关键点也称为兴趣点,它是2D图像或是3D点云或者曲面模型上,可以通过定义检测标准来获取的具有稳定性,区别性的点集,从技术上来说,关键点的数量相比于原始点云或图像的数据量减小很多,与局部特征描述子结合在一起,组成关键点描述子常用来形成原始数据的表示,而且不失代表性和描述性,从而加快了后续的识别,追踪等对数据的处理了速度,故而,关键点技术成为在2D和3D 信息处理中非常关键的技术
NARF(Normal Aligned Radial Feature)关键点是为了从深度图像中识别物体而提出的,对NARF关键点的提取过程有以下要求:
a) 提取的过程考虑边缘以及物体表面变化信息在内;
b)在不同视角关键点可以被重复探测;
c)关键点所在位置有足够的支持区域,可以计算描述子和进行唯一的估计法向量。
其对应的探测步骤如下:
(1) 遍历每个深度图像点,通过寻找在近邻区域有深度变化的位置进行边缘检测。
(2) 遍历每个深度图像点,根据近邻区域的表面变化决定一测度表面变化的系数,及变化的主方向。
(3) 根据step(2)找到的主方向计算兴趣点,表征该方向和其他方向的不同,以及该处表面的变化情况,即该点有多稳定。
(4) 对兴趣值进行平滑滤波。
(5) 进行无最大值压缩找到的最终关键点,即为NARF关键点。
关于NARF的更为具体的描述请查看这篇博客www.cnblogs.com/ironstark/p/5051533.html。
PCL中keypoints模块及类的介绍
(1)class pcl::Keypoint<PointInT,PointOutT> 类keypoint是所有关键点检测相关类的基类,定义基本接口,具体实现由子类来完成,其继承关系时下图:
具体介绍:
Public Member Functions | |
---|---|
virtual void | setSearchSurface (const PointCloudInConstPtr &cloud) |
设置搜索时所用搜索点云,cloud为指向点云对象的指针引用 | |
void | setSearchMethod (const KdTreePtr &tree) 设置内部算法实现时所用的搜索对象,tree为指向kdtree或者octree对应的指针 |
void | setKSearch (int k) 设置K近邻搜索时所用的K参数 |
void | setRadiusSearch (double radius) 设置半径搜索的半径的参数 |
int | searchForNeighbors (int index, double parameter, std::vector< int > &indices, std::vector< float > &distances) const |
采用setSearchMethod设置搜索对象,以及setSearchSurface设置搜索点云,进行近邻搜索,返回近邻在点云中的索引向量,indices以及对应的距离向量distance其中为查询点的索引,parameter为搜索时所用的参数半径或者K |
(2)class pcl::HarrisKeypoint2D<PointInT,PointOutT,IntensityT>
类HarrisKeypoint2D实现基于点云的强度字段的harris关键点检测子,其中包括多种不同的harris关键点检测算法的变种,其关键函数的说明如下:
Public Member Functions | |
---|---|
HarrisKeypoint2D (ResponseMethod method=HARRIS, int window_width=3, int window_height=3, int min_distance=5, float threshold=0.0) | |
重构函数,method需要设置采样哪种关键点检测方法,有HARRIS,NOBLE,LOWE,WOMASI四种方法,默认为HARRIS,window_width window_height为检测窗口的宽度和高度min_distance 为两个关键点之间 容许的最小距离,threshold为判断是否为关键点的感兴趣程度的阀值,小于该阀值的点忽略,大于则认为是关键点 | |
void | setMethod (ResponseMethod type)设置检测方式 |
void | setWindowWidth (int window_width) 设置检测窗口的宽度 |
void | setWindowHeight (int window_height) 设置检测窗口的高度 |
void | setSkippedPixels (int skipped_pixels) 设置在检测时每次跳过的像素的数目 |
void | setMinimalDistance (int min_distance) 设置候选关键点之间的最小距离 |
void | setThreshold (float threshold) 设置感兴趣的阀值 |
void | setNonMaxSupression (bool=false) 设置是否对小于感兴趣阀值的点进行剔除,如果是true则剔除,否则返回这个点 |
void | setRefine (bool do_refine)设置是否对所得的关键点结果进行优化, |
void | setNumberOfThreads (unsigned int nr_threads=0) 设置该算法如果采用openMP并行机制,能够创建线程数目 |
(3)pcl::HarrisKeypoint3D< PointInT, PointOutT, NormalT >
类HarrisKeypoint3D和HarrisKeypoint2D类似,但是没有在点云的强度空间检测关键点,而是利用点云的3D空间的信息表面法线向量来进行关键点检测,关于HarrisKeypoint3D的类与HarrisKeypoint2D相似,除了
HarrisKeypoint3D (ResponseMethod method=HARRIS, float radius=0.01f, float threshold=0.0f)
重构函数,method需要设置采样哪种关键点检测方法,有HARRIS,NOBLE,LOWE,WOMASI四种方法,默认为HARRIS,radius为法线估计的搜索半径,threshold为判断是否为关键点的感兴趣程度的阀值,小于该阀值的点忽略,大于则认为是关键点。
(4)pcl::HarrisKeypoint6D< PointInT, PointOutT, NormalT >
类HarrisKeypoint6D和HarrisKeypoint2D类似,只是利用了欧式空间域XYZ或者强度域来候选关键点,或者前两者的交集,即同时满足XYZ域和强度域的关键点为候选关键点,
