From d0916058f4260f17619628b898649a9fd1d917b6 Mon Sep 17 00:00:00 2001
From: Philemon Brakel <philemon@google.com>
Date: Thu, 7 Apr 2022 17:14:54 +0100
Subject: [PATCH] Fix outer derivative by replacing cwiseproduct with explicit
 matvec.

---
 towr/src/height_map.cc | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/towr/src/height_map.cc b/towr/src/height_map.cc
index d5883e4e0..1e89dfcd4 100644
--- a/towr/src/height_map.cc
+++ b/towr/src/height_map.cc
@@ -86,8 +86,12 @@ HeightMap::GetDerivativeOfNormalizedBasisWrt (Direction basis, Dim2D dim,
 
   // outer derivative
   Vector3d v = GetBasis(basis, x,y, {});
-  Vector3d dn_norm_wrt_n = GetDerivativeOfNormalizedVectorWrtNonNormalizedIndex(v, dim);
-  return dn_norm_wrt_n.cwiseProduct(dv_wrt_dim);
+  Vector3d result = Vector3d::Zero();
+  for (auto inner_dim : {X,Y,Z}) {
+    auto dn_norm_wrt_n = GetDerivativeOfNormalizedVectorWrtNonNormalizedIndex(v, inner_dim);
+    result = result + dv_wrt_dim(inner_dim) * dn_norm_wrt_n;
+  }
+  return result;
 }
 
 HeightMap::Vector3d