Determine bug in bezier curve rendering

shader/bezierQuad.geom

@@ -1,6 +1,6 @@
 #version 430 core
 layout (triangles) in;
-layout (triangle_strip, max_vertices = 4) out;
+layout (triangle_strip, max_vertices = 5) out;
 in vec4 vColor[];
 in float vRad[];
 out vec4 gColor;
@@ -8,8 +8,11 @@ out float gRadStart;
 out float gRadEnd;
 out vec3 gBoundingBox;
 
-vec3 normV (vec3 v)
-{   return vec3(v.y,-v.x,v.z) ;
+vec2 normV2 (vec2 v)
+{   return vec2(-v.y,v.x) ;
+}
+float lhs (vec2 da, vec2 db)
+{   return sign(dot(da, normV2(db))) ;
 }
 
 void main()
@@ -21,21 +24,31 @@ void main()
   gRadStart = vRad[0];
   gRadEnd   = vRad[2];
 
+  vec2 ab = (pa.xy - pb.xy);
+  vec2 cb = (pc.xy - pb.xy);
+  float dir = lhs( cb,ab);
 
+  gBoundingBox = vec3 (0.9,0.1,0);
+  gColor = vColor[0];
+  gl_Position = vec4 (pa.xy + dir * 2 * gRadStart * normV2(ab) , pa.z , 1);
+  EmitVertex();
+  gBoundingBox = vec3 (0.1,0.9,0);
+  gColor = vColor[2];
+//  gl_Position = vec4 (pc.xy + dir * gRadEnd * (normV2 (pb.xy-pc.xy)), pa.z , 1);
+  gl_Position = vec4 (pc.xy - dir * 2 * gRadEnd * normV2(cb) , pc.z , 1);
+  EmitVertex();
   gBoundingBox = vec3 (1,0,1);
   gColor = vColor[0];
-  gl_Position = vec4 (pa + normV (pa-pb) , 1);
-  EmitVertex();
   gl_Position = vec4 (pa, 1);
   EmitVertex();
-  gBoundingBox = vec3 (0,0,0);
-  gColor = vColor[1];
-  gl_Position = vec4 (pb, 1);
-  EmitVertex();
   gBoundingBox = vec3 (0,1,1);
   gColor = vColor[2];
   gl_Position = vec4 (pc, 1);
   EmitVertex();
+  gBoundingBox = vec3 (0,0,0);
+  gColor = vColor[1];
+  gl_Position = vec4 (pb, 1);
+  EmitVertex();
 
   EndPrimitive();
 }