Each drawing primitive has its own header file, usually compiled with opc89.
There are typically two kind of drawing primitives: stroked and centerline.

TODO

The intersection of two primitives of the same type is in the primitive's
implementation; TODO:intersect.h hosts the intersection code between different
primitive types. Rationale: in some cases (e.g. box) the
"self type intersection" call is needed by other functions of the primitive;
including intersect.h for this would result in cyclic #includes.

The intersection of a primitive and a box is also implemented at the
primitive, not in TODO:intersect.h. Rationale: primitives need to
calculate bounding box so they already depend on box.h. (Box is not
really a drawing primitive but a concept like vector.)

The intersection of a stroked primitive and the centerline version of the
same primtiive is implemented at the stroked primitive, not in
TODO:intersect.h. Rationale: stroked primitives depend on the centerline
version heavily for other calculations already.