From: Michael Neumann <mneumann@...>
Date: 2004-11-16T08:15:07+09:00
Subject: Re: bitmaps

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Joe Van Dyk wrote:
> How can I create a bitmap in Ruby?  I have a bunch of RGBA values, and need
> to convert them into a bitmap or some other picture format.

You can use my appended PNG writer class. Generating PNM images (there 
very different to PNG!!!) is even simpler.

Regards,

   Michael

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#
# PNG writer class for true-color images
#
# Author:: Michael Neumann
# Copyright:: (c) 2004 by Michael Neumann
# 

class Array2
  attr_reader :values
  attr_reader :rows, :cols

  def initialize(rows, cols, &init_block)
    @rows, @cols = rows, cols 
    @values = (0...rows).collect do |i|
      (0...cols).collect do |j|
        if init_block.nil?
          0
        else
          init_block.call(i, j)
        end
      end
    end
  end

  def each_row(&block)
    @values.each(&block)
  end

  def [](row,col)
    raise IndexError if row < 0 or col < 0 or row >= @rows or col >= @cols
    @values[row][col]
  end

  def []=(row,col, value)
    raise IndexError if row < 0 or col < 0 or row >= @rows or col >= @cols
    @values[row][col] = value
  end
end

class RGB
  attr_accessor :r, :g, :b
  def initialize(r,g,b)
    @r, @g, @b = r, g, b
  end
end

class PNG_Writer
  Signature = [137, 80, 78, 71, 13, 10, 26, 10].pack("C*")

  # pixel_data is an Array2
  def write_png(pixel_data, transparent_col=nil,  stream="")
    stream << Signature
    stream << dump_truecolor_ihdr(pixel_data.cols, pixel_data.rows)
    if transparent_col != nil
      t = transparent_col
      stream << dump_truecolor_tRNS(t.r, t.g, t.b) 
    end

    stream << dump_truecolor_nofilter_idat(pixel_data)
    stream << dump_iend

    return stream
  end

  private 

  def dump_chunk(type, data)
    sz = data.size
    raise if sz > 2**31-1

    crc = CRC.instance.crc(type + data)

    [sz, type, data, crc].pack("Na*a*N") 
  end

  COL_TYPE_PALETTE_USED = 1
  COL_TYPE_COLOR_USED = 2
  COL_TYPE_ALPHA_CHANNEL_USED = 4

  def dump_ihdr(width, height, bit_depth, color_type, 
                      compression_method, filter_method, interlace_method)
    data = [width, height, bit_depth, color_type, compression_method, filter_method,
            interlace_method].pack("NNCCCCC")
    dump_chunk('IHDR', data)
  end

  def dump_truecolor_ihdr(w, h)
    dump_ihdr(w, h, 8, COL_TYPE_COLOR_USED, 0, 0, 0)
  end

  # transparency
  def dump_truecolor_tRNS(r, g, b)
    data = [r, g, b].pack("nnn")
    dump_chunk('tRNS', data)
  end

  def dump_iend
    dump_chunk('IEND', "")
  end

  require 'zlib'

  def dump_truecolor_nofilter_idat(data)
    # no filter = 0

    # produce raw image data string (with filter method byte before each scanline)
    str = ""
    data.each_row do |row|
      str << [0].pack("C") # no filter
      row.each {|c| str << [c.r, c.g, c.b].pack("CCC") }
    end
    data = Zlib::Deflate.deflate(str)
    dump_chunk('IDAT', data)
  end

  class CRC
    require 'singleton'
    include Singleton

    def initialize
      @crc_table = Array.new(256)
      make_crc_table
    end

    def self.crc(data)
      instance.crc(data)
    end

    def crc(data)
      update_crc(0xffffffff, data) ^ 0xffffffff
    end

    private

    def update_crc(crc, data)
      c = crc
      data.each_byte { |b| c = @crc_table[(c ^ b) & 0xff] ^ (c >> 8) }
      return c
    end
    
    def make_crc_table
      (0..255).each { |n|
        c = n
        8.times { c = if c & 1 == 1 then 0xedb88320 ^ (c >> 1) else c >> 1 end }
        @crc_table[n] = c
      }
    end
  end

end

if __FILE__ == $0
  data = Array2.new(100,100) {|y,x|
    RGB.new((x+y) % 255 ,0,0)
  }
  print PNG_Writer.new.write_png(data)
end

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