scrot.rb

# % ruby scrot.rb /tmp/screenshot.png 
# scrot.rb uses xlib + imlib via ffi to capture screenshots.
require 'ffi'

module X11
  extend FFI::Library
  ffi_lib 'X11'
  attach_function :XOpenDisplay, [:string], :pointer
  attach_function :XFlush, [:pointer], :pointer

  module_function
  def get_display(display = nil); XOpenDisplay(display || ENV['DISPLAY']); end

  @ptr_size = FFI.type_size(:pointer)
  def ptr_size; @ptr_size; end

  def ScreenOfDisplay(d, i);   d.get_pointer( @ptr_size == 4 ? 35*4 : 58*4 );  end
  def RootWindow(d, i=0);      ScreenOfDisplay(d,i).get_int(2*@ptr_size);      end
  def DefaultVisual(d, i=0);   ScreenOfDisplay(d,i).get_pointer( @ptr_size == 4 ? 40 : 48); end
  def DefaultColormap(d, i=0); ScreenOfDisplay(d,i).get_int(12*@ptr_size);     end
end

module Imlib
  extend FFI::Library
  ffi_lib 'Imlib2'
  attach_function :context_set_display,  :imlib_context_set_display,  [:pointer], :int
  attach_function :context_set_visual,   :imlib_context_set_visual,   [:pointer], :int
  attach_function :context_set_colormap, :imlib_context_set_colormap, [:int],     :int
  attach_function :context_set_color_modifier, :imlib_context_set_color_modifier, [:pointer], :int
  attach_function :context_set_operation, :imlib_context_set_operation, [:int],   :int
  IMLIB_OP_COPY = 0

  attach_function :context_set_drawable, :imlib_context_set_drawable, [:int], :int
  attach_function :create_image_from_drawable,
    :imlib_create_image_from_drawable, [:int, :int, :int, :int, :int, :int], :pointer
  attach_function :context_set_image, :imlib_context_set_image, [:pointer], :int
  attach_function :image_set_format, :imlib_image_set_format, [:string], :int
  attach_function :save_image, :imlib_save_image, [:string], :int

  module_function
  def setup_context(display, visual, colormap, colormap_mod=nil, operation=nil)
    context_set_display(display)
    context_set_visual(visual)
    context_set_colormap(colormap)
    context_set_color_modifier(colormap_mod || nil)
    context_set_operation(operation || IMLIB_OP_COPY)
  end
end


if $0 == __FILE__
  fileout = ARGV.first || File.join(ENV['HOME'], 'screenshot.png')

  $d = X11.get_display

  screen, root = X11.ScreenOfDisplay($d, 0), X11.RootWindow($d, 0)
  visual, cm   = X11.DefaultVisual($d, 0),   X11.DefaultColormap($d, 0)

  Imlib.setup_context($d, visual, cm, nil, Imlib::IMLIB_OP_COPY)
  Imlib.context_set_drawable(root)

  s_width, s_height = *screen.get_array_of_int(3*X11.ptr_size, 2)
  image = Imlib.create_image_from_drawable(0, 0, 0, s_width, s_height, 1)
  Imlib.context_set_image(image)

  Imlib.image_set_format('png')
  Imlib.save_image(fileout) if File.directory?(File.dirname(fileout))
end



__END__
  Imlib_Color color; int x, y;
  for (x = 0; x < screen->width; x++) {
    for (y = 0; y < screen->height; y++) {
      imlib_image_query_pixel(x, y, &color);
      printf("#%02x%02x%02x\n", color.red, color.green, color.blue);
    }
  }