[http://mini.net/files/chatcanvas.jpg] "cool" by [dkf], "very" by [RS], "Bye" by [stevel], all in a few minutes...
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n the [Tcl chatroom] we wanted to exchange drawings too, so here's an ultra quick shot we came up with:
 proc transmit {w id} {
	    set ::entry "\$white_board create line [string map {.0 {}} [$w coords $id]]"
	    .e selection range 0 end
 }
 entry .e -textvar entry -bg white
 bind .e <Return> {eval $entry}
 set white_board [canvas .c -bg white]
 bind .c <1> {set entry ""; set id [%W create line %x %y %x %y]}
 bind .c <B1-Motion> {%W coords $id [concat [%W coords $id] %x %y]}
 bind .c <ButtonRelease-1> {transmit %W $id}
 pack .e .c -fill both -expand 1
Test data:
 $white_board create line 190.0 142.0 190.0 142.0 189.0 142.0 187.0 143.0 186.0 143.0  184.0 144.0 180.0 145.0 177.0 146.0 175.0 148.0 173.0 150.0 171.0 152.0 171.0 154.0 171.0 156.0 171.0 157.0 171.0 158.0 171.0 160.0 172.0 161.0 173.0 162.0 174.0 163.0 175.0 163.0 176.0 164.0 178.0 164.0 179.0 164.0 181.0 164.0 183.0 163.0 184.0 162.0 186.0 162.0 188.0 161.0 189.0 161.0 190.0 160.0 191.0 159.0 192.0 158.0 193.0 158.0 194.0 157.0 194.0 156.0 194.0 154.0 195.0 153.0 196.0 152.0 197.0 151.0 198.0 150.0 200.0 150.0 202.0 149.0 203.0 149.0 204.0 148.0 205.0 148.0 206.0 148.0 204.0 148.0 202.0 148.0 199.0 148.0 196.0 148.0 195.0 149.0 193.0 150.0 191.0 150.0 191.0 152.0 191.0 153.0 191.0 154.0 191.0 156.0 192.0 157.0 193.0 158.0 194.0 160.0 195.0 161.0 196.0 162.0 198.0 162.0 199.0 163.0 200.0 163.0 202.0 163.0 204.0 162.0 206.0 162.0 207.0 160.0 208.0 159.0 208.0 157.0 208.0 156.0 208.0 155.0 208.0 154.0 207.0 153.0 206.0 152.0 205.0 151.0 204.0 150.0 203.0 150.0 203.0 149.0 204.0 149.0 205.0 149.0 206.0 149.0 207.0 149.0 209.0 149.0 211.0 150.0 214.0 150.0 217.0 150.0 219.0 150.0 220.0 150.0 221.0 149.0 223.0 149.0 224.0 149.0 223.0 150.0 222.0 150.0 221.0 151.0 219.0 152.0 218.0 153.0 217.0 154.0 217.0 155.0 217.0 156.0 217.0 157.0 218.0 158.0 218.0 159.0 219.0 159.0 219.0 160.0 219.0 161.0 220.0 161.0 221.0 161.0 222.0 161.0 223.0 161.0 224.0 160.0 225.0 159.0 226.0 158.0 227.0 157.0 227.0 156.0 227.0 155.0 226.0 154.0 226.0 153.0 225.0 152.0 223.0 151.0 222.0 150.0 221.0 150.0 220.0 150.0 221.0 150.0 222.0 150.0 223.0 150.0 224.0 150.0 225.0 150.0 226.0 150.0 227.0 150.0 228.0 150.0 229.0 150.0 230.0 150.0 231.0 150.0 233.0 149.0 233.0 148.0 235.0 147.0 235.0 146.0 235.0 145.0 236.0 144.0 237.0 141.0 238.0 139.0 239.0 137.0 240.0 135.0 240.0 134.0 240.0 132.0 239.0 130.0 238.0 128.0 238.0 127.0 238.0 125.0 237.0 124.0 236.0 126.0 235.0 129.0 235.0 132.0 235.0 135.0 235.0 138.0 235.0 140.0 236.0 142.0 236.0 144.0 236.0 147.0 237.0 149.0 237.0 151.0 238.0 153.0 238.0 155.0 239.0 157.0 239.0 158.0 240.0 159.0 241.0 160.0 242.0 161.0 242.0 160.0 242.0 159.0