ov_core::TrackDescriptor class

Descriptor-based visual tracking.

Reference

Here we use descriptor matching to track features from one frame to the next. We track both temporally, and across stereo pairs to get stereo constraints. Right now we use ORB descriptors as we have found it is the fastest when computing descriptors. Tracks are then rejected based on a ratio test and ransac.

Base classes

class TrackBase: Visual feature tracking base class.

Constructors, destructors, conversion operators

TrackDescriptor(): Public default constructor.
TrackDescriptor(int numfeats, int numaruco, int fast_threshold, int gridx, int gridy, double knnratio) explicit: Public constructor with configuration variables.

Public functions

void feed_monocular(double timestamp, cv::Mat& img, size_t cam_id) override: Process a new monocular image.
void feed_stereo(double timestamp, cv::Mat& img_left, cv::Mat& img_right, size_t cam_id_left, size_t cam_id_right) override: Process new stereo pair of images.

Protected functions

void perform_detection_monocular(const cv::Mat& img0, std::vector<cv::KeyPoint>& pts0, cv::Mat& desc0, std::vector<size_t>& ids0): Detects new features in the current image.
void perform_detection_stereo(const cv::Mat& img0, const cv::Mat& img1, std::vector<cv::KeyPoint>& pts0, std::vector<cv::KeyPoint>& pts1, cv::Mat& desc0, cv::Mat& desc1, size_t cam_id0, size_t cam_id1, std::vector<size_t>& ids0, std::vector<size_t>& ids1): Detects new features in the current stereo pair.
void robust_match(std::vector<cv::KeyPoint>& pts0, std::vector<cv::KeyPoint> pts1, cv::Mat& desc0, cv::Mat& desc1, size_t id0, size_t id1, std::vector<cv::DMatch>& matches): Find matches between two keypoint+descriptor sets.

Function documentation

ov_core::TrackDescriptor::TrackDescriptor(int numfeats, int numaruco, int fast_threshold, int gridx, int gridy, double knnratio) explicit

Public constructor with configuration variables.

Parameters
numfeats	number of features we want want to track (i.e. track 200 points from frame to frame)
numaruco	the max id of the arucotags, so we ensure that we start our non-auroc features above this value
fast_threshold	FAST detection threshold
gridx	size of grid in the x-direction / u-direction
gridy	size of grid in the y-direction / v-direction
knnratio	matching ratio needed (smaller value forces top two descriptors during match to be more different)

void ov_core::TrackDescriptor::feed_monocular(double timestamp, cv::Mat& img, size_t cam_id) override

Process a new monocular image.

Parameters
timestamp	timestamp the new image occurred at
img	new cv:Mat grayscale image
cam_id	the camera id that this new image corresponds too

void ov_core::TrackDescriptor::feed_stereo(double timestamp, cv::Mat& img_left, cv::Mat& img_right, size_t cam_id_left, size_t cam_id_right) override

Process new stereo pair of images.

Parameters
timestamp	timestamp this pair occured at (stereo is synchronised)
img_left	first grayscaled image
img_right	second grayscaled image
cam_id_left	first image camera id
cam_id_right	second image camera id

void ov_core::TrackDescriptor::perform_detection_monocular(const cv::Mat& img0, std::vector<cv::KeyPoint>& pts0, cv::Mat& desc0, std::vector<size_t>& ids0) protected

Detects new features in the current image.

Parameters
img0	image we will detect features on
pts0	vector of extracted keypoints
desc0	vector of the extracted descriptors
ids0	vector of all new IDs

Given a set of images, and their currently extracted features, this will try to add new features. We return all extracted descriptors here since we DO NOT need to do stereo tracking left to right. Our vector of IDs will be later overwritten when we match features temporally to the previous frame's features. See robust_match() for the matching.

void ov_core::TrackDescriptor::perform_detection_stereo(const cv::Mat& img0, const cv::Mat& img1, std::vector<cv::KeyPoint>& pts0, std::vector<cv::KeyPoint>& pts1, cv::Mat& desc0, cv::Mat& desc1, size_t cam_id0, size_t cam_id1, std::vector<size_t>& ids0, std::vector<size_t>& ids1) protected

Detects new features in the current stereo pair.

Parameters
img0	left image we will detect features on
img1	right image we will detect features on
pts0	left vector of new keypoints
pts1	right vector of new keypoints
desc0	left vector of extracted descriptors
desc1	left vector of extracted descriptors
cam_id0	id of the first camera
cam_id1	id of the second camera
ids0	left vector of all new IDs
ids1	right vector of all new IDs

This does the same logic as the perform_detection_monocular() function, but we also enforce stereo contraints. We also do STEREO matching from the left to right, and only return good matches that are found in both the left and right. Our vector of IDs will be later overwritten when we match features temporally to the previous frame's features. See robust_match() for the matching.

void ov_core::TrackDescriptor::robust_match(std::vector<cv::KeyPoint>& pts0, std::vector<cv::KeyPoint> pts1, cv::Mat& desc0, cv::Mat& desc1, size_t id0, size_t id1, std::vector<cv::DMatch>& matches) protected

Find matches between two keypoint+descriptor sets.

Parameters
pts0	first vector of keypoints
pts1	second vector of keypoints
desc0	first vector of descriptors
desc1	second vector of decriptors
id0	id of the first camera
id1	id of the second camera
matches	vector of matches that we have found

This will perform a "robust match" between the two sets of points (slow but has great results). First we do a simple KNN match from 1to2 and 2to1, which is followed by a ratio check and symmetry check. Original code is from the "RobustMatcher" in the opencv examples, and seems to give very good results in the matches. https://github.com/opencv/opencv/blob/master/samples/cpp/tutorial_code/calib3d/real_time_pose_estimation/src/RobustMatcher.cpp