gallery/path/knot.py

   1 from pyx import *
   2 unit.set(uscale=3, wscale=3)
   3
   4 dist = 0.15           # distance between the ropes
   5 thick = 0.08          # thickness of the ropes
   6
   7 # build the basepath of the knot. This is the curve that lies
   8 # between both ropes.
   9 A = 1.0, 0.1    # point where the knot starts after the straight line
  10 B = 1.3, 0      # point where the knot is curved, overlying the straight line
  11 t = -0.8, -0.1  # direction at A
  12 s =  0.3, 1.2   # direction at B
  13 pts = [
  14   ( A[0]-t[0], A[1]-t[1]), ( A[0]     , A[1]     ),
  15   ( A[0]+t[0], A[1]+t[1]), (-B[0]-s[0],-B[1]-s[1]),
  16   (-B[0]     ,-B[1]     ), (-B[0]+s[0],-B[1]+s[1]),
  17   ( B[0]-s[0], B[1]-s[1]), ( B[0]     , B[1]     ),
  18   ( B[0]+s[0], B[1]+s[1]), (-A[0]-t[0],-A[1]-t[1]),
  19   (-A[0]     ,-A[1]     ), (-A[0]+t[0],-A[1]+t[1]) ]
  20 seam = path.path(
  21   path.moveto(*(pts[0])),
  22   path.lineto(*(pts[1])),
  23   path.curveto(pts[2][0],pts[2][1],pts[3][0],pts[3][1],pts[4][0],pts[4][1]),
  24   path.curveto(pts[5][0],pts[5][1],pts[6][0],pts[6][1],pts[7][0],pts[7][1]),
  25   path.curveto(pts[8][0],pts[8][1],pts[9][0],pts[9][1],pts[10][0],pts[10][1]),
  26   path.lineto(*(pts[11])) )
  27 # We smooth this curve a little because we used curveto together with lineto
  28 seam = deformer.smoothed(0.6).deform(seam)
  29
  30 # The ropes, when drawn later, will have to overlap in a very specific way.
  31 l = seam.arclen()
  32 epsilon = 0.002*l # small overlap between the segments
  33 seam_segs = seam.split([0.28*l+epsilon, 0.28*l-epsilon,
  34                         0.42*l+epsilon, 0.42*l-epsilon,
  35                         0.58*l+epsilon, 0.58*l-epsilon,
  36                         0.72*l+epsilon, 0.72*l-epsilon])[::2]
  37
  38 # For each segment, two shifted paths build the boundaries of each rope
  39 ropes_segs = []
  40 for seam_seg in seam_segs:
  41     ropes_segs.append([])
  42     for ropeshift in [-0.5*dist, 0.5*dist]:
  43         ropes_segs[-1].append([])
  44         for edgeshift in [-0.5*thick, 0.5*thick]:
  45             ropes_segs[-1][-1].append(
  46               deformer.parallel(ropeshift + edgeshift).deform(seam_seg))
  47 # Now, ropes_segs is a list of segments, containing a list of ropes,
  48 # each containing the two bounding paths of the rope segment
  49
  50 rope_colors = [color.rgb.blue, color.rgb.red]
  51 c = canvas.canvas()
  52 # Because the segments should overlap in a very specific manner
  53 # we have to draw them in the correct order
  54 for index in [1, 4, 2, 0, 3]:
  55     for rope_seg, col in zip(ropes_segs[index], rope_colors):
  56         seg = rope_seg[0].joined(rope_seg[1].reversed())
  57         seg.append(path.closepath())
  58         c.fill(seg, [col])
  59         c.stroke(rope_seg[0], [style.linecap.round, style.linejoin.round])
  60         c.stroke(rope_seg[1], [style.linecap.round, style.linejoin.round])
  61
  62 c.writeEPSfile("knot")
  63 c.writePDFfile("knot")