-# Blit $img to $dispimg, with scaling.
-proc update_dispimg { h } {
- set img [dict get $h img]
+# Blit $img to $dispimg, with scaling. If the optional img parameter
+# is given, blit that image to $dispimg instead, else use the normal
+# img stored in the handle.
+proc update_dispimg { h { img - } } {
+ if { "$img" eq "-" } {
+ set img [dict get $h img]
+ }