gtk-tree-model-filter-set-modify-func (filter column-types func)

filter -- a gtk-tree-model-filter object
column-types -- a list of g-type types of the columns
func -- a gtk-tree-model-filter-modify-func callback function

With types parameters, you give a list of column types for this model, which will be exposed to the parent model/view. The func parameter specifies the modify function. The modify function will get called for each data access, the goal of the modify function is to return the data which should be displayed at the location specified using the parameters of the modify function.