Sorry but at the moment most of the plotter simply don't support transparent images... for *some* of them it could be fixed, for others there is no hope (postscript simply doesn't allow transparency without some optional extentions)
Sorry but at the moment most of the plotter simply don't support transparent images... for *some* of them it could be fixed, for others there is no hope (postscript simply doesn't allow transparency without some optional extentions)