The [Xlib] Emulation Layer is Tk's traditional mechanism for
factoring cross-platform support.  Replacement of XLEL is,
in one sense, the main aim of [TkGS], a project initiated by [Frederic Bonnet].

See [http://wiki.tcl.tk/12672] for a discussion of the limitations and problems with the XLEL.

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[Lars H], 8 March 2007:
I just encountered an awkward misfeature of this XLEL: Coordinates of lines drawn on the [canvas] will be rounded to integers! Sure enough, the canvas stores item coordinates as floats ([double]s, even), and [Aqua] (or Quartz 2D [http://developer.apple.com/referencelibrary/GraphicsImaging/idxQuartz-date.html], if we're technical) thinks a point is a pair of floats too, but on its way from one to the other it has to pass through the Xlib Emulation Layer, and Xlib thinks a point is a pair of integers (shorts, even), so any subpixel precision is lost.

While this is not a show-stopper, it does put some restrictions on the kind of things one can do on the canvas. It is probably also relevant for any discussion of [antialias]ing in Tk, but I don't know whether it would be pro or con...

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