From d79ccfed2146fbe216a8b3c7d781522bbc49a6d7 Mon Sep 17 00:00:00 2001
From: dellaert <dellaert@cc.gatech.edu>
Date: Thu, 22 Jan 2015 00:06:29 +0100
Subject: [PATCH] New example reads BAL file, creates expressions

---
 examples/SFMExampleExpressions_bal.cpp | 121 +++++++++++++++++++++++++
 1 file changed, 121 insertions(+)
 create mode 100644 examples/SFMExampleExpressions_bal.cpp

diff --git a/examples/SFMExampleExpressions_bal.cpp b/examples/SFMExampleExpressions_bal.cpp
new file mode 100644
index 000000000..66beefb35
--- /dev/null
+++ b/examples/SFMExampleExpressions_bal.cpp
@@ -0,0 +1,121 @@
+/* ----------------------------------------------------------------------------
+
+ * GTSAM Copyright 2010, Georgia Tech Research Corporation,
+ * Atlanta, Georgia 30332-0415
+ * All Rights Reserved
+ * Authors: Frank Dellaert, et al. (see THANKS for the full author list)
+
+ * See LICENSE for the license information
+
+ * -------------------------------------------------------------------------- */
+
+/**
+ * @file    SFMExampleExpressions_bal.cpp
+ * @brief   A structure-from-motion example done with Expressions
+ * @author  Frank Dellaert
+ * @date    January 2015
+ */
+
+/**
+ * This is the Expression version of SFMExample
+ * See detailed description of headers there, this focuses on explaining the AD part
+ */
+
+// The two new headers that allow using our Automatic Differentiation Expression framework
+#include <gtsam/slam/expressions.h>
+#include <gtsam/nonlinear/ExpressionFactorGraph.h>
+
+// Header order is close to far
+#include <gtsam/inference/Symbol.h>
+#include <gtsam/nonlinear/LevenbergMarquardtOptimizer.h>
+#include <gtsam/slam/dataset.h> // for loading BAL datasets !
+#include <vector>
+
+using namespace std;
+using namespace gtsam;
+using namespace noiseModel;
+using symbol_shorthand::C;
+using symbol_shorthand::P;
+
+// An SfM_Camera is defined in datase.h as a camera with unknown Cal3Bundler calibration
+// and has a total of 9 free parameters
+
+/* ************************************************************************* */
+int main(int argc, char* argv[]) {
+
+  // Find default file, but if an argument is given, try loading a file
+  string filename = findExampleDataFile("dubrovnik-3-7-pre");
+  if (argc > 1)
+    filename = string(argv[1]);
+
+  // Load the SfM data from file
+  SfM_data mydata;
+  readBAL(filename, mydata);
+  cout
+      << boost::format("read %1% tracks on %2% cameras\n")
+          % mydata.number_tracks() % mydata.number_cameras();
+
+  // Create a factor graph
+  ExpressionFactorGraph graph;
+
+  // Here we don't use a PriorFactor but directly the ExpressionFactor class
+  // First, we create an expression to the pose from the first camera
+  Expression<SfM_Camera> camera0_(C(0));
+  // Then, to get its pose:
+  Pose3_ pose0_(&SfM_Camera::getPose, camera0_);
+  // Finally, we say it should be equal to first guess
+  graph.addExpressionFactor(pose0_, mydata.cameras[0].pose(),
+      noiseModel::Isotropic::Sigma(6, 0.1));
+
+  // similarly, we create a prior on the first point
+  Point3_ point0_(P(0));
+  graph.addExpressionFactor(point0_, mydata.tracks[0].p,
+      noiseModel::Isotropic::Sigma(3, 0.1));
+
+  // We share *one* noiseModel between all projection factors
+  noiseModel::Isotropic::shared_ptr noise = noiseModel::Isotropic::Sigma(2,
+      1.0); // one pixel in u and v
+
+  // Simulated measurements from each camera pose, adding them to the factor graph
+  size_t j = 0;
+  BOOST_FOREACH(const SfM_Track& track, mydata.tracks) {
+    // Leaf expression for j^th point
+    Point3_ point_('p', j);
+    BOOST_FOREACH(const SfM_Measurement& m, track.measurements) {
+      size_t i = m.first;
+      Point2 uv = m.second;
+      // Leaf expression for i^th camera
+      Expression<SfM_Camera> camera_(C(i));
+      // Below an expression for the prediction of the measurement:
+      Point2_ predict_ = project2<SfM_Camera>(camera_, point_);
+      // Again, here we use an ExpressionFactor
+      graph.addExpressionFactor(predict_, uv, noise);
+    }
+    j += 1;
+  }
+
+  // Create initial estimate
+  Values initial;
+  size_t i = 0;
+  j = 0;
+  BOOST_FOREACH(const SfM_Camera& camera, mydata.cameras)
+    initial.insert(C(i++), camera);
+  BOOST_FOREACH(const SfM_Track& track, mydata.tracks)
+    initial.insert(P(j++), track.p);
+
+  /* Optimize the graph and print results */
+  Values result;
+  try {
+    LevenbergMarquardtParams params;
+    params.setVerbosity("ERROR");
+    LevenbergMarquardtOptimizer lm(graph, initial, params);
+    result = lm.optimize();
+  } catch (exception& e) {
+    cout << e.what();
+  }
+  cout << "final error: " << graph.error(result) << endl;
+
+  return 0;
+}
+/* ************************************************************************* */
+