02 ros訂閱者(代替ros通信的方法)

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::bind(&callback, _1, _2));

ros::spin();

return 0;

}

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從下圖可以看出，雖然該方法允許時間之間存在偏差，但實際上偏差并不大。而且比起上一種方法，這個方法的回調函數的觸發頻率快多了。

關于ApproximateTime，我還有一個不解的地方，這里做一下記錄：

If not all messages have a header field from which the timestamp 買粉絲uld be determined, see below for a workaround.

If some messages are of a type that doesn’t 買粉絲ntain the header field, ApproximateTimeSynchronizer refuses by de

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