Fix for Sinus Displacement and 2D Offset stroke geometry modifiers.

These modifiers were not working properly when they were applied to strokes
whose backbone was already modified by other geometry shaders.  This problem
was due to the use of Normal2DF0D that compute 2D vertex normals based on
the underlying FEdges up on which initial stroke geometry is defined.  Now vertex
normals are computed on the basis of modified stroke vertices.

A helper function 'stroke_normal' for computing normals of stroke vertices was
added to the 'freestyle.utils' API module.

3 files changed, 59 insertions, 12 deletions

diff --git a/release/scripts/freestyle/modules/freestyle/chainingiterators.py b/release/scripts/freestyle/modules/freestyle/chainingiterators.py
index c4e22192a89..ae0a8a04294 100644
--- a/release/scripts/freestyle/modules/freestyle/chainingiterators.py
+++ b/release/scripts/freestyle/modules/freestyle/chainingiterators.py
@@ -39,8 +39,10 @@ from freestyle.types import (
 from freestyle.predicates import (
     ExternalContourUP1D,
     )
-from freestyle.utils import ContextFunctions as CF
-
+from freestyle.utils import (
+    ContextFunctions as CF,
+    stroke_normal,
+    )
 import bpy
 
 
diff --git a/release/scripts/freestyle/modules/freestyle/utils.py b/release/scripts/freestyle/modules/freestyle/utils.py
index 6d2c9da8146..24ff86d5ef6 100644
--- a/release/scripts/freestyle/modules/freestyle/utils.py
+++ b/release/scripts/freestyle/modules/freestyle/utils.py
@@ -26,3 +26,44 @@ from _freestyle import (
     getCurrentScene,
     integrate,
     )
+
+# constructs for definition of helper functions in Python
+from freestyle.types import (
+    StrokeVertexIterator,
+    )
+import mathutils
+
+
+def stroke_normal(it):
+    """
+    Compute the 2D normal at the stroke vertex pointed by the iterator
+    'it'.  It is noted that Normal2DF0D computes normals based on
+    underlying FEdges instead, which is inappropriate for strokes when
+    they have already been modified by stroke geometry modifiers.
+    """
+    # first stroke segment
+    it_next = StrokeVertexIterator(it)
+    it_next.increment()
+    if it.is_begin:
+        e = it_next.object.point_2d - it.object.point_2d
+        n = mathutils.Vector((e[1], -e[0]))
+        n.normalize()
+        return n
+    # last stroke segment
+    it_prev = StrokeVertexIterator(it)
+    it_prev.decrement()
+    if it_next.is_end:
+        e = it.object.point_2d - it_prev.object.point_2d
+        n = mathutils.Vector((e[1], -e[0]))
+        n.normalize()
+        return n
+    # two subsequent stroke segments
+    e1 = it_next.object.point_2d - it.object.point_2d
+    e2 = it.object.point_2d - it_prev.object.point_2d
+    n1 = mathutils.Vector((e1[1], -e1[0]))
+    n2 = mathutils.Vector((e2[1], -e2[0]))
+    n1.normalize()
+    n2.normalize()
+    n = n1 + n2
+    n.normalize()
+    return n
diff --git a/release/scripts/freestyle/modules/parameter_editor.py b/release/scripts/freestyle/modules/parameter_editor.py
index f3dd66b78cc..ce15028d0d7 100644
--- a/release/scripts/freestyle/modules/parameter_editor.py
+++ b/release/scripts/freestyle/modules/parameter_editor.py
@@ -76,6 +76,7 @@ from freestyle.shaders import (
 from freestyle.utils import (
     ContextFunctions,
     getCurrentScene,
+    stroke_normal,
     )
 from _freestyle import (
     blendRamp,
@@ -582,13 +583,15 @@ class SinusDisplacementShader(StrokeShader):
         self._wavelength = wavelength
         self._amplitude = amplitude
         self._phase = phase / wavelength * 2 * math.pi
-        self._getNormal = Normal2DF0D()
 
     def shade(self, stroke):
+        # separately iterate over stroke vertices to compute normals
+        buf = []
         for it, distance in iter_distance_along_stroke(stroke):
-            v = it.object
-            n = self._getNormal(Interface0DIterator(it))
-            n = n * self._amplitude * math.cos(distance / self._wavelength * 2 * math.pi + self._phase)
+            buf.append((it.object, distance, stroke_normal(it)))
+        # iterate over the vertices again to displace them
+        for v, distance, normal in buf:
+            n = normal * self._amplitude * math.cos(distance / self._wavelength * 2 * math.pi + self._phase)
             v.point = v.point + n
         stroke.update_length()
 
@@ -639,18 +642,19 @@ class Offset2DShader(StrokeShader):
         self.__start = start
         self.__end = end
         self.__xy = mathutils.Vector((x, y))
-        self.__getNormal = Normal2DF0D()
 
     def shade(self, stroke):
+        # first iterate over stroke vertices to compute normals
+        buf = []
         it = stroke.stroke_vertices_begin()
         while not it.is_end:
-            v = it.object
-            u = v.u
-            a = self.__start + u * (self.__end - self.__start)
-            n = self.__getNormal(Interface0DIterator(it))
+            buf.append((it.object, stroke_normal(it)))
+            it.increment()
+        # again iterate over the vertices to add displacement
+        for v, n in buf:
+            a = self.__start + v.u * (self.__end - self.__start)
             n = n * a
             v.point = v.point + n + self.__xy
-            it.increment()
         stroke.update_length()