from pyx import *
unit.set(uscale=3, wscale=3)

dist = 0.15           # distance between the ropes
thick = 0.08          # thickness of the ropes

# build the basepath of the knot. This is the curve that lies
# between both ropes.
A = 1.0, 0.1    # point where the knot starts after the straight line
B = 1.3, 0      # point where the knot is curved, overlying the straight line
t = -0.8, -0.1  # direction at A
s =  0.3, 1.2   # direction at B
pts = [
  ( A[0]-t[0], A[1]-t[1]), ( A[0]     , A[1]     ),
  ( A[0]+t[0], A[1]+t[1]), (-B[0]-s[0],-B[1]-s[1]),
  (-B[0]     ,-B[1]     ), (-B[0]+s[0],-B[1]+s[1]),
  ( B[0]-s[0], B[1]-s[1]), ( B[0]     , B[1]     ),
  ( B[0]+s[0], B[1]+s[1]), (-A[0]-t[0],-A[1]-t[1]),
  (-A[0]     ,-A[1]     ), (-A[0]+t[0],-A[1]+t[1]) ]
seam = path.path(
  path.moveto(*(pts[0])),
  path.lineto(*(pts[1])),
  path.curveto(pts[2][0],pts[2][1],pts[3][0],pts[3][1],pts[4][0],pts[4][1]),
  path.curveto(pts[5][0],pts[5][1],pts[6][0],pts[6][1],pts[7][0],pts[7][1]),
  path.curveto(pts[8][0],pts[8][1],pts[9][0],pts[9][1],pts[10][0],pts[10][1]),
  path.lineto(*(pts[11])) )
# We smooth this curve a little because we used curveto together with lineto
seam = deformer.smoothed(0.6).deform(seam)

# The ropes, when drawn later, will have to overlap in a very specific way.
l = seam.arclen()
epsilon = 0.002*l # small overlap between the segments
seam_segs = seam.split([0.28*l+epsilon, 0.28*l-epsilon,
                        0.42*l+epsilon, 0.42*l-epsilon,
                        0.58*l+epsilon, 0.58*l-epsilon,
                        0.72*l+epsilon, 0.72*l-epsilon])[::2]

# For each segment, two shifted paths build the boundaries of each rope
ropes_segs = []
for seam_seg in seam_segs:
    ropes_segs.append([])
    for ropeshift in [-0.5*dist, 0.5*dist]:
        ropes_segs[-1].append([])
        for edgeshift in [-0.5*thick, 0.5*thick]:
            ropes_segs[-1][-1].append(
              deformer.parallel(ropeshift + edgeshift).deform(seam_seg))
# Now, ropes_segs is a list of segments, containing a list of ropes,
# each containing the two bounding paths of the rope segment

rope_colors = [color.rgb.blue, color.rgb.red]
c = canvas.canvas()
# Because the segments should overlap in a very specific manner
# we have to draw them in the correct order
for index in [1, 4, 2, 0, 3]:
    for rope_seg, col in zip(ropes_segs[index], rope_colors):
        seg = rope_seg[0].joined(rope_seg[1].reversed())
        seg.append(path.closepath())
        c.fill(seg, [col])
        c.stroke(rope_seg[0], [style.linecap.round, style.linejoin.round])
        c.stroke(rope_seg[1], [style.linecap.round, style.linejoin.round])

c