MATLAB ROS Transform Error

I'm getting the following error when atempting to use getTransform():

"Lookup would require extrapolation into the past. Requested time Wed Feb 13 14:23:57 2019 but the earliest data is at time Wed Feb 13 14:23:57 2019 when looking up transform from frame [base_link] to frame [world]"

This error makes it seem as though the transform should be available... but it's not.

Additionally, canTransform() returns a logical 0 when requesting a transform at that time. What's going on here?

Code below:

bagfile = 'multiradar_2019-02-13_rangeRes_0-04_velRes_0-09_loop.bag'
bag = rosbag(bag_file);

% topic information
radar_topic = 'radar_fwd/mmWaveDataHdl/RScan';

% Understand how to use ROS TFs in MATLAB
radar_bag = select(bag,'Topic',radar_topic);
radar_messages = readMessages(radar_bag);
radar_struct = readMessages(radar_bag,'DataFormat','struct');

radar_x = readField(radar_messages{1}, 'x');
radar_y = readField(radar_messages{1}, 'y');
radar_z = readField(radar_messages{1}, 'z');

for i=1:radar_struct{1}.Width
    %transform each point in radar message to 'world' frame
    pt = rosmessage('geometry_msgs/PointStamped');
    pt.Header.FrameId = radar_struct{1}.Header.FrameId;
    pt.Header.Stamp = rostime(radar_struct{1}.Header.Stamp.Sec, ...
        radar_struct{1}.Header.Stamp.Nsec);
    pt.Point.X = radar_x(i);
    pt.Point.Y = radar_y(i);
    pt.Point.Z = radar_z(i);
    
    % transform point in 'radar_fwd_link' frame to 'world' frame
    tfTime = rostime(radar_struct{attempting 1}.Header.Stamp.Sec, ...
        radar_struct{1}.Header.Stamp.Nsec);
    if canTransform(bag,'world',radar_struct{1}.Header.FrameId,tfTime)
        disp('TF available!')
        tf = getTransform(bag,'world',radar_struct{1}.Header.FrameId,tfTime, ...
            'Timeout',5);
        tfpt = apply(tf,pt);
    else
        disp('TF not available!')
    end
    
    disp([pt.Point.X, pt.Point.Y, pt.Point.Z])
    tf = getTransform(bag,'world',radar_struct{1}.Header.FrameId,tfTime)
    tfpt2 = apply(tf,pt)
    
end

[Cam Salzberger]

Hey Carl,

Check the /tf topic in the rosbag, or just list the first several messages. Had it already published on the /tf topic before the timestamp of the message you are trying to get the transform at? It could simply be round-off confusion in the error message (e.g. where the earliest available time is at 57.2 seconds, and you're requesting at 57.0 seconds).

-Cam

[Carl Stahoviak]

Thank you Cam. The problem was that only the very first radar message had a timestamp that just barely (in the Nsecs range) pre-dated the first transfrom message. So all other messages in the bag can be properly transformed.

I have a follow up question though. I think there is an error in the following documentation page: https://www.mathworks.com/help/robotics/ref/apply.html#buqbf3f-entity

The list of transformable entities includes the type: geometry_msgs/PointCloud2Stamped

However, in ROS, there is no such thing as a PointCloud2Stamped message. Did the documentation mean to refer to the sensor_msgs/PointCloud2 message type?

This message type is listed in the entity types under the documentation page that describes the transform() function: https://www.mathworks.com/help/robotics/ref/transform.html#buqbzo2-entity

[Cam Salzberger]

Hey Carl,

Jeeze this took me a long time to get back to you. Yes, that is a typo. If you check the help text for the "apply" method, you'll notice it is correct there:

m = rosmessage('geometry_msgs/TransformStamped');
help apply

I'll put in a note to the appropriate team to update the documentation. Thanks again!

-Cam