Static transforms for fixed frames in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output behavior of a static_transform_publisher node?

In ROS, the static_transform_publisher node is used to publish a fixed transform between two frames. What does this node continuously publish once started?

A It publishes transforms only when the robot moves.

B It publishes dynamic transforms that change based on sensor input.

C It continuously publishes the same fixed transform at a regular rate.

D It publishes a single transform message once and then stops.

💡 Hint

Think about what 'static' means in this context and how ROS handles transforms.

The static_transform_publisher continuously broadcasts the same fixed transform at a regular rate so that other nodes can listen to it anytime.

📝 Syntax

intermediate

Identify the correct command syntax to publish a static transform between frames 'base_link' and 'camera_link'

Which of the following commands correctly publishes a static transform with translation (1, 2, 3) and rotation (0, 0, 0, 1) between 'base_link' and 'camera_link' using static_transform_publisher?

A ros2 run tf2_ros static_transform_publisher base_link camera_link 1 2 3 0 0 0 1

B ros2 run tf2_ros static_transform_publisher 1 2 3 base_link camera_link 0 0 0 1

C ros2 run tf2_ros static_transform_publisher 0 0 0 1 1 2 3 base_link camera_link

D ros2 run tf2_ros static_transform_publisher 1 2 3 0 0 0 1 base_link camera_link

💡 Hint

Remember the order: translation xyz, rotation xyzw, then parent and child frames.

The correct syntax is translation (x y z), rotation (x y z w), then parent frame, then child frame.

❓ state_output

advanced

What is the value of the transform between 'map' and 'odom' after running a static transform publisher with zero translation and rotation?

If you run static_transform_publisher 0 0 0 0 0 0 1 map odom, what will be the transform from 'map' to 'odom'?

A A transform with zero translation and identity rotation (no change).

B A transform with zero translation but a 180-degree rotation.

C No transform will be published because rotation is invalid.

D A transform with translation (0,0,1) and identity rotation.

💡 Hint

Quaternion (0,0,0,1) represents which rotation?

Quaternion (0,0,0,1) is the identity rotation, so the transform is zero translation and no rotation.

🔧 Debug

advanced

Why does this static_transform_publisher command fail to publish the transform?

Consider the command: ros2 run tf2_ros static_transform_publisher 1 2 3 0 0 0 base_link camera_link. Why does this command fail?

A Because the translation values must be integers, not floats.

B Because the rotation quaternion is incomplete; it requires 4 values but only 3 are given.

C Because the parent and child frames are reversed in order.

D Because static_transform_publisher requires a YAML file, not command line arguments.

💡 Hint

Check the number of rotation parameters provided.

The rotation quaternion must have 4 components (x, y, z, w). Missing one causes the command to fail.

🧠 Conceptual

expert

What is the main advantage of using static_transform_publisher over dynamic transform publishers in ROS?

Why would you choose to use static_transform_publisher for certain frame transforms instead of publishing transforms dynamically?

A Because static_transform_publisher uses less CPU by broadcasting fixed transforms at a low rate without recalculating.

B Because static_transform_publisher can publish transforms only when requested by other nodes.

C Because static_transform_publisher automatically updates transforms based on sensor data.

D Because static_transform_publisher supports dynamic reconfiguration of transforms during runtime.

💡 Hint

Think about fixed vs changing transforms and resource usage.

Static transforms do not change, so publishing them at a fixed low rate saves CPU compared to recalculating dynamic transforms.

Practice

1. What is the main purpose of using static_transform_publisher in ROS?

easy

A. To dynamically update the position of a robot part during movement

B. To define a fixed position and orientation between two frames that do not move relative to each other

C. To visualize sensor data in RViz

D. To launch multiple ROS nodes simultaneously

Solution

1. Step 1: Understand the role of static transforms

   Static transforms are used to represent fixed relationships between frames that do not change over time.

2. Step 2: Identify the function of static_transform_publisher

   This command publishes a fixed transform between two frames, meaning the position and orientation remain constant.

3. Final Answer:

   To define a fixed position and orientation between two frames that do not move relative to each other -> Option B

4. Quick Check:

   Static transform = fixed frame relation [OK]

Hint: Static transforms fix frame relations that never change [OK]

Common Mistakes:

• Confusing static with dynamic transforms
• Thinking it updates during robot movement
• Mixing it up with sensor data visualization

2. Which of the following is the correct syntax to publish a static transform from frame base_link to camera_link with translation (1, 0, 0) and no rotation using static_transform_publisher?

easy

A. static_transform_publisher base_link camera_link 1 0 0 0 0 0 1 100

B. static_transform_publisher 0 0 0 1 0 0 0 base_link camera_link 100

C. static_transform_publisher 1 0 0 0 0 0 1 base_link camera_link 100

D. static_transform_publisher 1 0 0 0 0 0 0 base_link camera_link 100

Solution

1. Step 1: Recall the static_transform_publisher argument order

   The syntax is: static_transform_publisher x y z qx qy qz qw frame_id child_frame_id period_in_ms.

