Marker display for custom visualization in ROS - Mini Project: Build & Apply

Marker display for custom visualization

📖 Scenario: You are building a simple ROS node to display a custom marker in RViz. This marker will help visualize a point of interest in a robot's environment.

🎯 Goal: Create a ROS node that publishes a visualization_msgs/Marker message to display a red sphere marker at a fixed position in RViz.

📋 What You'll Learn

Create a ROS publisher for the topic /visualization_marker

Define a visualization_msgs/Marker message with type SPHERE

Set the marker's position to (1.0, 2.0, 0.0)

Set the marker's scale to (0.2, 0.2, 0.2)

Set the marker's color to red with full opacity

Publish the marker message continuously at 1 Hz

💡 Why This Matters

🌍 Real World

Robots often need to visualize points, paths, or objects in their environment. Markers in RViz help developers and operators see what the robot 'sees' or plans.

💼 Career

Understanding how to publish visualization markers is essential for robotics developers working with ROS to debug and demonstrate robot perception and planning.

1

Create ROS node and publisher

Write Python code to initialize a ROS node named marker_publisher and create a publisher called marker_pub that publishes visualization_msgs/Marker messages on the topic /visualization_marker.

ROS

# Initialize ROS node and create publisher
# Your code here

Hint

Use rospy.init_node to start the node and rospy.Publisher to create the publisher.

2

Create and configure the Marker message

Create a Marker message named marker. Set its header.frame_id to "map", ns to "basic_shapes", id to 0, and type to Marker.SPHERE. Set action to Marker.ADD.

ROS

import rospy
from visualization_msgs.msg import Marker

rospy.init_node('marker_publisher')
marker_pub = rospy.Publisher('/visualization_marker', Marker, queue_size=10)

# Create and configure Marker message
# Your code here

Hint

Use the Marker class and set the required fields exactly as specified.

3

Set marker position, scale, and color

Set the marker's position to x=1.0, y=2.0, z=0.0. Set the scale to x=0.2, y=0.2, z=0.2. Set the color to red with full opacity by setting color.r = 1.0, color.g = 0.0, color.b = 0.0, and color.a = 1.0.

ROS

import rospy
from visualization_msgs.msg import Marker

rospy.init_node('marker_publisher')
marker_pub = rospy.Publisher('/visualization_marker', Marker, queue_size=10)

marker = Marker()
marker.header.frame_id = "map"
marker.ns = "basic_shapes"
marker.id = 0
marker.type = Marker.SPHERE
marker.action = Marker.ADD

# Set position, scale, and color of the marker
# Your code here

Hint

Set the pose.position, scale, and color fields of the marker exactly as described.

4

Publish the marker continuously

Write a loop that runs while ROS is not shutdown. Inside the loop, update the marker's header.stamp with the current time using rospy.Time.now(), publish the marker using marker_pub.publish(marker), and sleep at 1 Hz using rospy.Rate(1).sleep().

ROS

import rospy
from visualization_msgs.msg import Marker

rospy.init_node('marker_publisher')
marker_pub = rospy.Publisher('/visualization_marker', Marker, queue_size=10)

marker = Marker()
marker.header.frame_id = "map"
marker.ns = "basic_shapes"
marker.id = 0
marker.type = Marker.SPHERE
marker.action = Marker.ADD

marker.pose.position.x = 1.0
marker.pose.position.y = 2.0
marker.pose.position.z = 0.0

marker.scale.x = 0.2
marker.scale.y = 0.2
marker.scale.z = 0.2

marker.color.r = 1.0
marker.color.g = 0.0
marker.color.b = 0.0
marker.color.a = 1.0

# Publish the marker continuously at 1 Hz
# Your code here

Hint

Use a while not rospy.is_shutdown() loop, update the timestamp, publish the marker, and sleep at 1 Hz.

Practice

1. What is the main purpose of using markers in ROS visualization tools like RViz?

easy

A. To control the robot's movement directly

B. To send commands to the robot's motors

C. To draw shapes in 3D space to help visualize robot data

D. To store robot configuration files

Solution

1. Step 1: Understand marker usage in ROS

   Markers are used to create visual shapes like arrows, cubes, or spheres in RViz to represent data.

2. Step 2: Differentiate markers from control commands

   Markers do not control robot movement or store files; they only help visualize data.

