Get original index from th element in the tfoot

pablop

I'm using the datatables-responsive plugin which hides columns when the screen is big enough.
I'm using server side processing and sorting.

I have filters in the tfoot.
When getting the index of the column similar to how it is done in this example:
https://github.com/Comanche/datatables-responsive

When some of the columns are hidden, the index is not the original index which confuses the server sorting.
oTable.fnFilter( this.value, $("tfoot input").index(this) );

How can I get the original column index from the th element?
fnGetPosition is almost what I need but it only works on the tbody.
Is there a function that does the same for tfoot and thead?
http://datatables.net/ref#fnGetPosition

Thanks

pablop

Using _fnVisibleToColumnIndex works.
Thanks