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Bipartite graph of factors and nodes


A factorGraph object stores a bipartite graph consisting of factors connected to variable nodes. The nodes represent the unknown random variables in an estimation problem, and the factors represent probabilistic constraints on those variables, derived from measurements or prior knowledge. Add factors and nodes to the factor graph by using the addFactor function.




G = factorGraph creates an empty factorGraph object.


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This property is read-only.

Number of nodes in the factor graph, specified as a positive integer.

This property is read-only.

Number of factors in the factor graph, specified as a positive integer.

Object Functions

addFactorAdd factor to factor graph
fixNodeFix or free node in factor graph
hasNodeCheck if node ID exists in factor graph
isConnectedCheck if factor graph is connected
isNodeFixedCheck if node is fixed
nodeIDsGet all node IDs in factor graph
nodeStateGet or set node state in factor graph
nodeTypeGet node type of node in factor graph
optimizeOptimize factor graph


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Create and optimize a factor graph with custom solver options.

Create Factor Graph and Solver Settings

Create a factor graph and solver options with custom settings. Set the maximum number of iterations to 1000 and set the verbosity of the optimize output to 2.

G = factorGraph;
optns = factorGraphSolverOptions(MaxIterations=1000,VerbosityLevel=2)
optns = 
  factorGraphSolverOptions with properties:

              MaxIterations: 1000
          FunctionTolerance: 1.0000e-06
          GradientTolerance: 1.0000e-10
              StepTolerance: 1.0000e-08
             VerbosityLevel: 2
    TrustRegionStrategyType: 1

Add GPS Factor

Create a GPS factor with node identification number of 1 with NED ReferenceFrame and add it to the factor graph.

fgps = factorGPS(1,ReferenceFrame="NED");

Optimize Factor Graph

Optimize the factor graph with the custom settings. The results of the optimization are displayed with the level of detail depending on the VerbosityLevel.

iter      cost      cost_change  |gradient|   |step|    tr_ratio  tr_radius  ls_iter  iter_time  total_time
   0  0.000000e+00    0.00e+00    0.00e+00   0.00e+00   0.00e+00  1.00e+04        0    1.41e-05    5.51e-05
Terminating: Gradient tolerance reached. Gradient max norm: 0.000000e+00 <= 1.000000e-10

Solver Summary (v 2.0.0-eigen-(3.3.4)-no_lapack-eigensparse-no_openmp-no_custom_blas)

                                     Original                  Reduced
Parameter blocks                            1                        1
Parameters                                  7                        7
Effective parameters                        6                        6
Residual blocks                             1                        1
Residuals                                   3                        3

Minimizer                        TRUST_REGION

Sparse linear algebra library    EIGEN_SPARSE
Trust region strategy                  DOGLEG (TRADITIONAL)

                                        Given                     Used
Threads                                     1                        1
Linear solver ordering              AUTOMATIC                        1

Initial                          0.000000e+00
Final                            0.000000e+00
Change                           0.000000e+00

Minimizer iterations                        1
Successful steps                            1
Unsuccessful steps                          0

Time (in seconds):
Preprocessor                         0.000041

  Residual only evaluation           0.000000 (0)
  Jacobian & residual evaluation     0.000006 (1)
  Linear solver                      0.000000 (0)
Minimizer                            0.002897

Postprocessor                        0.000004
Total                                0.002942

Termination:                      CONVERGENCE (Gradient tolerance reached. Gradient max norm: 0.000000e+00 <= 1.000000e-10)

Create a matrix of positions of the landmarks to use for localization, and the real positions of the robot to compare your factor graph estimate against. Use the exampleHelperPlotPositionsAndLandmarks helper function to visualize the landmark points and the real path of the robot..

landmarks = [0 -3  0;
             3  4  0;
             7  1  0];
realpos = [0  0  0;
           2 -2  0;
           5  3  0;
           10 2  0];

Figure contains an axes object. The axes object contains 2 objects of type line, scatter. These objects represent Ground Truth, Landmarks.

Use Landmarks and Other Data as Factors

Create a factor graph, and add a prior factor to loosely fix the start pose of the robot by providing an estimate pose.

fg = factorGraph;
pf = factorPoseSE3Prior(1,Measurement=[0 0 0 1 0 0 0]);

Create factorPoseSE3AndXYZ landmark factor objects that relate the first and second pose nodes to the first landmark point, and then add the landmark factors to the factor graph. The landmark factors used here are for SE(3) state space but the process is identical for landmark factors for SE(2) state space. Add some random number to the relative position between the landmark and the ground truth position to simulate real sensor measurements.

