bmp_writer.rb

bmp_writer.rb

# coding: binary

class BMP 
  class Writer
    PIXEL_ARRAY_OFFSET = 54
    BITS_PER_PIXEL     = 24
    DIB_HEADER_SIZE    = 40
    PIXELS_PER_METER   = 2835 # 2835 pixels per meter is basically 72dpi

    def initialize(width, height)
      @width, @height = width, height

      @pixels = Array.new(@height) { Array.new(@width) { "000000" } }
    end

    def [](x,y)
      @pixels[y][x]
    end

    def []=(x,y,value)
      @pixels[y][x] = value
    end

    def save_as(filename)
      File.open(filename, "wb") do |file|
        write_bmp_file_header(file)
        write_dib_header(file)
        write_pixel_array(file)
      end
    end

    private

    def write_bmp_file_header(file)
      file << ["BM", file_size, 0, 0, PIXEL_ARRAY_OFFSET].pack("A2Vv2V")
    end

    def file_size
      PIXEL_ARRAY_OFFSET + pixel_array_size 
    end

    def pixel_array_size
      ((BITS_PER_PIXEL*@width)/32.0).ceil*4*@height
    end
   
    def write_dib_header(file)
      file << [DIB_HEADER_SIZE, @width, @height, 1, BITS_PER_PIXEL,
               0, pixel_array_size, PIXELS_PER_METER, PIXELS_PER_METER, 
               0, 0].pack("Vl<2v2V2l<2V2")
    end

    def write_pixel_array(file)
      @pixels.reverse_each do |row|
        row.each do |color|
          file << pixel_binstring(color)
        end

        file << row_padding
      end
    end

    def pixel_binstring(rgb_string)
      raise ArgumentError unless rgb_string =~ /\A\h{6}\z/
      [rgb_string].pack("H6")
    end

    def row_padding
      "\x0" * (@width % 4)
    end
  end
end

bmp = BMP::Writer.new(2,2)

bmp[0,0] = "ff0000"
bmp[1,0] = "00ff00"
bmp[0,1] = "0000ff"
bmp[1,1] = "ffffff"

bmp.save_as("example_generated.bmp")