This is a method for the generic function as.data.frame
for the class "psp" of line segment patterns.
It extracts the coordinates of the endpoints of the line segments,
and returns them as columns named x0, y0, x1
and y1 in a data frame. If the line segments were marked,
the marks are appended as an extra column or columns to the
data frame which is returned. If the marks are a vector then a
single column named marks is appended. in the data frame,
with the same type as in the line segment pattern dataset. If the
marks are a data frame, then the columns of this data frame are
appended (retaining their names).