Continue changes to quadratic bezier rendering

shader/bezierQuad.frag

@@ -1,8 +1,12 @@
 #version 430 core
 in vec4 gColor;
 in vec3 gBoundingBox;
+in vec2 box2;
 in float wStart;
 in float wEnd;
+// the width is calculated along the line a-c
+// this should instead be along the lines a-b (for start width)
+// and b-c (for end width)
 
 out vec4 fColor;
 
@@ -19,8 +23,10 @@ void main()
 {
   //float d = x - y - 1 + 2* sqrt(y);
   float d = sqrt(x) + sqrt(y) - 1;
-  float e = sqrt(f(x,wStart)) + sqrt(f(y,wEnd)) - 1.0;
+//  float e = sqrt(f(x,wStart)) + sqrt(f(y,wEnd)) - 1.0;
+  float e = sqrt(box2.x) + sqrt(box2.y) - 1.0;
 //  if ( d < 0 || e > 0) { discard; }
   fColor = gColor;
+//  if ( d < 0 || e > 0) { fColor = vec4(0,0,1,1); }
   if ( d < 0 || e > 0) { fColor = vec4(0,0,1,1); }
 }

shader/bezierQuad.geom

@@ -9,6 +9,8 @@ out float wStart;
 out float wEnd;
 out vec3 gBoundingBox;
 
+out vec2 box2 ;
+
 vec2 normV2 (vec2 v)
 {   return vec2(-v.y,v.x) ;
 }
@@ -16,6 +18,19 @@ float lhs (vec2 da, vec2 db)
 {   return sign(dot(da, normV2(db))) ;
 }
 
+float extrapolate (vec2 inOne, vec2 inZeroA, vec2 inZeroB, vec2 xy)
+{ 
+    float det = inOne.x * (inZeroA.y - inZeroB.y)
+                + inZeroA.x * (inZeroB.y - inOne.y)
+                + inZeroB.x * (inOne.y   - inZeroA.y)
+                ;
+    float r = (inZeroA.y - inZeroB.y) ;
+    float s = (inZeroB.x - inZeroA.x) ;
+    float t = (inZeroA.x * inZeroB.y - inZeroB.x * inZeroA.y) ;
+    return r * xy.x + s * xy.y + t
+            / det ;
+}
+
 void main()
 {
   vec3 pa = gl_in[0].gl_Position.xyz;
@@ -25,22 +40,33 @@ void main()
   wStart = vRadMag [0];
   wEnd   = vRadMag [2];
 
+  box2 = vec2 (1,0);
   gBoundingBox = vec3 (0.9,0.1,0);
   gColor = vColor[0];
   gl_Position = vec4 (vPosOff[0] , pa.z , 1);
   EmitVertex();
+  box2 = vec2 (0,1);
   gBoundingBox = vec3 (0.1,0.9,0);
   gColor = vColor[2];
   gl_Position = vec4 (vPosOff[2] , pc.z , 1);
   EmitVertex();
+  box2 = vec2( extrapolate(vPosOff[0],vPosOff[2],pb.xy,pa.xy)
+             , extrapolate(vPosOff[2],vPosOff[0],pb.xy,pa.xy)
+             ) ;
+  box2 = vec2(0.2,0);
   gBoundingBox = vec3 (1,0,1);
   gColor = vColor[0];
   gl_Position = vec4 (pa, 1);
   EmitVertex();
+  box2 = vec2( extrapolate(vPosOff[0],vPosOff[2],pb.xy,pc.xy)
+             , extrapolate(vPosOff[2],vPosOff[0],pb.xy,pc.xy)
+             ) ;
+  box2 = vec2(0,0.2);
   gBoundingBox = vec3 (0,1,1);
   gColor = vColor[2];
   gl_Position = vec4 (pc, 1);
   EmitVertex();
+  box2 = vec2( 0,0 ) ;
   gBoundingBox = vec3 (0,0,0);
   gColor = vColor[1];
   gl_Position = vec4 (pb, 1);