ellipse
Ellipses use dx and dy to define the semi-major and
semi-minor axis lengths with the center of the ellipse at (cx,cy).
Of note, the commonly used phantom described by
Shepp and Logan[2] uses only ellipses.
rectangle
Rectangles use (cx,cy) to define the position of
the center of the rectangle with respect to the origin. dx
and dy are the half-width and half-height of the rectangle.
triangle
Triangles are drawn with the center of the base at (cx,cy)
and a base half-width of dx and a height of dy.
Rotations are then applied about the center of the base.
segment
Segments are complex. They are the portion of an circle between a
chord and the perimeter of the circle. dy sets the
radius of the circle. Segments start with the center of the chord
located at (0,0) and the chord horizontal. The half-width
of the chord is set by dx. The portion of an circle
lying below the chord is then added. The imaginary center of this
circle is located at (0,-dy). The segment is then rotated
by r and then translated by (cx,cy).
sector
Sectors are the like a "pie slice'' from a circle. The radius of
the circle is set by dy. Sectors are defined similarly to
segments. In this case, though, a chord is not drawn. Instead,
the lines are drawn from the origin of the circle (0,-dy)
to the points (-dx,0) and (dx,0). The perimeter
of the circle is then drawn between those two points and lies
below the x-axis. The sector is then rotated and translated the
same as a segment.