HarrisKeypoint6D (float radius=0.01, float threshold=0.0) 重构函数,此处并没有方法选择的参数,而是默认采用了Tomsai提出的方法实现关键点的检测,radius为法线估计的搜索半径,threshold为判断是否为关键点的感兴趣程度的阀值,小于该阀值的点忽略,大于则认为是关键点。
(5)pcl::SIFTKeypoint< PointInT, PointOutT >
类SIFTKeypoint是将二维图像中的SIFT算子调整后移植到3D空间的SIFT算子的实现,输入带有XYZ坐标值和强度的点云,输出为点云中的SIFT关键点,其关键函数的说明如下:
void | setScales (float min_scale, int nr_octaves, int nr_scales_per_octave) |
---|---|
设置搜索时与尺度相关的参数,min_scale在点云体素尺度空间中标准偏差,点云对应的体素栅格中的最小尺寸 | |
int nr_octaves是检测关键点时体素空间尺度的数目,nr_scales_per_octave为在每一个体素空间尺度下计算高斯空间的尺度所需要的参数 | |
void | setMinimumContrast (float min_contrast) 设置候选关键点对应的对比度下限 |
(6)还有很多不再一一介绍
实例分析
实验实现提取NARF关键点,并且用图像和3D显示的方式进行可视化,可以直观的观察关键点的位置和数量 narf_feature_extraction.cpp:
#include <iostream>
#include <boost/thread/thread.hpp>
#include <pcl/range_image/range_image.h>
#include <pcl/io/pcd_io.h>
#include <pcl/visualization/range_image_visualizer.h>
#include <pcl/visualization/pcl_visualizer.h>
#include <pcl/features/range_image_border_extractor.h>
#include <pcl/keypoints/narf_keypoint.h>
#include <pcl/features/narf_descriptor.h>
#include <pcl/console/parse.h>
typedef pcl::PointXYZ PointType;
float angular_resolution = 0.5f; //angular_resolution为模拟的深度传感器的角度分辨率,即深度图像中一个像素对应的角度大小
float support_size = 0.2f; //点云大小的设置
pcl::RangeImage::CoordinateFrame coordinate_frame = pcl::RangeImage::CAMERA_FRAME; //设置坐标系
bool setUnseenToMaxRange = false;
bool rotation_invariant = true;
void printUsage (const char* progName)
{
std::cout << "\n\nUsage: "<<progName<<" [options] <scene.pcd>\n\n"
<< "Options:\n"
<< "-------------------------------------------\n"
<< "-r <float> angular resolution in degrees (default "<<angular_resolution<<")\n" << "-c <int> coordinate frame (default "<< (int)coordinate_frame<<")\n"
<< "-m Treat all unseen points to max range\n"
<< "-s <float> support size for the interest points (diameter of the used sphere - ""default "<<support_size<<")\n"
<< "-o <0/1> switch rotational invariant version of the feature on/off"
<< " (default "<< (int)rotation_invariant<<")\n"
<< "-h this help\n"
<< "\n\n";
}void setViewerPose (pcl::visualization::PCLVisualizer& viewer, const Eigen::Affine3f& viewer_pose) //设置视口的位姿{
Eigen::Vector3f pos_vector = viewer_pose * Eigen::Vector3f (0, 0, 0);
//视口的原点pos_vector
Eigen::Vector3f look_at_vector = viewer_pose.rotation () * Eigen::Vector3f (0, 0, 1) + pos_vector; //旋转+平移look_at_vector
Eigen::Vector3f up_vector = viewer_pose.rotation () * Eigen::Vector3f (0, -1, 0); //up_vector
viewer.setCameraPosition (pos_vector[0], pos_vector[1], pos_vector[2], look_at_vector[0], look_at_vector[1], look_at_vector[2], up_vector[0], up_vector[1], up_vector[2]);
}
int main (int argc, char** argv)
{
if (pcl::console::find_argument (argc, argv, "-h") >= 0)
{
printUsage (argv[0]); return 0;
} if (pcl::console::find_argument (argc, argv, "-m") >= 0)
{
setUnseenToMaxRange = true;
cout << "Setting unseen values in range image to maximum range readings.\n";
} if (pcl::console::parse (argc, argv, "-o", rotation_invariant) >= 0)
