Quick and simple BMP writer for Ruby

bmp.rb

# This code is meant for debugging purposes.  Its performance is probably bad,
# it's just meant to work for some hacky scripts you throw out tomorrow :)
#   Create an instance of `Bitmap`, passing the file (IO or string), the width and height.
#   Then push one (or more) 32bit pixels in ARGB format using `Bitmap#<<`.
#   Pixel format is thus: 0xAARRGGBB (Alpha is highest byte, blue lowest).
#   (Alpha is ignored by most programs however)

class Bitmap
  MAGIC = "BM"
  
  def initialize(io, width, height)
    io = File.open(io, "wb") if io.is_a?(String)
    @io = io
    @width = width
    @height = height
    
    write_header
  end
  
  private def write_header
    bitmap_size = @width * @height * 4
    file_size = 54 + bitmap_size
    
    # Magic
    @io.write MAGIC
    
    # File size, Reserved, Offset to bitmap:
    @io.write [ file_size, 0, 54 ].pack("l<*")
    
    info = [
      40, # u32, biSize
      @width, # i32, biWidth
      -@height, # i32, biHeight
      1, # u16, biPlanes
      32, # u16, biBitCount
      0, # u32, biCompression
      bitmap_size, # u32, biSizeImage
      0, # i32, biXPelsPerMeter
      0, # i32, biYPelsPerMeter
      0, # u32, biClrUsed
      0, # u32, biClrImportant
    ]
    
    @io.write info.pack("L<l<l<S<S<l<l<L<L<l<l<")
  end
  
  def <<(argb)
    @io.write Array(argb).pack("l<*")
    self
  end
  
  def close
    @io.flush
    @io.close
  end
end

# Usage sample: 256x256 bitmap with a color gradient:
# Best piped through display: ruby bmp.rb | display -
# Or put something else than `STDOUT`.

pic = Bitmap.new(STDOUT, 256, 256)

256.times do |y|
  c = y << 8 # Use the y as green channel
  pic << Array.new(256){|i| c | i} # And x as blue channel.
end

pic.close # Done.