Marker display for custom visualization in ROS - Step-by-Step Execution

Concept Flow - Marker display for custom visualization

Create Marker Object

↓

Set Marker Properties

↓

Publish Marker to Topic

↓

RViz Subscribes to Topic

↓

RViz Displays Marker

↓

Update Marker Properties (optional)

↓

Publish Updated Marker

↓

RViz Updates Display

The flow shows creating a marker, setting its properties, publishing it to a ROS topic, and RViz subscribing and displaying it. Updates follow the same publish-subscribe cycle.

Execution Sample

ROS

marker = Marker()
marker.header.frame_id = "map"
marker.type = Marker.SPHERE
marker.pose.position.x = 1.0
marker.scale.x = 0.2
publisher.publish(marker)

This code creates a sphere marker at position x=1.0 in the 'map' frame and publishes it for RViz to display.

Execution Table

Step	Action	Marker Property Set	Publish Status	RViz Display
1	Create Marker Object	None	Not Published	No Display
2	Set frame_id to 'map'	frame_id='map'	Not Published	No Display
3	Set type to SPHERE	type=SPHERE	Not Published	No Display
4	Set position x=1.0	pose.position.x=1.0	Not Published	No Display
5	Set scale x=0.2	scale.x=0.2	Not Published	No Display
6	Publish marker	All properties set	Published	Marker appears as sphere at x=1.0
7	Update position x=2.0	pose.position.x=2.0	Not Published	Marker at x=1.0
8	Publish updated marker	Updated position	Published	Marker moves to x=2.0
9	End	No changes	No further publish	Display stable at x=2.0

💡 Execution stops after publishing updated marker and RViz displays it; no further updates.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	Final
marker.header.frame_id	''	'map'	'map'	'map'	'map'	'map'	'map'	'map'	'map'
marker.type	None	None	SPHERE	SPHERE	SPHERE	SPHERE	SPHERE	SPHERE	SPHERE
marker.pose.position.x	0.0	0.0	0.0	1.0	1.0	1.0	2.0	2.0	2.0
marker.scale.x	0.0	0.0	0.0	0.0	0.2	0.2	0.2	0.2	0.2
published	False	False	False	False	False	True	False	True	True

Key Moments - 3 Insights

Why doesn't the marker appear in RViz before publishing?

Because RViz only shows markers that are published to the topic. Until step 6, the marker is created and properties set but not published, so RViz has no data to display.

What happens if you update marker properties but don't publish again?

RViz will continue showing the last published marker. Changes in properties are not visible until the updated marker is published again, as seen between steps 7 and 8.

Why do we need to set the frame_id property?

frame_id tells RViz which coordinate frame to use for the marker's position. Without it, RViz cannot place the marker correctly in the 3D space.

Visual Quiz - 3 Questions

Look at the execution table, what is the marker's position x after step 4?

A 0.0

B 1.0

C 2.0

D Not set yet

💡 Hint

Check the 'Marker Property Set' column at step 4 in the execution table.

At which step does RViz first display the marker?

A Step 6

B Step 5

C Step 7

D Step 8

💡 Hint

Look for the first 'Published' status and marker appearance in the 'RViz Display' column.

If you change the scale.x property after step 6 but do not publish again, what will RViz show?

A Marker with new scale

B No marker

C Marker with old scale

D Error in RViz

💡 Hint

Refer to the key moment about updating properties without publishing.

Concept Snapshot

Marker display in ROS:
- Create Marker object
- Set frame_id, type, pose, scale
- Publish marker to a topic
- RViz subscribes and displays marker
- Update properties and re-publish to change display
- frame_id is essential for correct placement

Full Transcript

This visual execution shows how to display a custom marker in ROS using the Marker message. First, a Marker object is created. Then, properties like frame_id, type (e.g., sphere), position, and scale are set. The marker is published to a ROS topic. RViz listens to this topic and displays the marker accordingly. If you update the marker's properties, you must publish again for RViz to update the display. The frame_id property is crucial because it tells RViz the coordinate frame to use for positioning the marker. Without publishing, changes are not visible in RViz. This step-by-step trace helps beginners understand the flow from code to visualization.

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