cout << "Switching rotation invariant feature version "<< (rotation_invariant ? "on" : "off")<<".\n"; int tmp_coordinate_frame; if (pcl::console::parse (argc, argv, "-c", tmp_coordinate_frame) >= 0)
{
coordinate_frame = pcl::RangeImage::CoordinateFrame (tmp_coordinate_frame);
cout << "Using coordinate frame "<< (int)coordinate_frame<<".\n";
} if (pcl::console::parse (argc, argv, "-s", support_size) >= 0)
cout << "Setting support size to "<<support_size<<".\n"; if (pcl::console::parse (argc, argv, "-r", angular_resolution) >= 0)
cout << "Setting angular resolution to "<<angular_resolution<<"deg.\n";
angular_resolution = pcl::deg2rad (angular_resolution);
// -Read pcd file or create example point cloud if not given-- //
pcl::PointCloud<PointType>::Ptr point_cloud_ptr (new pcl::PointCloud<PointType>);
pcl::PointCloud<PointType>& point_cloud = *point_cloud_ptr;
pcl::PointCloud<pcl::PointWithViewpoint> far_ranges;
Eigen::Affine3f scene_sensor_pose (Eigen::Affine3f::Identity ());
std::vector<int> pcd_filename_indices = pcl::console::parse_file_extension_argument (argc, argv, "pcd"); if (!pcd_filename_indices.empty ())
{
std::string filename = argv[pcd_filename_indices[0]]; if (pcl::io::loadPCDFile (filename, point_cloud) == -1)
{
cerr << "Was not able to open file \""<<filename<<"\".\n";
printUsage (argv[0]); return 0;
}
scene_sensor_pose = Eigen::Affine3f (Eigen::Translation3f (point_cloud.sensor_origin_[0], point_cloud.sensor_origin_[1],point_cloud.sensor_origin_[2])) * Eigen::Affine3f (point_cloud.sensor_orientation_);
std::string far_ranges_filename = pcl::getFilenameWithoutExtension (filename)+"_far_ranges.pcd"; if (pcl::io::loadPCDFile (far_ranges_filename.c_str (), far_ranges) == -1)
std::cout << "Far ranges file \""<<far_ranges_filename<<"\" does not exists.\n";
} else
{
setUnseenToMaxRange = true;
cout << "\nNo *.pcd file given => Genarating example point cloud.\n\n"; for (float x=-0.5f; x<=0.5f; x+=0.01f)
{ for (float y=-0.5f; y<=0.5f; y+=0.01f)
{
PointType point; point.x = x; point.y = y; point.z = 2.0f - y;
point_cloud.points.push_back (point);
}
}
point_cloud.width = (int) point_cloud.points.size (); point_cloud.height = 1;
}
// -----Create RangeImage from the PointCloud----- //
float noise_level = 0.0; float min_range = 0.0f; int border_size = 1;
boost::shared_ptr<pcl::RangeImage> range_image_ptr (new pcl::RangeImage);
pcl::RangeImage& range_image = *range_image_ptr;
range_image.createFromPointCloud (point_cloud, angular_resolution, pcl::deg2rad (360.0f), pcl::deg2rad (180.0f),
scene_sensor_pose, coordinate_frame, noise_level, min_range, border_size);
range_image.integrateFarRanges (far_ranges); if (setUnseenToMaxRange)
range_image.setUnseenToMaxRange (); // -----Open 3D viewer and add point cloud----- //
pcl::visualization::PCLVisualizer viewer ("3D Viewer");
viewer.setBackgroundColor (1, 1, 1);
pcl::visualization::PointCloudColorHandlerCustom<pcl::PointWithRange> range_image_color_handler (range_image_ptr, 0, 0, 0);
