jqGrid computed columns

Question

Anyone have an idea of how to create computed columns. For e.g. I have a grid like:

$("#myProductGrid").jqGrid({
  url:DUMMY_URL,
  dataType:"local",
  width:285,
  colNames:['Item', 'Unit Price', 'Qty', 'Discount', 'Sale Price'],
  colModel:[ 
    {name:'description', index:'description', summaryType:'count'}, 
    {name:'unit_price', index:'unit_price', align:"right", sorttype:"float"}, 
    {name:'quantity', index:'quantity', formatter:'number', summaryType:'sum'}, 
    {name:'item_id', index:'item_id', hidden:true}, 
    {name:'discount_percent', index:'discount_percent', align:"right", sorttype:"float"},
    {name:'sale_price', index:'sale_price', align:"right", sorttype:"float", summaryType:'sum'},
   ],
 ... 
 });

I would like the "sale price" column to display quantity*(unit_price - (unit_price*discount))

Thanks in advance....