What is the Visitor extension:
  The visitors extension was made to enable users to add "C" operations on
  canvas graphics items without having to constantly be changing the
  Tk core to do so (yes it required a core change :-). There is a sample
  visitor included in the release which performs coord rotation on each
  specified canvas item. This may be used as an example for further visitors.

  Of course, this won't help you create items in C. I've included an example
  of how I did some creation routines (quickly) once.


Getting the patch:
  Presently, the patch is available from:
        ftp://pgw.on.ca/pub/tcl

  I've also uploaded it to:
	ftp://ftp.neosoft.com/pub/tcl/incoming
 

Applying the patch:
  The tk4.1visitors.patch is a patch for tk4.1 (final).
  Run "patch -p <tk4.1visitors.patch" from the top directory
  of the tk4.1 distribution. This info is also supplied in the patch
  file itself.

  Once you have this patch compiled there is a little sample of what
  the rotation visitor does. run rotate_test.tk with the new wish. The
  usage is:
      canvas visit rotate tagOrID -angle angle -xcenter x -ycenter y


More information:
  Nobody's reported any problems installing the patch so far, except for
  some minor incompatibilty with Jeffrey Hobbs' "Tk entry widget
  validation patch" (in tkEntry.c - see below).

  However, there is a mailing list for sharing information/problems with
  the visitors extension as well as any other extensions (preferably)
  related to the tk canvas widget.

  You can subscribe to the list by sending e-mail to:
        Majordomo@pgw.on.ca

  with the mail body:
        subscribe canvas-visitors

  You can also send mail directly to Matthew Rice at:
        Matthew.Rice@pgw.on.ca


What is a canvas extension doing with tkEntry.c ????:
  This visitor patch includes Jan Nijtmans' "dash patch". Part of the
  dash patch is support for "states". Hope this explains everything :-)
  If not, do a diff on tkEntry.c and tkEntry.c.orig.


What visitor specific changes were required:
  For interest's sake (and because there is no supporting documentation
  other than the code) here are the major file changes:

  tkCanvas.c    - Added the canvas visit operation in the command list. There
                    is also a Tk_CreateVisitorType function so that you
                    may add your own without needing core changes.
  tkVisRotate.c - (new file) This is the module that implements the rotation
  tkVisitor.h   - (new file) This is the export canvas item structures.
  tk.h          - Added Tk_Visitor and Tk_VisitorType structures. 
                  Added a VisitorProc & BboxProc to the Tk_Item structure.


Where's the C API for creating canvas items:
  There is also a file called C_Canvas_eg.c which shows a very simple 
  (read hacked) way to create canvas items in C only. Don't expect it
  to compile. I used this file to map an existing API of graphic objects
  to canvas items to speed up rendering. Basically it was just a copy'n'paste
  of the important parts of the CreateItem and ConfigureItem routines
  in each item module. (It hasn't even been updated for tk4.1 final)

  A true C API for item creation is in the works but it may take a little while.