trouble getting black lines with linesmooth

[RonCagenello at UCBVAX.BERKELEY.EDU]

Using the manual's example to generate anti-aliased grey-scale
lines (in which 16 grey levels from black to white are
used to smooth the line) I have not been able to produce lines
of the two darkest grey levels.  This is the relevant code:

#define RMPBASE 64
#define RMPSIZE 16
#define RMPSTEP (255/(RMPSIZE-1))

for(i=0; i<RMPSIZE; i++)
	  mapcolor(i+RMPBASE, 255-i*RMPSTEP, 255-i*RMPSTEP, 255-i*RMPSTEP);
	
In the drawing routine:

subpixel(TRUE);
linesmooth(SML_ON);
color(RMPBASE);  /*background, white*/
clear();
color(RMPBASE+RMPSIZE-1); /*should be cmap entry 79, which has value 0,black*/
bgnline()...endline()

Visually, this gives a darkish grey line, and upon inspection with
rectread, the cmap entry used for black is 77, never 79, even when
explicitly set. 77 corresponds to a grey level two increments lighter
than black, and so the contrast of the resulting line is less than it
could be.  

I use a Personal Iris running 3.3, and colormap mode in this example.
Any insights would be appreciated.

Ron Cagenello
The Lighthouse (Low Vision Research)
rbc at xp.psych.edu