helixish: ensure s and t are >0

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/helixish.py b/helixish.py
index b1eac1ce34bab6881bd4f2cfb49205816a7e9466..bf8083fbd35dcc137f044027461e59bd0610003a 100644 (file)
--- a/helixish.py
+++ b/helixish.py
@@ -161,6 +161,7 @@ class HelixishCurve():
         def railway_ST(C, start, end, railway_r):
           delta = atan2(*(end - C)[0:2]) - atan2(*(start - C)[0:2])
           dbg('railway_ST C', C, 'start', start, 'end', end, 'delta', delta)
+          if delta < 0: delta += tau
           s = delta * railway_r
           dbg('railway_ST delta', delta, 'r', railway_r, 's', s)
           return s