That's confusing. Make it clear we're working with 3 points == 9 scalars.

src/core/SkGeometry.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkGeometry.h"
 #include "SkMatrix.h"
 
@@ skipping 1318 matching lines @@
 
     void set(SkScalar x, SkScalar y, SkScalar z) {
         fX = x; fY = y; fZ = z;
     }
 
     void projectDown(SkPoint* dst) const {
         dst->set(fX / fZ, fY / fZ);
     }
 };
 
-// we just return the middle 3 points, since the first and last are dups of src
-//
-static void p3d_interp(const SkScalar src[3], SkScalar dst[3], SkScalar t) {
+// We only interpolate one dimension at a time (the first, at +0, +3, +6).
+static void p3d_interp(const SkScalar src[7], SkScalar dst[7], SkScalar t) {
     SkScalar ab = SkScalarInterp(src[0], src[3], t);
     SkScalar bc = SkScalarInterp(src[3], src[6], t);
     dst[0] = ab;
     dst[3] = SkScalarInterp(ab, bc, t);
     dst[6] = bc;
 }
 
 static void ratquad_mapTo3D(const SkPoint src[3], SkScalar w, SkP3D dst[]) {
     dst[0].set(src[0].fX * 1, src[0].fY * 1, 1);
     dst[1].set(src[1].fX * w, src[1].fY * w, w);
@@ skipping 168 matching lines @@
     }
     if (this->findYExtrema(&t)) {
         this->evalAt(t, &pts[count++]);
     }
     bounds->set(pts, count);
 }
 
 void SkConic::computeFastBounds(SkRect* bounds) const {
     bounds->set(fPts, 3);
 }