EssentialViewGraphExample.py

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"""
  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
"""
 
"""
Python version of EssentialViewGraphExample.cpp
View-graph calibration with essential matrices.
Author: Frank Dellaert
Date: October 2024
"""
 
import numpy as np
from gtsam.examples import SFMdata
 
import gtsam
from gtsam import Cal3f, EdgeKey, EssentialMatrix
from gtsam import EssentialTransferFactorCal3f as Factor
from gtsam import (LevenbergMarquardtOptimizer, LevenbergMarquardtParams,
                   NonlinearFactorGraph, PinholeCameraCal3f, Values)
 
# For symbol shorthand (e.g., X(0), L(1))
K = gtsam.symbol_shorthand.K
 
 
# Formatter function for printing keys
def formatter(key):
    sym = gtsam.Symbol(key)
    if sym.chr() == ord("k"):
        return f"k{sym.index()}"
    else:
        edge = EdgeKey(key)
        return f"({edge.i()},{edge.j()})"
 
 
def main():
    # Define the camera calibration parameters
    cal = Cal3f(50.0, 50.0, 50.0)
 
    # Create the set of 8 ground-truth landmarks
    points = SFMdata.createPoints()
 
    # Create the set of 4 ground-truth poses
    poses = SFMdata.posesOnCircle(4, 30)
 
    # Calculate ground truth essential matrices, 1 and 2 poses apart
    E1 = EssentialMatrix.FromPose3(poses[0].between(poses[1]))
    E2 = EssentialMatrix.FromPose3(poses[0].between(poses[2]))
 
    # Simulate measurements from each camera pose
    p = [[None for _ in range(8)] for _ in range(4)]
    for i in range(4):
        camera = PinholeCameraCal3f(poses[i], cal)
        for j in range(8):
            p[i][j] = camera.project(points[j])
 
    # Create the factor graph
    graph = NonlinearFactorGraph()
 
    for a in range(4):
        b = (a + 1) % 4  # Next camera
        c = (a + 2) % 4  # Camera after next
 
        # Collect data for the three factors
        tuples1 = []
        tuples2 = []
        tuples3 = []
 
        for j in range(8):
            tuples1.append((p[a][j], p[b][j], p[c][j]))
            tuples2.append((p[a][j], p[c][j], p[b][j]))
            tuples3.append((p[c][j], p[b][j], p[a][j]))
 
        # Add transfer factors between views a, b, and c.
        graph.add(Factor(EdgeKey(a, c), EdgeKey(b, c), tuples1))
        graph.add(Factor(EdgeKey(a, b), EdgeKey(b, c), tuples2))
        graph.add(Factor(EdgeKey(a, c), EdgeKey(a, b), tuples3))
 
    graph.print("graph", formatter)
 
    # Create a delta vector to perturb the ground truth (small perturbation)
    delta = np.ones(5) * 1e-2
 
    # Create the initial estimate for essential matrices
    initialEstimate = Values()
    for a in range(4):
        b = (a + 1) % 4  # Next camera
        c = (a + 2) % 4  # Camera after next
        initialEstimate.insert(EdgeKey(a, b).key(), E1.retract(delta))
        initialEstimate.insert(EdgeKey(a, c).key(), E2.retract(delta))
 
    # Insert initial calibrations
    for i in range(4):
        initialEstimate.insert(K(i), cal)
 
    # Optimize the graph and print results
    params = LevenbergMarquardtParams()
    params.setlambdaInitial(1000.0)  # Initialize lambda to a high value
    params.setVerbosityLM("SUMMARY")
    optimizer = LevenbergMarquardtOptimizer(graph, initialEstimate, params)
    result = optimizer.optimize()
 
    print("Initial error = ", graph.error(initialEstimate))
    print("Final error = ", graph.error(result))
 
    # Print final results
    print("Final Results:")
    result.print("", formatter)
 
    # Print ground truth essential matrices
    print("Ground Truth E1:\n", E1)
    print("Ground Truth E2:\n", E2)
 
 
if __name__ == "__main__":
    main()