% Landmark 1 Factors
measurementlmf1 = landmarks(1,:) - realpos(1,:) + 0.1*rand(1,3);
measurementlmf2 = landmarks(1,:) - realpos(2,:) + 0.1*rand(1,3);
lmf1 = factorPoseSE3AndPointXYZ([1 5],Measurement=measurementlmf1);
lmf2 = factorPoseSE3AndPointXYZ([2 5],Measurement=measurementlmf2);

Create landmark factors for the second and third landmark points, as well, relating them to the second and third pose nodes and third and fourth pose nodes, respectively. Use the exampleHelperAddNoiseAndAddToFactorGraph helper function to add noise to the measurement for each landmark factor and add the factors to the factor graph. Once you have added all landmark factors to the factor graph, the IDs of the pose nodes are 1, 2, 3, and 4, and the IDs of the landmark nodes are 5, 6, and 7.

% Landmark 2 Factors
lmf3 = factorPoseSE3AndPointXYZ([2 6],Measurement=landmarks(2,:)-realpos(2,:));
lmf4 = factorPoseSE3AndPointXYZ([3 6],Measurement=landmarks(2,:)-realpos(3,:));

% Landmark 3 Factors
lmf5 = factorPoseSE3AndPointXYZ([3 7],Measurement=landmarks(3,:)-realpos(3,:));
lmf6 = factorPoseSE3AndPointXYZ([4 7],Measurement=landmarks(3,:)-realpos(4,:));

exampleHelperAddNoiseAndAddToFactorGraph(fg,[lmf3 lmf4 lmf5 lmf6])

Use relative pose factors to relate consecutive poses, and add the factors to the factor graph. To simulate sensor readings for the measurements of each factor, take the difference between a consecutive pair of ground truth positions, append a quaternion of zero, and add noise.

zeroQuat = [1 0 0 0];
rp1 = factorTwoPoseSE3([1 2],Measurement=[realpos(2,:)-realpos(1,:) zeroQuat]);
rp2 = factorTwoPoseSE3([2 3],Measurement=[realpos(3,:)-realpos(2,:) zeroQuat]);
rp3 = factorTwoPoseSE3([3 4],Measurement=[realpos(4,:)-realpos(3,:) zeroQuat]);
exampleHelperAddNoiseAndAddToFactorGraph(fg,[rp1 rp2 rp3])

Optimize Factor Graph

Optimize the factor graph with the default solver options. The optimization updates the states of all nodes in the factor graph, so the positions of vehicle and the landmarks update.

fgso = factorGraphSolverOptions;
ans = struct with fields:
             InitialCost: 71.6462
               FinalCost: 0.0140
      NumSuccessfulSteps: 5
    NumUnsuccessfulSteps: 0
               TotalTime: 0.0328
         TerminationType: 0
        IsSolutionUsable: 1

Visualize and Compare Results

Get and store the updated node states for the vehicle and landmarks and plot the results, comparing the factor graph estimate of the robot path to the known ground truth of the robot.

fgposopt = [fg.nodeState(1); fg.nodeState(2); fg.nodeState(3); fg.nodeState(4)]
fgposopt = 4×7

   -0.0000    0.0000   -0.0000    1.0000   -0.0000    0.0000    0.0000
    2.0529   -2.0006    0.0528    0.9991    0.0415    0.0115    0.0053
    5.0501    3.0537    0.3775    0.9980    0.0569    0.0210    0.0197
   10.0939    2.3060    0.0984    0.9883    0.1423    0.0465    0.0274

fglmopt = [fg.nodeState(5); fg.nodeState(6); fg.nodeState(7)]
fglmopt = 3×3

    0.0753   -2.9889    0.0527
    3.0294    4.0345    0.5962
    7.1825    1.2102    0.1122


Figure contains an axes object. The axes object contains 4 objects of type line, scatter. These objects represent Ground Truth, Landmarks, FG-Estimated Position., FG-Estimated Landmarks.


[1] Dellaert, Frank. Factor graphs and GTSAM: A Hands-On Introduction. Georgia: Georgia Tech, September, 2012.

Extended Capabilities

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Version History

Introduced in R2022a