gistfile1.rb

#!/usr/bin/env ruby

require "chunky_png"
#require "oily_png"

def screenshot(display = ":0")
  mine_pipe, xwd_pipe = IO.pipe
  png = nil
  
  begin
    pid = Process.spawn "xwd", "-root", "-display", display, in: :close, out: xwd_pipe
    xwd_pipe.close
    xwd_pipe = nil

    
    header_size, file_version,
    pixmap_format, pixmap_depth, pixmap_width, pixmap_height,
    xoffset, byteorder, bitmap_unit, bitmap_bit_order, bitmap_pad,
    bits_per_pixel, bytes_per_line, visual_class,
    red_mask, green_mask, blue_mask, bits_per_rgb,
    number_of_colors, color_map_entires,
    window_width, window_height, window_x, window_y, window_border_width =
      mine_pipe.read(4 * 25).unpack("N*")

    raise "Only direct color images are supported" if (visual_class != 5 && visual_class != 4)
    
    mine_pipe.read((header_size + number_of_colors * 12) - 4 * 25)

    initial = Array.new pixmap_width, pixmap_height
    
    case bits_per_pixel
    when 24
      0.upto(pixmap_height - 1) do |y|
        line = mine_pipe.read(bytes_per_line)
                        .slice(0, bits_per_pixel / 8 * pixmap_width)

        0.upto(pixmap_width - 1) do |x|
          initial[y * pixmap_width + x] =
              (line.getbyte(x * 3 + 2) << 24) |
              (line.getbyte(x * 3 + 1) << 16) |
              (line.getbyte(x * 3 + 0) << 8) |
              0xFF
        end
      end

    when 32
      0.upto(pixmap_height - 1) do |y|
        line = mine_pipe.read(bytes_per_line)
                        .slice(0, bits_per_pixel / 8 * pixmap_width)
        
        0.upto(pixmap_width - 1) do |x|
          pix, = line.slice(x * 4, 4).unpack("V")
          initial[y * pixmap_width + x] = ((pix & 0xFFFFFF) << 8) | 0xFF
        end
      end
      
    else
      raise "Unsupported bpp: #{bits_per_pixel}"
    end
    
    png = ChunkyPNG::Image.new pixmap_width, pixmap_height, initial
    
  ensure
    mine_pipe.close
    xwd_pipe.close unless xwd_pipe.nil?
  end

  png.to_datastream  
end

File.binwrite "shot.png", screenshot