[Richard Suchenwirth] 2004-09-22 - Here's very simple code for subtracting one polygon from another. The visual effect is that the bigger polygon has a hole in it, which makes it useful for clipping other [canvas] contents:

 proc poly'clip {p1 p2} {
    set res [lrange $p1 0 1]
    lappend res [lindex $p2 0] [lindex $p2 1]
    foreach {x y} $p2 {
        set res [linsert $res 4 $x $y]
    }
    concat $res $p1
 }
That's all. On a canvas where a picture of [JO] was already in place, this example code
 set poly1 {155 83 424 81 424 288 155 288}
 155 83 424 81 424 288 155 288
 set poly2 {228 147 330 147 334 222 228 222}
 228 147 330 147 334 222 228 222
 $canvas create poly [poly'clip $poly1 $poly2] -fill pink
made John look out of the box like this:

[http://mini.net/files/polyclip.gif]

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[Arts and crafts of Tcl-Tk programming]