Re: Clarification about the input synchronization step

Posted by matlabbe on
URL: http://official-rtab-map-forum.206.s1.nabble.com/Clarification-about-the-input-synchronization-step-tp5841p5842.html

Hi,

Yes, the approx_sync parameter refers to Approximate (when true) and Exact (when false) time policies: http://wiki.ros.org/message_filters#Policy-Based_Synchronizer_.5BROS_1.1.2B-.5D

When false, it means that the input topics should have exactly the same stamps. If odometry messages don't share exactly the same stamp than RGB and depth images, approx_sync would be set to true. I say "would" because it depends if we use odometry from the topic or TF. In case we are using TF by setting odom_frame_id (with odom_tf_angular_variance and odom_tf_linear_variance), approx_sync should be false. If odom_frame_id is not set, the odom topic will be synchronized. In this case approx_sync needs to be true. The problem is that we don't want to do approximate synchronization of RGB and Depth images. The rgbd_sync node can be used to pre-sync the image data before rtabmap node. So we exactly synchonize RGB and depth images to rgbd_sync (with approx_sync=false), then the output of rgbd_sync is fed to rtabmap synchronizing with odom topic (with approx_sync=true and subscribe_rgbd=true). See this example for details. See also Figure 6 in this paper.

cheers,
Mathieu