Fast global registration

The RANSAC based Global registration solution may take a long time due to countless model proposals and evaluations. [Zhou2016] introduced a faster approach that quickly optimizes line process weights of few correspondences. As there is no model proposal and evaluation involved for each iteration, the approach proposed in [Zhou2016] can save a lot of computational time.

This script compares the running time of RANSAC based Global registration and implementation of [Zhou2016].

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# examples/Python/Advanced/fast_global_registration.py

import open3d as o3d
from global_registration import *
import numpy as np
import copy

import time


def execute_fast_global_registration(source_down, target_down, source_fpfh,
                                     target_fpfh, voxel_size):
    distance_threshold = voxel_size * 0.5
    print(":: Apply fast global registration with distance threshold %.3f" \
            % distance_threshold)
    result = o3d.registration.registration_fast_based_on_feature_matching(
        source_down, target_down, source_fpfh, target_fpfh,
        o3d.registration.FastGlobalRegistrationOption(
            maximum_correspondence_distance=distance_threshold))
    return result


if __name__ == "__main__":

    voxel_size = 0.05  # means 5cm for the dataset
    source, target, source_down, target_down, source_fpfh, target_fpfh = \
            prepare_dataset(voxel_size)

    start = time.time()
    result_ransac = execute_global_registration(source_down, target_down,
                                                source_fpfh, target_fpfh,
                                                voxel_size)
    print(result_ransac)
    print("Global registration took %.3f sec.\n" % (time.time() - start))
    draw_registration_result(source_down, target_down,
                             result_ransac.transformation)

    start = time.time()
    result_fast = execute_fast_global_registration(source_down, target_down,
                                                   source_fpfh, target_fpfh,
                                                   voxel_size)
    print("Fast global registration took %.3f sec.\n" % (time.time() - start))
    draw_registration_result(source_down, target_down,
                             result_fast.transformation)

Input

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    voxel_size = 0.05  # means 5cm for the dataset
    source, target, source_down, target_down, source_fpfh, target_fpfh = \
            prepare_dataset(voxel_size)

For the pair comparison, the script reuses the prepare_dataset function defined in Global registration. It produces a pair of downsampled point clouds as well as FPFH features.

Baseline

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    start = time.time()
    result_ransac = execute_global_registration(source_down, target_down,
                                                source_fpfh, target_fpfh,
                                                voxel_size)
    print(result_ransac)
    print("Global registration took %.3f sec.\n" % (time.time() - start))
    draw_registration_result(source_down, target_down,
                             result_ransac.transformation)

This script calls RANSAC based Global registration as a baseline. After registration it displays the following result.

../../_images/ransac.png 
RANSAC based global registration took 2.538 sec.

Fast global registration

With the same input used for a baseline, the next script calls the implementation of [Zhou2016].

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def execute_fast_global_registration(source_down, target_down, source_fpfh,
                                     target_fpfh, voxel_size):
    distance_threshold = voxel_size * 0.5
    print(":: Apply fast global registration with distance threshold %.3f" \
            % distance_threshold)
    result = o3d.registration.registration_fast_based_on_feature_matching(
        source_down, target_down, source_fpfh, target_fpfh,
        o3d.registration.FastGlobalRegistrationOption(
            maximum_correspondence_distance=distance_threshold))
    return result

This script displays the following result.

../../_images/fgr.png 
Fast global registration took 0.193 sec.

With proper configuration, the accuracy of fast global registration is even comparable with ICP. Please refer to [Zhou2016] for more experimental results.