3. Final Answer:

   To draw shapes in 3D space to help visualize robot data -> Option C

4. Quick Check:

   Markers visualize data = B [OK]

Hint: Markers show shapes for visualization, not control [OK]

Common Mistakes:

• Thinking markers control robot movement
• Confusing markers with configuration files
• Assuming markers send commands to motors

2. Which of the following is the correct way to set the type of a marker to a sphere in ROS using the visualization_msgs/Marker message?

easy

A. marker.type = visualization_msgs::Marker::SPHERE;

B. marker.type = visualization_msgs::Marker::CUBE;

C. marker.type = visualization_msgs::Marker::LINE_STRIP;

D. marker.type = visualization_msgs::Marker::TEXT_VIEW_FACING;

Solution

1. Step 1: Identify marker type constants

   ROS defines marker types like CUBE, SPHERE, LINE_STRIP, TEXT_VIEW_FACING as constants in visualization_msgs::Marker.

2. Step 2: Match sphere type

   The constant for a sphere is visualization_msgs::Marker::SPHERE.

3. Final Answer:

   marker.type = visualization_msgs::Marker::SPHERE; -> Option A

4. Quick Check:

   SPHERE type constant = A [OK]

Hint: Use Marker::SPHERE constant to set sphere type [OK]

Common Mistakes:

• Using CUBE instead of SPHERE
• Confusing LINE_STRIP with sphere
• Using text marker type for shapes

3. Given the following ROS Python code snippet, what will be the color of the marker displayed in RViz?

marker.color.r = 0.0
marker.color.g = 1.0
marker.color.b = 0.0
marker.color.a = 0.5

medium

A. Opaque red

B. Fully transparent (invisible)

C. Opaque blue

D. Semi-transparent green

Solution

1. Step 1: Analyze RGB color values

   Red = 0.0, Green = 1.0, Blue = 0.0 means pure green color.

2. Step 2: Check alpha (opacity) value

   Alpha = 0.5 means 50% transparency, so semi-transparent.

3. Final Answer:

   Semi-transparent green -> Option D

4. Quick Check:

   Green=1 and alpha=0.5 = D [OK]

Hint: Alpha <1 means transparency; RGB sets color [OK]

Common Mistakes:

• Ignoring alpha and assuming opaque
• Mixing up RGB values for color
• Thinking alpha=0.5 means fully transparent

4. You wrote this ROS C++ code to publish a marker, but the marker does not appear in RViz. What is the likely error?

visualization_msgs::Marker marker;
marker.header.frame_id = "map";
marker.type = visualization_msgs::Marker::SPHERE;
marker.action = visualization_msgs::Marker::ADD;
marker.pose.position.x = 1.0;
marker.pose.position.y = 2.0;
marker.pose.position.z = 0.0;
marker.scale.x = 0.5;
marker.scale.y = 0.5;
marker.scale.z = 0.5;
marker.color.r = 1.0;
marker.color.g = 0.0;
marker.color.b = 0.0;
marker.color.a = 1.0;
marker_pub.publish(marker);

medium

A. Incorrect marker type used

B. Missing setting of marker.header.stamp to current time

C. Scale values are too small to see

D. Color alpha is zero making it invisible

Solution

1. Step 1: Check marker header completeness

   ROS markers require header.stamp to be set to current time for RViz to display them properly.

2. Step 2: Verify other parameters

   Marker type, scale, and color are valid and visible; alpha is 1.0 (opaque).

3. Final Answer:

   Missing setting of marker.header.stamp to current time -> Option B

4. Quick Check:

   Header stamp missing = A [OK]

Hint: Always set header.stamp to ros::Time::now() [OK]

Common Mistakes:

• Forgetting to set header.stamp
• Using zero alpha making marker invisible
• Assuming scale too small to see

5. You want to display multiple markers at different positions using a single ROS publisher. Which approach correctly updates and publishes markers to show all of them in RViz?

hard

A. Publish each marker separately with unique IDs and the same topic

B. Publish only the last marker repeatedly, ignoring others

C. Publish markers with the same ID to overwrite previous ones

D. Publish markers without setting frame_id to avoid errors

Solution

1. Step 1: Understand marker IDs and topics

   Each marker must have a unique ID to be displayed simultaneously on the same topic.

2. Step 2: Avoid overwriting markers

   Publishing markers with the same ID overwrites previous ones, so unique IDs are needed.

3. Final Answer:

   Publish each marker separately with unique IDs and the same topic -> Option A

4. Quick Check:

   Unique IDs per marker = C [OK]

Hint: Use unique IDs for each marker on one topic [OK]

Common Mistakes:

• Using same ID for multiple markers
• Publishing only one marker and expecting all to show
• Not setting frame_id causing display errors