6.3.1 Slope Map

From [pov:6.7.2.1]:
Each of the various pattern types available is in fact a mathematical function that takes any x, y, z location and turns it into a number between 0.0 and 1.0 inclusive. That number is used to specify where the various high and low spots are. The slope_map lets you further shape the contours.

This is like in a color_map: We have some function and a table called slope_map:

normal {
  FUNCTION
  slope_map {
    [ T1, <H1,K1> ]
    [ T2, <H2,K2> ]
       ...
    [ TN, <HN,KN> ]
  }
}

where $T1$, $T2$,...,$Tn$ are the special return values of the function and the $H$'s and $K$'s are the corresponding heights and slopes. The slopes inbetween are interpolated by cubic splines.