postgis - Create closest point on multilinestring to use in shortest_path()

I am working a routing based application where I need to create a route between two points of interest. The problem which I am facing occurs after calculating network topology using assign_vertex_id(). When I run shortest_path I get the path between vertexes and not from the point of interest (as shown in image).

Can you please help me out with this: how can I use this query or any other function to do it?

Thanks for any tips.

EDIT 1

I am using this function called multiline_locate_point() similar to line_locate_point() which helps me to find nearest LINESTRING from MULTILINESTRING to the point.

here it is:

-- Function: multiline_locate_point(geometry, geometry)

-- DROP FUNCTION multiline_locate_point(geometry, geometry);

CREATE OR REPLACE FUNCTION multiline_locate_point(amultils geometry, apoint geometry)
  RETURNS geometry AS

$BODY$
DECLARE
    mindistance float8;
nearestlinestring geometry;
nearestpoint geometry;
i integer;

BEGIN
    mindistance := (distance(apoint,amultils)+100);
    FOR i IN 1 .. NumGeometries(amultils) LOOP

    if distance(apoint,GeometryN(amultils,i)) < mindistance THEN
        mindistance:=distance(apoint,GeometryN(amultils,i));
        nearestlinestring:=GeometryN(amultils,i);
    END IF;
END LOOP;

nearestpoint:=line_interpolate_point(nearestlinestring,line_locate_point(nearestlinestring,apoint));
    RETURN nearestpoint;
END;
$BODY$

  LANGUAGE plpgsql IMMUTABLE STRICT
  COST 100;
ALTER FUNCTION multiline_locate_point(geometry, geometry) OWNER TO postgres;

this gives me the_geom() of point on line. Now the problem is shortest_path() only takes vertex (integer number) as input to create route. So how can I pass this point to shortest_path() so that it creates route from the point and not from any vertex ??