blue = 0
                        rgb_column[y].red = MaxRGB
                    end
                    if yf >= HISTOGRAM_ROWS - (green[x] * scale)
                        green_column[y].green = MaxRGB
                        green_column[y].red = 0
                        green_column[y].blue = 0
                        rgb_column[y].green = MaxRGB
                    end
                    if yf >= HISTOGRAM_ROWS - (blue[x] * scale)
                        blue_column[y].blue = MaxRGB
                        blue_column[y].red = 0
                        blue_column[y].green = 0
                        rgb_column[y].blue = MaxRGB
                    end
                    if yf >= HISTOGRAM_ROWS - (int[x] * scale)
                        int_column[y].opacity = TransparentOpacity
                    end
                end
    
                rgb_histogram.store_pixels(  x, 0, 1, HISTOGRAM_ROWS, rgb_column)
                red_histogram.store_pixels(  x, 0, 1, HISTOGRAM_ROWS, red_column)
                green_histogram.store_pixels(x, 0, 1, HISTOGRAM_ROWS, green_column)
                blue_histogram.store_pixels( x, 0, 1, HISTOGRAM_ROWS, blue_column)
                int_histogram.store_pixels(  x, 0, 1, HISTOGRAM_ROWS, int_column)
                rgb_column.reset(bg)
                red_column.reset(bg)
                green_column.reset(bg)
                blue_column.reset(bg)
                int_column.reset(bg)
            end

           return red_histogram, green_histogram, blue_histogram, rgb_histogram, int_histogram
        end

        # Make the color histogram. Quantize the image to 256 colors if necessary.
        def color_hist(fg, bg)
            img = number_colors > 256 ? quantize(256) : self

            begin
                hist = img.color_histogram
            rescue NotImplementedError
                $stderr.puts "The color_histogram method is not supported by this version "+
                             "of ImageMagick/GraphicsMagick"

            else
                pixels = hist.keys.sort_by {|pixel| hist[pixel] }
                scale = HISTOGRAM_ROWS / (hist.values.max*AIR_FACTOR)

                histogram = Image.new(HISTOGRAM_COLS, HISTOGRAM_ROWS) {
                    self.background_color = bg
                    self.border_color = fg
                    }

                x = 0
                pixels.each do |pixel|
                    column = Array.new(HISTOGRAM_ROWS).fill {Pixel.new}
                    HISTOGRAM_ROWS.times do |y|
                        if y >= HISTOGRAM_ROWS - (hist[pixel] * scale)
                            column[y] = pixel;
                        end
                    end
                    histogram.store_pixels(x, 0, 1, HISTOGRAM_ROWS, column)
                    x = x.succ
                end

                histogram['Label'] = 'Color Frequency'
                return histogram
            end
        end

        # Use AnnotateImage to write the stats.
        def info_text(fg, bg)
            klass = class_type == DirectClass ? "DirectClass" : "PsuedoClass"

            text = <<-END_TEXT
Format: #{format}
Geometry: #{columns}x#{rows}
Class: #{klass}
Depth: #{depth} bits-per-pixel component
Colors: #{number_colors}
            END_TEXT

            info = Image.new(HISTOGRAM_COLS, HISTOGRAM_ROWS) {
                self.background_color = bg
                self.border_color = fg
                }

            gc = Draw.new

            gc.annotate(info, 0, 0, 0, 0, text) {
                self.stroke = 'transparent'
                self.fill = fg
                self.gravity = CenterGravity
                }
            info['Label'] = 'Info'

            return info
        end

        def intensity_hist(int_histogram)

            gradient = (Image.read("gradient:#ffff80-#ff9000") { self.size="#{HISTOGRAM_COLS}x#{HISTOGRAM_ROWS}" }).first
            int_histogram = gradient.composite(int_histogram, CenterGravity, OverCompositeOp)

            int_histogram['Label'] = 'Intensity'

