I just tested this and it works perfectly
click on effects / add custom effect group and define >
click Add an effect to add a row
type > user select
length > 2
strat value> user define
end value > user define
name> user define
click ok and exit
open display group draw selection box >
click custom
execution of custom dialog pops up
select random top right
adjust coverage > user define %
select random intensity > if using ramp < otherwise uncheck all boxes on left hand side
if you want rgb color other than pre defined > click use rgb effect color and ignore cuctom color button
if not a segment channel > select > segment channel treat as individual channels
> your good to go <
Bookmarks