viewer.addPointCloud (range_image_ptr, range_image_color_handler, "range image");
viewer.setPointCloudRenderingProperties (pcl::visualization::PCL_VISUALIZER_POINT_SIZE, 1, "range image");
//viewer.addCoordinateSystem (1.0f, "global");
//PointCloudColorHandlerCustom<PointType> point_cloud_color_handler (point_cloud_ptr, 150, 150, 150);
//viewer.addPointCloud (point_cloud_ptr, point_cloud_color_handler, "original point cloud");
viewer.initCameraParameters ();
setViewerPose (viewer, range_image.getTransformationToWorldSystem ()); // -----Show range image----- //
pcl::visualization::RangeImageVisualizer range_image_widget ("Range image");
range_image_widget.showRangeImage (range_image);
/* 创建RangeImageBorderExtractor对象,它是用来进行边缘提取的,因为NARF的第一步就是需要探测出深度图像的边缘*/ // -----Extract NARF keypoints----- //
pcl::RangeImageBorderExtractor range_image_border_extractor; //用来提取边缘
pcl::NarfKeypoint narf_keypoint_detector; //用来检测关键点
narf_keypoint_detector.setRangeImageBorderExtractor (&range_image_border_extractor); // narf_keypoint_detector.setRangeImage (&range_image);
narf_keypoint_detector.getParameters ().support_size = support_size; //设置NARF的参数
pcl::PointCloud<int> keypoint_indices;
narf_keypoint_detector.compute (keypoint_indices);
std::cout << "Found "<<keypoint_indices.points.size ()<<" key points.\n"; // -----Show keypoints in 3D viewer----- //
pcl::PointCloud<pcl::PointXYZ>::Ptr keypoints_ptr (new pcl::PointCloud<pcl::PointXYZ>);
pcl::PointCloud<pcl::PointXYZ>& keypoints = *keypoints_ptr;
keypoints.points.resize (keypoint_indices.points.size ()); for (size_t i=0; i<keypoint_indices.points.size (); ++i)
keypoints.points[i].getVector3fMap () = range_image.points[keypoint_indices.points[i]].getVector3fMap ();
pcl::visualization::PointCloudColorHandlerCustom<pcl::PointXYZ> keypoints_color_handler (keypoints_ptr, 0, 255, 0);
viewer.addPointCloud<pcl::PointXYZ> (keypoints_ptr, keypoints_color_handler, "keypoints");
viewer.setPointCloudRenderingProperties (pcl::visualization::PCL_VISUALIZER_POINT_SIZE, 7, "keypoints"); // -----Extract NARF descriptors for interest points----- //
std::vector<int> keypoint_indices2;
keypoint_indices2.resize (keypoint_indices.points.size ()); for (unsigned int i=0; i<keypoint_indices.size (); ++i) // This step is necessary to get the right vector type
keypoint_indices2[i]=keypoint_indices.points[i];
pcl::NarfDescriptor narf_descriptor (&range_image, &keypoint_indices2);
narf_descriptor.getParameters ().support_size = support_size;
narf_descriptor.getParameters ().rotation_invariant = rotation_invariant;
pcl::PointCloud<pcl::Narf36> narf_descriptors;
narf_descriptor.compute (narf_descriptors);
cout << "Extracted "<<narf_descriptors.size ()<<" descriptors for "
<<keypoint_indices.points.size ()<< " keypoints.\n";
while (!viewer.wasStopped ())
{
range_image_widget.spinOnce (); // process GUI events viewer.spinOnce ();
pcl_sleep(0.01);
}
}
运行结果: