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readFrame

Read point cloud frame from ROS message

Description

example

ptCloud = readFrame(veloReader) reads the next point cloud frame from the Velodyne ROS messages and returns a pointCloud object. The next point cloud frame is the first point cloud available at or after the value of the CurrentTime property of the Velodyne ROS message reader object veloReader.

ptCloud = readFrame(veloReader,frameNumber) reads the point cloud with the specified frame number from the Velodyne ROS messages.

ptCloud = readFrame(veloReader,frameTime) reads the first point cloud available at or after the specified timestamp frameTime.

Examples

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Velodyne ROS messages store data in a format that requires some interpretation before it can be used for further processing. MATLAB® can help you by formatting Velodyne ROS messages for easy use. In this example, you can explore how VelodyneScan messages from a Velodyne LiDAR are handled.

Prerequisites: Work with Basic ROS Messages

Load Sample Messages

Load sample Velodyne messages. These messages are populated with data gathered from Velodyne LiDAR sensor.

load('lidarData_ConstructionRoad.mat')

VelodyneScan Messages

VelodyneScan messages are ROS messages that contain Velodyne LIDAR scan packets. You can see the standard ROS format for a VelodyneScan message by creating an empty message of the appropriate type.

emptyveloScan = rosmessage('velodyne_msgs/VelodyneScan')
emptyveloScan = 
  ROS VelodyneScan message with properties:

    MessageType: 'velodyne_msgs/VelodyneScan'
         Header: [1x1 Header]
        Packets: [0x1 VelodynePacket]

  Use showdetails to show the contents of the message

Since you created an empty message, emptyveloScan does not contain any meaningful data. For convenience, the loaded lidarData_ConstructionRoad.mat filecontains a set of VelodyneScan messages that is fully populated and is stored in the msgs variable. Each element in the msgs cell array is a VelodyneScan ROS message struct. The primary data in each VelodyneScan message is in the Packets property, it contains multiple VelodynePacket messages. You can see the standard ROS format for a VelodynePacket message by creating an empty message of the appropriate type.

emptyveloPkt = rosmessage('velodyne_msgs/VelodynePacket')
emptyveloPkt = 
  ROS VelodynePacket message with properties:

    MessageType: 'velodyne_msgs/VelodynePacket'
          Stamp: [1x1 Time]
           Data: [1206x1 uint8]

  Use showdetails to show the contents of the message

Create Velodyne ROS Message Reader

The velodyneROSMessageReader object reads point cloud data from VelodyneScan ROS messages based on their specified model type. Note that providing an incorrect device model may result in improperly calibrated point clouds. This example uses messages from the 'HDL32E' model.

veloReader = velodyneROSMessageReader(msgs,'HDL32E')
veloReader = 
  velodyneROSMessageReader with properties:

    VelodyneMessages: {28x1 cell}
         DeviceModel: 'HDL32E'
     CalibrationFile: '/mathworks/devel/bat/Bdoc21a/build/matlab/toolbox/shared/pointclouds/utilities/velodyneFileReaderConfiguration/HDL32E.xml'
      NumberOfFrames: 55
            Duration: 2.7477 sec
           StartTime: 1145.2 sec
             EndTime: 1147.9 sec
          Timestamps: [1x55 duration]
         CurrentTime: 1145.2 sec

Extract Point Clouds

You can extract point clouds from the raw packets message with the help of this velodyneROSMessageReader object. By providing a specific frame number or timestamp, one point cloud can be extracted from velodyneROSMessageReader object using the readFrame object function. If you call readFrame without a frame number or timestamp, it extracts the next point cloud in the sequence based on the CurrentTime property.

Create a duration scalar that represents one second after the first point cloud reading.

timeDuration = veloReader.StartTime + duration(0,0,1,'Format','s');

Read the first point cloud recorded at or after the given time duration.

ptCloudObj = readFrame(veloReader,timeDuration);

Access Location data in the point cloud.

ptCloudLoc = ptCloudObj.Location;

Reset the CurrentTime property of veloReader to the default value

reset(veloReader)

Display All Point Clouds

You can also loop through all point clouds in the input Velodyne ROS messages.

Define x-, y-, and z-axes limits for pcplayer in meters. Label the axes.

xlimits = [-60 60];
ylimits = [-60 60];
zlimits = [-20 20];

Create the point cloud player.

player = pcplayer(xlimits,ylimits,zlimits);

Label the axes.

xlabel(player.Axes,'X (m)');
ylabel(player.Axes,'Y (m)');
zlabel(player.Axes,'Z (m)');

The first point cloud of interest is captured at 0.3 second into the input messages. Set the CurrentTime property to that time to begin reading point clouds from there.

veloReader.CurrentTime = veloReader.StartTime + seconds(0.3);

Display the point cloud stream for 2 seconds. To check if a new frame is available and continue past 2 seconds, remove the last while condition. Iterate through the file by calling readFrame to read in point clouds. Display them using the point cloud player.

while(hasFrame(veloReader) && isOpen(player) && (veloReader.CurrentTime < veloReader.StartTime + seconds(2)))
    ptCloudObj = readFrame(veloReader);
    view(player,ptCloudObj.Location,ptCloudObj.Intensity);
    pause(0.1);
end

Figure Point Cloud Player contains an axes. The axes contains an object of type scatter.

Input Arguments

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Velodyne ROS message reader, specified as a velodyneROSMessageReader object.

Frame number of the desired point cloud, specified as a positive integer. Frame numbers are sequential in the velodyneROSMessageReader object.

Frame time of the desired point cloud, specified as a duration scalar in seconds. The function return the first frame available at or after the specified timestamp.

Output Arguments

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Point cloud, returned as pointCloud object.

Introduced in R2020b