The ARROW and TRIANGLE markers should follow the director path that could be a curve. Use the METAPOST solution suggested by John Hobby, that is given angle and size, rotates the director path by half angle in both directions, cuts it with a circle centered at the end point of the director path with radius size and close the resulting path (eventually filling it).
The algorithm has been taken from "A user’s manual for METAPOST" by J. D. Hobby, page 42. Technical Report 162, AT&T Bell Laboratories, Murray Hill, New
Jersey, 1992. Available [online](http://www.tug.org/docs/metapost/mpman.pdf).