[MG] Apr 29th 2004 - A friend of mine who I introduced to Tcl just started learning the [canvas] widget, and now that I've finally (started to get/) got it figured out myself, I wrote this to give her a hand. 
It's very basic; the canvas itself is dark green, and the window (.)'s title shows the height and width of the canvas as you resize it.

If you left-click the canvas, it adds a (playing) card image. Left-clicking a card makes that the 'active card'. (The Active Card is the Ace of Spaces, all others are the Ace of Hearts; she's writing a card-game:) If you right-click on the Active Card, it displays (via [puts]) that card's coordinates. 
Right-clicking anywhere else will show the cursor's coordinates.

It's fairly basic, like I said, but the main point was to help her figure out the coordinates for all the rows of cards (hence those images being used). Ideally, there would be keyboard bindings for moving the cards up/down/left/right by a small amount each key-press, and perhaps raising or lowering the cards, too; 
if someone wants to add that, please go ahead :) 
Hope someone finds this useful. (If you do, and want something with a little more substantial to play with, see [Spider Solitaire], my first real venture into the canvas widget.)

The code:
----
 pack [canvas .c -bg darkgreen -borderwidth 0 -highlightthickness 0] -expand 1 -fill both
 namespace eval ::img {}
  image create photo ::img::ah -format gif -data {
       R0lGODlhRwBgAKEAAH//1AAAAP////8AACH5BAEAAAAALAAAAABHAGAAAAL/BIKpy+0PYzBH2I
       uz3rz7L0wBSJamiZzquqbawMZyOL7zfbrYwOP+p7vwYL+iJijo9YxMWkZJbBaDy2RUiqMOh9if
       bgvuZmvW51XMcm2FXHRM3bZW3SokfUq+G+36MRsW15dTs7YmOGgBZnhYAqd4xujhqBjZSPZYab
       mzmAmUJ9ep+RQqStryaVqaioK66umKCKsqK9lKe2R7i8Gnu5vb6wTMwStMLLPI6WdESen1u4KJ
       6WMM/Sit/GN9fUOtot2M7fMdNv1cPY7HND7HbX5uvef+Tp4ute3cRR8vFrjP39XtVkBaA2UVhH
       XQVcJVC1M1NPWQVMRQEztVzHTxDqSMYm76ceTH6SOWayKbaLNQUh28YLROspRVqE1KlYCqzLxz
       k05ONztnIJMp79DPJT19nrEZVOjKl7C2FTXKxlcvKBmeQmVn1ejGpJH6MW25IavPsFwZlnVYQV
       hVChLaun3b1kABADs=}
  image create photo ::img::as -format gif -data {
       R0lGODlhRwBgAKEAAH//1AAAAP///////yH5BAEAAAAALAAAAABHAGAAAAL/BIKpy+0PYzBH2I
       uz3rz7L0wBSJamiZzquqbZyMZyCGP1jJfuleQ+uLP0fsRNMBUsFo+jpPK3i96ePumCuoQ9sFDt
       1MllIYc0cPg0tk7PKjO7un535VRnnI7+uvHA25XfUtMA2OY1SKhjyICYKOSQcwfH00QDuTeTRI
       apUKcXibKodBnDyZn1VFr2GSja46qJM5qXRjvXRfsq23fblNur6wEc7BuiarxpWUtsSrrap9xr
       zMwqU/paDC1s49xhPVZW6c2toT2ZDb4MS1Iu7Ut5JV4YiPuLq1oLOn+fi02fllfoHzx099Cp6z
       bOXMFvDKPBA6bNnTSC4n7lA1XPkUCD3y68NcLI0d5DfgeNJOTYLkJKcOtOkhQJ02JJci7xLcRW
       UKPFlrP2jQw5aOaLmvUWbYHAchfGou6MBhW6LaBORSGnary41FPHrbD+8AyI9OrUR1hnkc3pc9
       pXMaHMZWoLsJNXIuxazrXV6hDdk4m27GVkCXAsvoKHFrZr+DAQmooR9pvU2OShunL8Un5jmTAf
       vZcrT+vsWU/kYIxHlzX9ATQj1XIFDWGNiowp2LETV0Kd1jXusX4005ESdXc40cK/BS9uxzcedb
       S5xGmO5bnywtB/VxDOYYIBCdy7e+duoAAAOw==}
 
 bind .c <ButtonPress-1> {bPress1 %W %x %y}
 bind .c <ButtonPress-3> {bPress3 %W %x %y}
 bind . <Configure> {wm title . "Canvas Example (Height = [winfo height .c] / Width = [winfo width .c])"}
 
 proc bPress1 {w x y} {
 
   set i [lindex [$w find overlapping $x $y $x $y] end]
   if { $i == "" } {
        set i [$w create image $x $y -anchor nw -image ::img::ah -tags card]
        setFocus $w $i
      } else {
        setFocus $w $i
      }
 
 };# bPress1
 
 proc bPress3 {w x y} {
 
   set close [lindex [$w find overlapping $x $y $x $y] end]
   if { $close != "" && [lsearch [.c gettags $close] "hasfocus"] != "-1" } {
        puts "Coords of Card: [.c coords $close] ($close)"
      } else {
        puts "Coords of Click: $x $y ($close)"
      }
   
 };# bPress3
 
 proc setFocus {w i} {
 
   if { [.c find withtag hasfocus] != "" } {
        $w itemconfigure [.c find withtag hasfocus] -image ::img::ah
        $w dtag [.c find withtag hasfocus] hasfocus
      }
   $w focus $i
   $w itemconfigure $i -image ::img::as
   $w addtag hasfocus withtag $i
 
 };# setFocus
 
----
For another canvas example, see [A minimal doodler explained] or [Simple Canvas Demo]
----
[[
[Category Example] - [Category Graphics] 
]]