xwd

The X-standard xwd utility nicely complements Tcl in at least a couple of ways. It can do useful image manipulation without Tk, and in particular it can be a handy way to convert formats. DKF, for example, suggests

      # Does not work if anything is overlapping the canvas,
      #     and only does the currently visible area.
    exec xwd -id [winfo id $canvaswidget] | convert - foobar.png

[But only available for X, that is, essentially, Unix.]