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message_filters::sync::ApproximateTime does not work properly [closed]

asked 2016-01-11 14:13:33 -0600

Gren8 gravatar image

Good evening, i have an important question. I try to subscribe to two image topics and want them to be synchronized and ready to be processed in one callback. In the callback function i want to convert them via cv_bridge to do some image processing. I've already learned the best way to do this is using the message_filters::sync::ApproximateTime class. The synchronizer works and i can see the images with imshow(). The problem is that the callback function only get called each second so i get a very SLOW image stream. If i move an object i'll see it seconds later moving on the screen.

   void callback(const ImageConstPtr& sub1, const ImageConstPtr& sub2){

        double stamp_1 =sub1->header.stamp.toSec();
        double stamp_2 =sub2->header.stamp.toSec();
        ROS_INFO("\nCALLBACK GETS CALLED\n TIME_STAMP_IMAGE1: %f\n TIME_STAMP_IMAGE2: %f\n", stamp_1, stamp_2);

        Mat image_front, image_back;

        //Converting Image 1
        image1 = *(sub1.get());
        frameptr1=cv_bridge::toCvCopy(bild1, sensor_msgs::image_encodings::BGR8);

        //Converting Image 2
        image2 = *(sub2.get());
        frameptr2=cv_bridge::toCvCopy(bild2, sensor_msgs::image_encodings::BGR8);

        //Show images
        imshow("window1", image_front);
        imshow("window2", image_back);


    int main(int argc, char** argv) {

        ros::init( argc, argv, "sync_test" );
        ros::NodeHandle nh;
        namedWindow("window1", CV_WINDOW_NORMAL);
        namedWindow("window2", CV_WINDOW_NORMAL);
        message_filters::Subscriber<Image> sub1(nh, "/camera1/image_raw", 1);
        message_filters::Subscriber<Image> sub2(nh, "/camera2/image_raw", 1);

        typedef sync_policies::ApproximateTime<Image, Image> MySyncPolicy;
        // ApproximateTime takes a queue size as its constructor argument, hence MySyncPolicy(10)

        Synchronizer<MySyncPolicy> sync(MySyncPolicy(100), sub1, sub2);
        sync.registerCallback(boost::bind(&callback, _1, _2));



        return 0;


Could anybody see a possibly mistake? I really need help, it's very important for me.

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Closed for the following reason question is not relevant or outdated by tfoote
close date 2017-04-20 14:02:01.512242

1 Answer

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answered 2016-01-11 16:34:11 -0600

Thomas D gravatar image

In main() where you create sub1 and sub2 try increasing the queue_size from 1 to 5 or 10. This will allow the approximate algorithm enough samples to find which messages are closest in time. Alternatively, try reducing the queue_size passed to the sync_policy. You are using 100 and could try dropping that to about 10.

Also, instead of wrapping spinOnce() inside a while loop you can replace the entire while loop with a single call to ros::spin().

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Thanks for the advice, but i already tried it with different values for the queue_size as you said, but it didn't change anything.

Gren8 gravatar image Gren8  ( 2016-01-12 01:35:43 -0600 )edit

Did you ever figure out how to fix this? I have a similar problem.

reben gravatar image reben  ( 2018-01-23 18:20:58 -0600 )edit

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Asked: 2016-01-11 14:13:33 -0600

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Last updated: Jan 11 '16