proc main {} {
    set w .c
    pack [canvas $w] -fill both -expand 1
    $w create text 189 132 -text "Hello, world!"
    iris $w 0 0 [$w cget -width] [$w cget -height] 100
 }
if 0 {The ''iris'' command first constructs the smallest circle that fully
encloses the given rectangle (normally, the canvas itself), and creates
a square polygon ''frame'' which will enclose
the round hole. It then starts the timed animation.}
 proc iris {w xa ya xb yb ms} {
    set radius [expr {hypot($ya-$yb,$xa-$xb)/2}]
    set xm [expr {($xa + $xb)/2.}]
    set ym [expr {($ya + $yb)/2.}]
    set x0 [expr {$xm - $radius}]
    set x1 [expr {$xm + $radius}]
    set y0 [expr {$ym - $radius}]
    set y1 [expr {$ym + $radius}]
    set frame [list $x0 $y0 $x1 $y0 $x1 $y1 $x0 $y1]
    set id [$w create poly $frame -fill black]
    iris'close $w $id $radius $xm $ym $ms
 }
if 0 {This draws the iris hole by clipping a 32-gon with given radius
from the enclosing frame:}
 proc iris'set {w tag radius xm ym} {
    set frame [lrange [$w coords $tag] end-7 end]
    set iris {}
    for {set i 0} {$i<32} {incr i} {
        set angle [expr {acos(-1)*$i/16}]
        lappend iris [expr {$xm + $radius * cos($angle)}]
        lappend iris [expr {$ym + $radius * sin($angle)}]
    }
    $w coords $tag [poly'clip $frame $iris]
 }
if 0 {This reduces the radius by a certain amount and redraws the iris.
This is continued in ''ms'' millisecond intervals until the radius is
small enough that the hole appears fully closed.}
 proc iris'close {w tag radius xm ym ms} {
    if {$radius > 0} {
        iris'set $w $tag $radius $xm $ym 
        after $ms [list iris'close $w $tag [expr {$radius-2}] $xm $ym $ms] 
    }
 }
#-- code borrowed from [Polygon clipping]
  proc poly'clip {p1 p2} {
     set p2_ {}
     foreach {x y} $p2 {set p2_ [linsert $p2_ 0 $x $y]}
     concat [lrange $p1 0 1] [lrange $p2 0 1] $p2_ $p1
 }
#-- Testing the "hole" thing:
 main