The FILL chunk defines a fill pattern.  This chunk is only valid
inside nested DR2D FORMs.  The GRUP object chunk section of this
article contans an example of the FILL chunk.

    struct FILLstruct {
        ULONG       ID;
        ULONG       Size;
        USHORT      FillID;                 /* ID of the fill */
    };

FillID is the number by which the ATTR object attribute chunk
references fill patterns.  The FILL chunk must be the first chunk
inside a nested DR2D FORM.  A FILL is followed by one DR2D object plus
any of the object attribute chunks (ATTR, BBOX) associated with the
object.

   Figure 2 - fill patterns 

DR2D makes a "tile" out of the fill pattern, giving it a virtual
bounding box based on the extreme X and Y values of the FILL's object
(Fig. A).  The bounding box shown in Fig. A surrounding the pattern
(the two ellipses) is invisible to the user.  In concept, this
rectangle is pasted on the page left to right, top to bottom like
floor tiles (Fig. B).  Again, the bounding boxes are not visible.  The
only portion of this tiled pattern that is visible is the part that
overlaps the object (Fig. C) being filled.  The object's path is
called a clipping path, as it "clips" its shape from the tiled
pattern (Fig. D).  Note that the fill is only masked on top of
underlying objects, so any "holes" in the pattern will act as a
window, leaving visible underlying objects.