if (( Within16RoundingErrors(t1s[0],m1->tstart) && Within16RoundingErrors(t1s[1],m1->tend)) ||
        ( Within16RoundingErrors(t2s[0],m2->tstart) && Within16RoundingErrors(t2s[1],m2->tend)) )
        /* It covers one of the monotonics entirely */;
    else if ( RealWithin(t1s[0],t1s[1],.01) )
return( false );        /* But otherwise, don't create a new tiny spline */

    /* Ok, if we've gotten this far we know that two of the end points are  */
    /*  on both splines.                                                    */
    t1s[2] = t2s[2] = -1;
    if ( !m1->s->knownlinear || !m2->s->knownlinear ) {
        if ( t1s[1]<t1s[0] ) {
            extended temp = t1s[1]; t1s[1] = t1s[0]; t1s[0] = temp;
            temp = t2s[1]; t2s[1] = t2s[0]; t2s[0] = temp;
        }
        diff = (t1s[1]-t1s[0])/16;
        for ( t=t1s[0]+diff; t<t1s[1]-diff/4; t += diff ) {
            BasePoint here, slope;
            here.x = ((m1->s->splines[0].a*t+m1->s->splines[0].b)*t+m1->s->splines[0].c)*t+m1->s->splines[0].d;
            here.y = ((m1->s->splines[1].a*t+m1->s->splines[1].b)*t+m1->s->splines[1].c)*t+m1->s->splines[1].d;
            if ( (slope.x = (3*m1->s->splines[0].a*t+2*m1->s->splines[0].b)*t+m1->s->splines[0].c)<0 )
                slope.x = -slope.x;
            if ( (slope.y = (3*m1->s->splines[1].a*t+2*m1->s->splines[1].b)*t+m1->s->splines[1].c)<0 )
                slope.y = -slope.y;
            if ( slope.y>slope.x ) {
                t2 = IterateSplineSolveFixup(&m2->s->splines[1],t2s[0],t2s[1],here.y);
                if ( t2==-1 || !RealWithin(here.x,((m2->s->splines[0].a*t2+m2->s->splines[0].b)*t2+m2->s->splines[0].c)*t2+m2->s->splines[0].d,.1))
return( false );
            } else {
                t2 = IterateSplineSolveFixup(&m2->s->splines[0],t2s[0],t2s[1],here.x);
                if ( t2==-1 || !RealWithin(here.y,((m2->s->splines[1].a*t2+m2->s->splines[1].b)*t2+m2->s->splines[1].c)*t2+m2->s->splines[1].d,.1))
return( false );
            }
        }
    }

commit 21adfc22a566c9f6f39a070196dec4e24d20625d
Author: George Williams <pfaedit@users.sourceforge.net>
Date:   Sat Jan 22 03:19:44 2011 +0000

    tweak for greater precision.

diff --git a/fontforge/splineoverlap.c b/fontforge/splineoverlap.c
index 145a6ce1c..7e7bb0c6d 100644
--- a/fontforge/splineoverlap.c
+++ b/fontforge/splineoverlap.c
@@ -1526,7 +1526,7 @@ static int CoincidentIntersect(Monotonic *m1,Monotonic *m2,BasePoint *pts,
     extended t, t2, diff;
 
     if ( m1==m2 )
-return( false );               /* Can't be coincident. Adjacent */
+return( false );               /* Stupid question */
     /* Adjacent splines can double back on themselves */
     if ( m1->next==m2 || m1->prev==m2 ) {
        /* But normally they'll only intersect in one point, where they join */
@@ -1566,8 +1566,11 @@ return( false );
     if ( cnt!=2 )
 return( false );
 
-    if ( RealWithin(t1s[0],t1s[1],.01) )
-return( false );
+    if (( Within16RoundingErrors(t1s[0],m1->tstart) && Within16RoundingErrors(t1s[1],m1->tend)) ||
+       ( Within16RoundingErrors(t2s[0],m2->tstart) && Within16RoundingErrors(t2s[1],m2->tend)) )
+       /* It covers one of the monotonics entirely */;
+    else if ( RealWithin(t1s[0],t1s[1],.01) )
+return( false );       /* But otherwise, don't create a new tiny spline */