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/12673] 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...

[DKF]: Neutral for font rendering; we don't render individual characters but rather segments of text (i.e. all in the same font/color/style).
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[PT] 15-Sep-2003: Tk implements the following Xlib functions, which constitute the [XLEL]. These functions are in the xlib subdirectory of the Tk distribution. They map to corresponding functions on the different platforms and the original Xlib functions on systems using X11:

 xbytes.c:
 
 xcolors.c:
   XParseColor
 
 xdraw.c:
   XDrawLine -> maps to macosx/tkMacOSXDraw.c and win/tkWinDraw.c (called XDrawLines there) 
   XFillRectangle -> maps to macosx/tkMacOSXDraw.c and win/tkWinDraw.c (called XFillRectangles there)
 
 xgc.c:
   XCreateGC
   XChangeGC
   XFreeGC
   XSetForeground
   XSetBackground
   XSetDashes
   XSetFunction
   XSetFillRule
   XSetFillStyle
   XSetTSOrigin
   XSetFont
   XSetArcMode
   XSetStipple
   XSetLineAttributes
   XSetClipOrgin
   TkSetRegion
   XSetClipMask
   XCreateFontCursor /* not implemented */
   XDrawImageString  /* not implemented */
   XDrawPoint
   XDrawSegments /* not implemented */
   XFetchBuffer /* not implemented */
   XFetchName /* not implemented */
   XListProperties /* not implemented */
   XMapRaised /* not implemented */
   XPutImage /* not implemented */
   XQueryTextExtents /* not implemented */
   XReparentWindow /* not implemented */
   XRotateBuffers /* not implemented */
   XStoreBuffer /* not implemented */
   XUndefineCursor /* not implemented */
 
 ximage.c:
   XCreateBitmapFromData
 
 xutil.c:
   XInternAtom
   XGetVisualInfo

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