Re: How to center a circle?

Looking at your images, the rectangle and circle are closer to the
top-left corner of the canvas than the bottom-right - is this what you
mean ?

Yes, exactly. However, meanwhile I got convinced that it was only an
optical illusion (with the help of Tk itself): I enhanced my OP and it
showed that there is no difference.
---
package require Tk
package require tkpath

foreach ch [winfo children "."] {destroy $ch}

set useTK 0
if {$argc} {
    set useTK 1
}
if {$useTK} {
    set c [canvas .c -background gray50 -width 200 -height 200]
} else {
    set c [tkp::canvas .c -background gray50 -width 200 -height 200]
}
pack $c -padx 10 -pady 10

set width  [$c cget -width]
set height [$c cget -height]

set x1 [expr {$width * 0.04}]
set y1 $x1
set x2 [expr {$width - $width * 0.04}]
set y2 $x2
if {$useTK} {
    $c create oval $x1 $y1 $x2 $y2 -fill white -outline white
} else {
    set xc [expr {$x1 + ($x2 - $x1) / 2.0}]
    set yc [expr {$y1 + ($y2 - $y1) / 2.0}]
    set r  [expr {($x2 - $x1) / 2.0}]
    $c create circle $xc $yc -r $r -fill white -stroke white
}
$c create rectangle $x1 $y1 $x2 $y2 -outline red
puts "horizontal diff: [expr {$x1}] vs [expr {$width - $x2}]"
puts "vertical diff: [expr {$y1}] vs [expr {$height - $x2}]"
---
So, no matter what the monitor shows (or what we believe it shows) the
output says that all margins are 8 px.
Helmut