   setColor: (colorString)->
     @contexts.map (context)->
       context.strokeStyle = context.fillStyle = colorString
+
+  setOpacity: (fraction)->
+    colorString = context.fillStyle
+    colorString = colorString.split(',')
+    colorString[3] = "#{fraction})"
+    @setColor colorString.join(',')
 
   hasPreviousPlace: ->
     @lastPlace.x1 && @lastPlace.x2 && @lastPlace.y1 && @lastPlace.y2


    # a rectangle
    @tip = {
      width: 10
      height: 40
    }

    @lastPlace = {

    @lastPlace.y2 = y2

  updateCursor: ->
    @setOpacity(1)
    @place(@cursorContext)


