Marker display for custom visualization in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Marker message type for visualization.

ROS

from visualization_msgs.msg import [1]

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AMarker

BMarkerArray

CMarkerMsg

DMarkerDisplay

Attempts:

3 left

💡 Hint

Common Mistakes

Using MarkerArray instead of Marker

Using incorrect message names

The Marker message is used to create custom visualization markers in ROS.

2fill in blank

medium

Complete the code to create a publisher for Marker messages on the 'visualization_marker' topic.

ROS

marker_pub = rospy.Publisher('visualization_marker', [1], queue_size=10)

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AMarkerArray

BMarkerDisplay

CMarkerMsg

DMarker

Attempts:

3 left

💡 Hint

Common Mistakes

Using MarkerArray instead of Marker

Incorrect topic name or message type

The publisher must use the Marker message type to send individual markers.

3fill in blank

hard

Fix the error in setting the marker type to a cube shape.

ROS

marker.type = [1]

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AMarker.CUBE

BMarker.CYLINDER

CMarker.ARROW

DMarker.SPHERE

Attempts:

3 left

💡 Hint

Common Mistakes

Using Marker.SPHERE or other shapes by mistake

The cube shape is specified by Marker.CUBE constant.

4fill in blank

hard

Fill both blanks to set the marker's position and orientation.

ROS

marker.pose.position.[1] = 1.0
marker.pose.orientation.[2] = 0.0

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Ax

By

Cz

Dw

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing position and orientation components

Using y or z for orientation w

The position coordinate is set with x, and orientation uses quaternion component w.

5fill in blank

hard

Fill all three blanks to create a dictionary for marker color with red, green, and alpha values.

ROS

marker.color = [1](r=1.0, g=[2], b=0.0, a=[3])

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AColorRGBA

B0.0

C1.0

DColorRGB

Attempts:

3 left

💡 Hint

Common Mistakes

Using ColorRGB which lacks alpha

Swapping green and alpha values

The ColorRGBA class is used for colors with red, green, blue, and alpha channels. Here green is 0.0 and alpha is 1.0 for full opacity.

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