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.

See [http://tronche.com/gui/x/xlib/] for an XLib manual and [http://tronche.com/gui/x/xlib/function-index.html] for a complete Xlib function index.

----
[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).
----
**Functions in XLEL**

[PT] 15-Sep-2003: Tk implements the following Xlib functions, which constitute the [XLEL]. Some 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. Other functions reside in the platform's subdirectories and the 'generic' subdirectory. Note, the functions in the 'mac' directory are no longer supported/included in Tk 8.5.

 xlib/xbytes.c: (defines a table to reverse bit order of bytes)

 xlib/xcolors.c: (contains the routines used to map from X color names to RGB and pixel values, all colors used by Tk are defined here)
   XParseColor
   FindColor (not an Xlib function)
 
 xlib/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)
 
 xlib/xgc.c: (contains generic routines for manipulating X graphics)
   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 */
 
 xlib/ximage.c: (contains X bitmap and image routines)
   XCreateBitmapFromData
 
 xlib/xutil.c: (contains generic X emulation routines)
   XInternAtom
   XGetVisualInfo

win/tkWinDraw.c | macosx/tkMacOSXDraw.c | mac/tkMacDraw.c:
  XCopyArea
  XCopyPlane
  XDrawLines (mapped from xlib/xdraw.c)
  XDrawSegments (only macosx and mac)
  XFillPolygon
  XDrawRectangle
  XDrawRectangles (only macosx)
  XFillRectangles (mapped from xlib/xdraw.c) *
  XDrawArc
  XDrawArcs (only macosx)
  XFillArc
  XFillArcs (only macosx)
  XMaxRequestSize (only macosx)
  
(Hmm, XFillArcs and XDrawArcs are never called anywhere in the core. Why are they there??)

generic/tkPointer.c:
  XGrabPointer
  XUngrabPointer
  XDefineCursor

mac/tkMacKeyboard.c:
  XKeycodeToKeysym
  XGetModifierMapping
  XFreeModifiermap
  XKeysymToString
  XStringToKeysym
  XKeysymToKeycode

mac/tkMacSubwindows.c:
  XDestroyWindow
  XMapWindow
  XUnmapWindow
  XResizeWindow
  XMoveResizeWindow
  XMoveWindow
  XRaiseWindow
  XConfigureWindow

mac/tkMacWindowMgr.c:
  XSetInputFocus
  XGrabKeyboard
  XUngrabKeyboard
  XQueryPointer

macosx/tkMacOSXKeyboard.c:
  XKeycodeToKeysym
  XGetModifierMapping
  XFreeModifiermap
  XKeysymToString
  XStringToKeysym
  XKeysymToMacKeycode (only macosx; not an Xlib function)
  XKeysymToKeycode

macosx/tkMacOSXKeyEvent.c:
  XGrabKeyboard
  XUngrabKeyboard

macosx/tkMacOSXMouseEvent.c:
  XQueryPointer

macosx/tkMacOSXSubwindows.c | win/tkWinWindow.c:
  XDestroyWindow
  XMapWindow
  XUnmapWindow
  XResizeWindow
  XMoveResizeWindow
  XMoveWindow
  XRaiseWindow
  XLowerWindow (only macosx)
  XConfigureWindow
  XClearWindow (only win)
  XChangeWindowAttributes (only win)

macosx/tkMacOSXWm.c:
  XSetInputFocus

macosx/tkMacOSXXStubs.c | win/tkWinImage.c:
  XCreateImage
  XGetImage
  XGetImageZPixmap (only win; not an Xlib function)

win/tkWinColor.c:
  XAllocColor
  XFreeColors
  XCreateColormap
  XFreeColormap

win/tkWinPixmap.c:
  XGetGeometry

win/tkWinPointer.c:
  XGrabKeyboard
  XUngrabKeyboard
  XQueryPointer
  XWarpPointer
  XGetInputFocus
  XSetInputFocus 

win/tkWinX.c:
  XBell


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