2. Step 2: Match values to the syntax

   Translation is (1, 0, 0), rotation quaternion is (0, 0, 0, 1) for no rotation, frames are base_link and camera_link, and period is 100 ms.

3. Final Answer:

   static_transform_publisher 1 0 0 0 0 0 1 base_link camera_link 100 -> Option C

4. Quick Check:

   Correct argument order and values = static_transform_publisher 1 0 0 0 0 0 1 base_link camera_link 100 [OK]

Hint: Remember translation then quaternion then frames [OK]

Common Mistakes:

• Swapping frame order
• Incorrect quaternion values for no rotation
• Placing frames before numbers

3. Given the command:
static_transform_publisher 0 0 1 0 0 0 1 world map 50
What does this static transform represent?

medium

A. A translation of 1 meter along the Y-axis from world to map with 180 degrees rotation

B. A translation of 1 meter along the X-axis from map to world with no rotation

C. A rotation of 1 radian around the Z-axis between world and map

D. A translation of 1 meter along the Z-axis from world to map with no rotation

Solution

1. Step 1: Analyze translation and rotation values

   Translation is (0, 0, 1), meaning 1 meter along Z-axis. Quaternion (0, 0, 0, 1) means no rotation.

2. Step 2: Identify frame order

   The transform is from world frame to map frame, so map is positioned 1 meter above world.

3. Final Answer:

   A translation of 1 meter along the Z-axis from world to map with no rotation -> Option D

4. Quick Check:

   Translation Z=1, no rotation, world to map [OK]

Hint: Check translation vector and quaternion carefully [OK]

Common Mistakes:

• Mixing up frame order
• Misreading quaternion as rotation angle
• Confusing axis directions

4. You run the command:
static_transform_publisher 0 0 0 0 0 0 0 base_link camera_link 100
but no transform appears in tf. What is the likely problem?

medium

A. The quaternion rotation is invalid because the w component is zero

B. The translation values are all zero, so no transform is published

C. The frame names are reversed; camera_link should be first

D. The period 100 is too short to publish the transform

Solution

1. Step 1: Check quaternion validity

   A quaternion must be normalized; here (0,0,0,0) is invalid. The w component cannot be zero for a valid rotation.

2. Step 2: Understand effect on transform publishing

   An invalid quaternion causes the transform publisher to fail silently, so no transform appears in tf.

3. Final Answer:

   The quaternion rotation is invalid because the w component is zero -> Option A

4. Quick Check:

   Quaternion w=0 invalid = no transform [OK]

Hint: Quaternion w must not be zero for valid rotation [OK]

Common Mistakes:

• Assuming zero translation means no transform
• Swapping frame order without checking syntax
• Thinking publish period affects visibility immediately

5. You want to create a static transform tree where base_link is fixed to odom with translation (0, 0, 0) and no rotation, and camera_link is fixed to base_link with translation (0.5, 0, 1) and a 90-degree rotation around the Y-axis. Which two commands correctly publish these static transforms?

hard

A. static_transform_publisher 0 0 0 0 0 0 1 odom base_link 100
static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 base_link camera_link 100

B. static_transform_publisher 0 0 0 0 0 0 1 base_link odom 100
static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 camera_link base_link 100

C. static_transform_publisher 0 0 0 0 0 0 1 odom base_link 100
static_transform_publisher 0.5 0 1 0 1 0 0 base_link camera_link 100

D. static_transform_publisher 0 0 0 0 0 0 0 odom base_link 100
static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 base_link camera_link 100

Solution

1. Step 1: Verify first transform from odom to base_link

   Translation is zero, no rotation quaternion is (0,0,0,1), frames ordered correctly as parent then child.

2. Step 2: Verify second transform from base_link to camera_link

   Translation is (0.5,0,1). A 90-degree rotation around Y-axis quaternion is approximately (0, 0.7071, 0, 0.7071). Frames ordered correctly.

3. Final Answer:

   static_transform_publisher 0 0 0 0 0 0 1 odom base_link 100 static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 base_link camera_link 100 -> Option A

4. Quick Check:

   Correct quaternions and frame order = static_transform_publisher 0 0 0 0 0 0 1 odom base_link 100 static_transform_publisher 0.5 0 1 0 0.7071 0 0.7071 base_link camera_link 100 [OK]

Hint: Check quaternion for 90° Y rotation and frame order carefully [OK]

Common Mistakes:

• Swapping parent and child frames
• Using wrong quaternion for rotation
• Setting quaternion w to zero