From 849f2a66ac3c0502cb18e550f56b62efb044bc93 Mon Sep 17 00:00:00 2001
From: gaschler <gaschlera@gmail.com>
Date: Fri, 15 Jun 2018 13:06:02 +0200
Subject: [PATCH] clarify comment (#1196)

---
 cartographer/mapping/internal/2d/pose_graph_2d.cc | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/cartographer/mapping/internal/2d/pose_graph_2d.cc b/cartographer/mapping/internal/2d/pose_graph_2d.cc
index 2b5d74c..9c10828 100644
--- a/cartographer/mapping/internal/2d/pose_graph_2d.cc
+++ b/cartographer/mapping/internal/2d/pose_graph_2d.cc
@@ -702,12 +702,15 @@ PoseGraph2D::GetLandmarkNodes() const {
 
 std::map<int, PoseGraphInterface::TrajectoryData>
 PoseGraph2D::GetTrajectoryData() const {
-  return {};  // Not implemented yet in 2D.
+  // The 2D optimization problem does not have any 'TrajectoryData'.
+  return {};
 }
 
 sensor::MapByTime<sensor::FixedFramePoseData>
 PoseGraph2D::GetFixedFramePoseData() const {
-  return {};  // Not implemented yet in 2D.
+  // FixedFramePoseData is not yet implemented for 2D. We need to return empty
+  // so serialization works.
+  return {};
 }
 
 std::vector<PoseGraphInterface::Constraint> PoseGraph2D::constraints() const {