Bruce provided extrusion fix.

Author: sspickle

lib/glow/extrude.js

@@ -546,15 +546,15 @@ function extrusion(args) {
     args.axis = (args.axis === undefined) ? null : args.axis
     args.up = (args.up === undefined) ? vec(0,1,0) : args.up.norm()
     let first_segment = (args.path[1].sub(args.path[0])).norm()
-    if (first_segment.cross(args.up).mag < 0.1)  { // construct args.up to be perpendicular to first_segment
-        if (args.up.cross(vec(0,1,0)).mag > 0.2) { // try making args.up be vec(0,1,0)
-            args.up = vec(0,1,0)
-        } else if (args.up.cross(vec(1,0,0)).mag > 0.2) { // try making args.up be vec(1,0,0)
-            args.up = vec(1,0,0)
-        } else if (args.up.cross(vec(0,0,1)).mag > 0.2) { // try making args.up be vec(0,0,1)
-            args.up = vec(0,0,1)
-        }
-    }
+    // if (first_segment.cross(args.up).mag < 0.1)  { // construct args.up to be perpendicular to first_segment
+    //     if (args.up.cross(vec(0,1,0)).mag > 0.2) { // try making args.up be vec(0,1,0)
+    //         args.up = vec(0,1,0)
+    //     } else if (args.up.cross(vec(1,0,0)).mag > 0.2) { // try making args.up be vec(1,0,0)
+    //         args.up = vec(1,0,0)
+    //     } else if (args.up.cross(vec(0,0,1)).mag > 0.2) { // try making args.up be vec(0,0,1)
+    //         args.up = vec(0,0,1)
+    //     }
+    // }
 
     args.color = (args.color === undefined) ? color.white : args.color
     args.twist = (args.twist === undefined) ? 0 : args.twist
@@ -693,11 +693,23 @@ function extrusion(args) {
     if (L < 2) throw new Error('An extrusion path must contain more than one distinct point.')
     let start_normal, end_normal, theta, A, extraA, axis, x, y, z
 
-    // Deal with normals to the end caps:
     if (!path_closed) {
+        // Adjust x, y, and z (z goes from one point to the next point)
+        // x, y, z are the vectors in a left-handed xyz set; x and y are unit vectors.
         z = p[1].sub(p[0]) // points inward
-        y = norm(args.up)
-        x = norm(z.cross(y))
+        let da = diff_angle(z, args.up)
+        if (da == 0) {
+            x = vec(1,0,0)
+            y = vec(0,0,1)
+        } else if (da == pi) {
+            x = vec(-1,0,0)
+            y = vec(0,0,1)
+        } else {
+            x = norm(z.cross(args.up))
+            y = norm(x.cross(z))
+        }
+
+        // Deal with normals to the end caps:
         start_normal = z
         if (args.start_normal !== null) start_normal = args.start_normal.multiply(-1) // points inward
         if (!norm(start_normal).equals(norm(z))) {
@@ -719,8 +731,8 @@ function extrusion(args) {
         }
     } else {
         z = p[0].sub(p[L-1])
-        y = norm(args.up)
-        x = norm(z.cross(y))
+        x = norm(z.cross(args.up))
+        y = norm(x.cross(z))
     }
 
     // Analyze the whole path before creating quads.