Color fringe hires behavior

["BLuRry" <brendan.robert@gmail.com>]

I've been trying to figure out a way to render hires to a double-hires
bitmap, but also taking into account the odd combinations of color
frindges.  See the following example paraphrased from Sather's book
below:

1 hgr2:for a=1 to 3:read c:hcolor=c:read n:for b=1 to n: read x:hplot
x,0 to x,191:next b,a:data
4,3,0,21,42,7,9,62,83,104,125,159,180,220,247,279,3,10,20,41,105,126,161,182,206,233,262,263


I know why it happens, I'm just trying to find the most efficient way
to programatically explain this behavior.  Any takers?

-B