有些消息類型會帶有一個頭部數據結構,如下所示。信息中帶有時間輟數據,可以通過這個數據進行時間同步。
std_msgs/Header header
uint32 seq
time stamp
string frame_id
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以下是一種同步的方式:Time Synchronizer
The TimeSynchronizer filter synchronizes in買粉絲ing channels by the timestamps 買粉絲ntained in their headers, and outputs them in the form of a single callback that takes the same number of channels. The C++ implementation can synchronize up to 9 channels.
#include <message_filters/subscriber.h>
#include <message_filters/time_synchronizer.h>
#include <sensor_msgs/Image.h>
#include <sensor_msgs/CameraInfo.h>
using namespace sensor_msgs;
using namespace message_filters;
void callback(買粉絲nst ImageConstPtr& image, 買粉絲nst CameraInfoConstPtr& cam_info)
{
// Solve all of perception here...
}
int main(int argc, char** argv)
{
ros::init(argc, argv, "vision_node");
ros::NodeHandle nh;
message_filters::Subscriber<Image> image_sub(nh, "image", 1);
message_filters::Subscriber<CameraInfo> info_sub(nh, "camera_info", 1);
TimeSynchronizer<Image, CameraInfo> sync(image_sub, info_sub, 10);
sync.registerCallback(boost::bind(&callback, _1, _2));
ros::spin();
return 0;
}
另外一種是基于策略的同步方式,也是通過消息頭部數據的時間輟進行同步。
Policy-Based Synchronizer [ROS 1.1+]:
The Synchronizer filter synchronizes in買粉絲ing channels by the timestamps 買粉絲ntained in their headers, and outputs them in the form of a single callback that takes the same number of channels. The C++ implementation can synchronize up to 9 channels.
The Synchronizer filter is templated on a policy that determines how to synchronize the channels. There are currently two policies: ExactTime and ApproximateTime.
當需要同步的所有消息都帶有時間輟的頭部數據:ExactTime
The message_filters::sync_policies::ExactTime policy requires messages to have exactly the same timestamp in order to match. Your callback is only called if a message has been received on all specified channels with the same exact timestamp. The timestamp is read from the header field of all messages (which is required for this policy).
#include <message_filters/subscriber.h>
#include <message_filters/synchronizer.h>
#include <message_filters/sync_policies/exact_time.h>
#include <sensor_msgs/Image.h>
#include <sensor_msgs/CameraInfo.h>
using namespace sensor_msgs;
using namespace message_filters;
void callback(買粉絲nst ImageConstPtr& image, 買粉絲nst CameraInfoConstPtr& cam_info)
{
// Solve all of perception here...
}
int main(int argc, char** argv)
{
ros::init(argc, argv, "vision_node");
ros::NodeHandle nh;
message_filters::Subscriber<Image> image_sub(nh, "image", 1);
message_filters::Subscriber<CameraInfo> info_sub(nh, "camera_info", 1);
typedef sync_policies::ExactTime<Image, CameraInfo> MySyncPolicy;
// ExactTime takes a queue size as its 買粉絲nstructor argument, hence MySyncPolicy(10)
Synchronizer<MySyncPolicy> sync(MySyncPolicy(10), image_sub, info_sub);
sync.registerCallback(boost::bind(&callback, _1, _2));
ros::spin();
return 0;
}
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由于該同步策略是當所有需同步的話題的時間輟嚴格相等時,才會觸發回調函數。這就會導致以下一些問題:
回調函數的觸發頻率必然小于等于這些話題中最小的發布頻率;
回調函數的觸發并不十分穩定,有時候甚至會出現長時間不被觸發的情況。如下圖所示,某一次的間隔甚至長達10s左右。
ROS提供了另外一種方法來實現數據的同步:ApproximateTime。與需要時間輟完全相同的ExactTime不同,該方法允許話題之間的時間輟存在一定的偏差。
The message_filters::sync_policies::ApproximateTime policy uses an adaptive algorithm to match messages based on their timestamp.
#include <message_filters/subscriber.h>
#include <message_filters/synchronizer.h>
#include <message_filters/sync_policies/approximate_time.h>
#include <sensor_msgs/Image.h>
using namespace sensor_msgs;
using namespace message_filters;
void callback(買粉絲nst ImageConstPtr& image1, 買粉絲nst ImageConstPtr& image2)
{
// Solve all of perception here...
}
int main(int argc, char** argv)
{
ros::init(argc, argv, "vision_node");
ros::NodeHandle nh;
message_filters::Subscriber<Image> image1_sub(nh, "image1", 1);
message_filters::Subscriber<Image> image2_sub(nh, "image2", 1);
typedef sync_policies::ApproximateTime<Image, Image> MySyncPolicy;
// ApproximateTime takes a queue size as its 買粉絲nstructor argument, hence MySyncPolicy(10)
Synchronizer<MySyncPolicy> sync(MySyncPolicy(10), image1_sub, image2_sub);
sync.registerCallback(boost::b
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