[George Peter Staplin] Fri Oct 26, 2001 - I had some trouble trying to figure out how to get the [canvas] view area 
in pixels for a game I'm writing called The Adventures of Baldo the Alien, so I devised the code below.  
Now it seems so obvious, but at the time it definitely wasn't.  
I guess I expected the canvas to have a builtin command that would tell me stuff like this, 
but I couldn't find a command to do it, so I wrote my own.

Enjoy!
----
  #!/usr/local/bin/wish8.3

  proc getCanvasViewArea {win} {
	
	# This foreach is used only as a "list assign", and has an empty body.
	foreach {junk junk totalXArea totalYArea} [$win cget -scrollregion] {break}
	set xview  [$win xview]
	set yview  [$win yview]
		
	set xstart [expr {int([lindex $xview 0] * $totalXArea)}]
	set xend   [expr {int([lindex $xview 1] * $totalXArea)}] 

	set ystart [expr {int([lindex $yview 0] * $totalYArea)}]
	set yend   [expr {int([lindex $yview 1] * $totalYArea)}] 

		
	return [list $xstart $xend $ystart $yend]
  }

  #Just some simple demonstration code
  proc main {} {
	pack [canvas    .c -scrollregion {0 0 5000 500} -xscrollcommand {.s set}]
	pack [scrollbar .s -command {.c xview} -orient horizontal] -fill x

	pack [label  .l -text [getCanvasViewArea .c]]
	pack [button .b -text "Get View Area" -command {
		.l config -text [getCanvasViewArea .c]
	}]
  }
  main
----
This Only works if the scrollregion is always {0 0 $X $Y} which is not neccessarily true. 
(If I'm plotting geographic data I might have a scroll region of {-180 -90 180 90} 

so I would replace
       foreach {junk junk totalXArea totalYArea} [$win cget -scrollregion]{break}
with the following:
       foreach {x1 y1 x2 y2} [$win cget -scrollregion]{break}
       set totalXArea [expr $x2 - $x1]
       set totalYArea [expr $y2 - $y1]

--''bbh''

----

GAF -- Yes, but the scrollregion is not the viewable area that the user can see right now.  
How do you find that?  For instance: 

 canvas .c  -height 100 -width 100 -bd 1
 pack .c
 .c create rectangle 4 5 103 104 -outline blue
 .c create rectangle 2 2 101 101 -outline green

In this example, on the Mac 10.4.3 ActiveState Tcl/Tk, the blue rectangle is the largest that shows all its lines.  Under X windows on the _same machine_ the green rectangle is the largest completely visible rectangle.  What gives?  Is there any way to really know the actual visible area of a canvas?
-- GAF

[George Peter Staplin] - I think perhaps the [[winfo width/height]] should be considered to account for that.  I'll try to think of a solution.

After some thought it seems, that perhaps a solution would be 

 .c configure -scrollregion [.c bbox all]

For use in combination with the proc above.

--

CJL, who nearly always has 0,0 as his origin, prefers the following to ensure a symmetrical margin around canvas items:

 foreach {top left bottom right} [.c bbox all] { break }
 incr bottom $top
 incr right $left
 .c configure -scrollregion [list 0 0 $right $bottom]


----
!!!!!!
%| [Category Graphics] |%
!!!!!!