readMessages

Read messages from rosbag

Syntax

msgs = readMessages(bag)

msgs = readMessages(bag,rows)

msgs = readMessages(___,"DataFormat","struct")

Description

msgs = readMessages(bag) returns data from all the messages in the BagSelection object, bag. The messages are returned in a cell array of messages.

To get a BagSelection object, use rosbag.

msgs = readMessages(bag,rows) returns data from messages in the rows specified by rows. The range of the rows is [1, bag.NumMessages].

msgs = readMessages(___,"DataFormat","struct") returns data as a cell array of structures using either set of the previous input arguments. Using structures can be significantly faster than using message objects, and custom message data can be read directly without loading message definitions using rosgenmsg.

Note

In a future release, ROS Toolbox will use message structures instead of objects for ROS messages.

To use message structures now, set the "DataFormat" name-value argument to "struct". For more information, see ROS Message Structures.

Examples

Return ROS Messages as a Cell Array

Read rosbag and filter by topic and time.

bagselect = rosbag('ex_multiple_topics.bag');
bagselect2 = select(bagselect,'Time',...
[bagselect.StartTime bagselect.StartTime + 1],'Topic','/odom');

Return all messages as a cell array.

allMsgs = readMessages(bagselect2);

Return the first ten messages as a cell array.

firstMsgs = readMessages(bagselect2,1:10);

Read Messages from a rosbag as a Structure

Load the rosbag.

bag = rosbag('ros_turtlesim.bag');

Select a specific topic.

bSel = select(bag,'Topic','/turtle1/pose');

Read messages as a structure. Specify the DataFormat name-value pair when reading the messages. Inspect the first structure in the returned cell array of structures.

msgStructs = readMessages(bSel,'DataFormat','struct');
msgStructs{1}

ans = struct with fields:
        MessageType: 'turtlesim/Pose'
                  X: 5.5016
                  Y: 6.3965
              Theta: 4.5377
     LinearVelocity: 1
    AngularVelocity: 0

Extract the xy points from the messages and plot the robot trajectory.

Use cellfun to extract all the X and Y fields from the structure. These fields represent the xy positions of the robot during the rosbag recording.

xPoints = cellfun(@(m) double(m.X),msgStructs);
yPoints = cellfun(@(m) double(m.Y),msgStructs);
plot(xPoints,yPoints)

Input Arguments

bag — Message of rosbag

BagSelection object

All the messages contained within a rosbag, specified as a BagSelection object.

rows — Rows of BagSelection object

n-element vector

Rows of BagSelection object, specified as an n-element vector, where n is the number of rows to retrieve messages from. Each entry in the vector corresponds to a numbered message in the bag. The range of the rows is [1, bag.NumMessage].

Output Arguments

msgs — ROS message data

object | cell array of message objects | cell array of structures

ROS message data, returned as an object, cell array of message objects, or cell array of structures. Data comes from the BagSelection object created using rosbag. You must specify "DataFormat","struct" in the function to get messages as a cell array of structures. Using structures can be significantly faster than using message objects, and custom message data can be read directly without loading message definitions using rosgenmsg.

Compatibility Considerations

ROS Message Structures

Behavior change in future release

You can now create messages as structures with fields matching the message object properties. Using structures typically improves performance of creating, updating, and using ROS messages, but message fields are no longer validated when set. Message types and corresponding field values from the structures are validated when sent across the network.

To use ROS messages as structures, use the "DataFormat" name-value argument when creating your publishers, subscribers, or other ROS objects. Any messages generated from these objects will utilize structures.

pub = rospublisher("/scan","sensor_msgs/LaserScan","DataFormat","struct")
msg = rosmessage(pub)

You can also create messages as structures directly, but make sure to specify the data format as "struct" for the publisher, subscriber, or other ROS objects as well. ROS objects still use message objects by default.

msg = rosmessage("/scan","sensor_msgs/LaserScan","DataFormat","struct")
...
pub = rospublisher("/scan","sensor_msgs/LaserScan","DataFormat","struct")

In a future release, ROS messages will use structures by default and ROS message objects will be removed.

For more information, see Improve Performance of ROS Using Message Structures.