            return int_histogram
        end

        # Returns a value between 0 and MAX_QUANTUM. Same as the PixelIntensity macro.
        def pixel_intensity(pixel)
            (306*(pixel.red & MAX_QUANTUM) + 601*(pixel.green & MAX_QUANTUM) + 117*(pixel.blue & MAX_QUANTUM))/1024
        end

      public
        # Create the histogram montage.
        def histogram(fg='white', bg='black')

            red   = Array.new(HISTOGRAM_COLS, 0)
            green = Array.new(HISTOGRAM_COLS, 0)
            blue  = Array.new(HISTOGRAM_COLS, 0)
            alpha = Array.new(HISTOGRAM_COLS, 0)
            int   = Array.new(HISTOGRAM_COLS, 0)

            rows.times do |row|
                pixels = get_pixels(0, row, columns, 1)
                pixels.each do |pixel|
                    red[pixel.red & MAX_QUANTUM] += 1
                    green[pixel.green & MAX_QUANTUM] += 1
                    blue[pixel.blue & MAX_QUANTUM] += 1

                    # Only count opacity channel if some pixels are not opaque.
                    if !opaque?
                        alpha[pixel.opacity & MAX_QUANTUM] += 1
                    end
                    v = pixel_intensity(pixel)
                    int[v] += 1
                end
            end

            # Scale to chart size. When computing the scale, add some "air" between
            # the max frequency and the top of the histogram. This makes a prettier chart.
            # The RGBA and intensity histograms are all drawn to the same scale.
            
            max = [red.max, green.max, blue.max, alpha.max, int.max].max
            scale = HISTOGRAM_ROWS / (max*AIR_FACTOR)

            charts = ImageList.new

            # Add the thumbnail.
            thumb = copy
            thumb['Label'] = File.basename(filename)
            charts << thumb

            # Compute the channel and intensity histograms.
            channel_hists = channel_histograms(red, green, blue, int, scale, fg, bg)

            # Add the red, green, and blue histograms to the list
            charts << channel_hists.shift
            charts << channel_hists.shift
            charts << channel_hists.shift

            # Add Alpha channel or image stats
            if !opaque?
                charts << alpha_hist(alpha, scale, fg, bg)
            else
                charts << info_text(fg, bg)
            end

            # Add the RGB histogram
            charts << channel_hists.shift

            # Add the intensity histogram.
            charts << intensity_hist(channel_hists.shift)

            # Add the color frequency histogram.
            charts << color_hist(fg, bg)

            # Make a montage.
            histogram = charts.montage {
                self.background_color = bg
                self.stroke = 'transparent'
                self.fill = fg
                self.border_width = 1
                self.tile         = "4x2"
                self.geometry     = "#{HISTOGRAM_COLS}x#{HISTOGRAM_ROWS}+10+10"
            }

            return histogram
        end
    end
end

puts <<END_INFO

This example shows how to get pixel-level access to an image.
Usage: histogram.rb <image-filename>

END_INFO

# Get filename from command line.
if !ARGV[0] then
    puts "No filename argument. Defaulting to Flower_Hat.jpg"
    filename = '../doc/ex/images/Flower_Hat.jpg'
else
    filename = ARGV[0]
end

# Only process first frame if multi-frame image
image = Magick::Image.read(filename)
if image.length > 1
    puts "Charting 1st image"
end
image = image.first

# Give the user something to look at while we're working.
name = File.basename(filename).sub(/\..*?$/,'')
$defout.sync = true
printf "Creating #{name}_Histogram.miff"

timer = Thread.new do
    loop do
        sleep(1)
        printf "."
    end
end

# Generate the histograms
histogram = image.histogram(Magick::Pixel.from_color('white'), Magick::Pixel.from_color('black'))

# Write output file
histogram.compression = Magick::ZipCompression
histogram.write("./#{name}_Histogram.miff")

Thread.kill(timer)
puts "Done!"
exit