# circumcenter.tcl
# Quick demo program about finding the circumcenter of three # points. E.g. given three points, find the center of the # circle which passes through all three points. Derived from # the book "A Programmer's Geometry". --willdye, 2004/11/23
set Kx 80.0 ; set Ky 50.0 ;# First point set Lx 200.0 ; set Ly 20.0 ;# Second point set Mx 230.0 ; set My 100.0 ;# Third point
set LKx expr { $Lx - $Kx } ; set LKy expr { $Ly - $Ky } set MKx expr { $Mx - $Kx } ; set MKy expr { $My - $Ky }
set determinant expr { $LKx * $MKy - $MKx * $LKy } if {expr { abs( $determinant ) < 0.0001 }} {
puts "Error: two or more points are coincident." ; exit}
set d2 expr { 0.5 / $determinant }
set LKr expr { $LKx * $LKx + $LKy * $LKy } set MKr expr { $MKx * $MKx + $MKy * $MKy } set Cx expr {( $LKr * $MKy - $MKr * $LKy ) * $d2 + $Kx } set Cy expr {( $LKx * $MKr - $MKx * $LKr ) * $d2 + $Ky }
# We'll probably want the radius, also. The straightforward # method should be good enough in this case, but of course in # general it is not very efficent, and has some accuracy issues.
set rad [expr { sqrt( pow( ( $Cx - $Kx ), 2 ) +
pow( ( $Cy - $Ky ), 2 ) ) }]
# Display the result.
puts "Circumcenter, as X/Y/Radius: $Cx $Cy $rad"
if { ! package present Tk} {exit} destroy .c ; canvas .c -background gray ; pack .c .c create polygon $Lx $Ly $Mx $My $Kx $Ky -fill white .c create oval $Cx $Cy $Cx $Cy -fill black .c create oval expr { $Cx - $rad } expr { $Cy - $rad } \
[expr { $Cx + $rad }] [expr { $Cy + $rad }] -outline red