ArUco marker landing in Drone Programming - Mini Project: Build & Apply

ArUco Marker Landing

📖 Scenario: You are programming a drone to land precisely on a special ArUco marker placed on the ground. The drone's camera detects markers and provides their IDs and positions. Your task is to write code that helps the drone identify the correct marker and calculate the landing position.

🎯 Goal: Build a program that stores detected ArUco markers with their positions, selects the landing marker by its ID, and outputs the exact coordinates where the drone should land.

📋 What You'll Learn

Create a dictionary with ArUco marker IDs as keys and their (x, y) positions as values.

Add a variable to store the target landing marker ID.

Use a dictionary comprehension to find the position of the target marker.

Print the landing coordinates for the drone.

💡 Why This Matters

🌍 Real World

Drones use ArUco markers to find exact landing spots in delivery, inspection, and rescue missions.

💼 Career

Understanding how to process marker data and control drone landing is important for robotics and drone programming jobs.

1

Create the ArUco markers dictionary

Create a dictionary called markers with these exact entries: 101: (5.0, 3.2), 102: (2.5, 7.8), 103: (9.1, 1.4), 104: (4.0, 4.0)

Drone Programming

# Create the markers dictionary with IDs and positions
# Your code here

Hint

Use curly braces {} to create a dictionary with keys as marker IDs and values as tuples for positions.

2

Set the target landing marker ID

Create a variable called target_id and set it to 103 to represent the marker where the drone should land.

Drone Programming

markers = {101: (5.0, 3.2), 102: (2.5, 7.8), 103: (9.1, 1.4), 104: (4.0, 4.0)}
# Set the target marker ID for landing
# Your code here

Hint

Just assign the number 103 to the variable target_id.

3

Find the landing position using dictionary comprehension

Use a dictionary comprehension to create a dictionary called landing_position that contains only the entry from markers where the key equals target_id.

Drone Programming

markers = {101: (5.0, 3.2), 102: (2.5, 7.8), 103: (9.1, 1.4), 104: (4.0, 4.0)}
target_id = 103
# Use dictionary comprehension to get landing_position
# Your code here

Hint

Use {k: v for k, v in markers.items() if k == target_id} to filter the dictionary.

4

Print the landing coordinates

Print the landing coordinates by accessing the position tuple from landing_position using target_id. Use print(f"Landing coordinates: {landing_position[target_id]}").

Drone Programming

markers = {101: (5.0, 3.2), 102: (2.5, 7.8), 103: (9.1, 1.4), 104: (4.0, 4.0)}
target_id = 103
landing_position = {k: v for k, v in markers.items() if k == target_id}
# Print the landing coordinates
# Your code here

Hint

Use an f-string to print the coordinates in the format shown.

Practice

1. What is the main purpose of using an ArUco marker in drone landing?

easy

A. To help the drone locate a precise landing spot using camera vision

B. To increase the drone's flying speed

C. To improve the drone's battery life

D. To enable the drone to fly higher

Solution

1. Step 1: Understand ArUco marker function

   ArUco markers are visual tags that drones detect with cameras to find exact positions.

2. Step 2: Connect marker use to landing

   The drone uses the marker to center itself and land precisely where the marker is placed.

3. Final Answer:

   To help the drone locate a precise landing spot using camera vision -> Option A

4. Quick Check:

   ArUco marker = precise landing spot [OK]

Hint: ArUco markers guide landing, not speed or battery [OK]

Common Mistakes:

• Thinking ArUco markers affect speed
• Confusing marker use with battery improvement
• Assuming marker helps flying altitude

2. Which of the following is the correct Python syntax to detect an ArUco marker using OpenCV?

easy

A. corners, ids, rejected = cv2.aruco.detectMarkers(image, dictionary)

B. corners, ids = cv2.aruco.detectMarkers(image dictionary)

C. corners, ids, rejected = cv2.detectMarkers(image, dictionary)

D. corners, ids, rejected = cv2.aruco.detectMarker(image, dictionary)

Solution

1. Step 1: Recall OpenCV ArUco detection syntax

   The correct function is cv2.aruco.detectMarkers with parameters (image, dictionary).

2. Step 2: Check returned values

   It returns corners, ids, and rejected candidates, so three outputs are expected.

3. Final Answer:

   corners, ids, rejected = cv2.aruco.detectMarkers(image, dictionary) -> Option A

4. Quick Check:

   Correct function and outputs = corners, ids, rejected = cv2.aruco.detectMarkers(image, dictionary) [OK]

Hint: Remember detectMarkers returns three values with cv2.aruco [OK]

Common Mistakes:

• Missing comma between arguments
• Using wrong function name detectMarker
• Not unpacking all three return values

3. Given this code snippet for ArUco marker detection, what will be printed if no markers are found?

corners, ids, rejected = cv2.aruco.detectMarkers(frame, dictionary)
if ids is None:
    print("No markers detected")
else:
    print(f"Detected {len(ids)} markers")

medium

A. Detected 0 markers

B. No markers detected

C. Error: ids is undefined

D. Detected None markers

Solution

1. Step 1: Understand detectMarkers output when no markers found

   When no markers are detected, ids is None, not an empty list.

2. Step 2: Follow the if condition

   The code checks if ids is None, then prints "No markers detected".

3. Final Answer:

   No markers detected -> Option B

4. Quick Check:

   ids is None means no markers [OK]

Hint: ids is None means no markers, not empty list [OK]

Common Mistakes:

• Assuming ids is empty list when no markers
• Expecting length of None causes error
• Printing zero markers instead of message

4. This code snippet is intended to center the drone over the detected ArUco marker. What is the bug?

corners, ids, _ = cv2.aruco.detectMarkers(frame, dictionary)
if ids is not None:
    cX = int(corners[0][0][0][0])
    cY = int(corners[0][0][0][1])
    move_drone_to(cX, cY)
else:
    print("Marker not found")

medium

A. Using move_drone_to with wrong parameters

B. Missing check if corners is empty

C. Incorrect indexing of corners to get center point

D. No bug, code works correctly

Solution

1. Step 1: Analyze corners indexing

   corners[0][0] is a list of marker corner points; [0][0] accesses first corner point, not center.

2. Step 2: Identify correct center calculation

   Center should be average of all four corners, not just first point.

3. Final Answer:

   Incorrect indexing of corners to get center point -> Option C

4. Quick Check:

   Center = average corners, not first corner [OK]

Hint: Center is average of corners, not first corner point [OK]

Common Mistakes:

• Using first corner as center
• Not averaging corner points
• Ignoring possibility of empty corners

5. You want the drone to land on an ArUco marker by centering it and then descending. Which sequence of steps in code best achieves this?

hard

A. Detect marker -> Descend quickly -> Calculate center -> Move drone to center

B. Descend slowly -> Detect marker -> Move drone randomly -> Confirm landing

C. Move drone randomly -> Detect marker -> Descend quickly -> Confirm landing

D. Detect marker -> Calculate center -> Move drone to center -> Descend slowly -> Confirm landing

Solution

1. Step 1: Understand safe landing sequence

   First detect the marker and find its center to position the drone accurately.

2. Step 2: Move drone to center and descend carefully

   After centering, descend slowly to avoid missing the marker or crashing.

3. Final Answer:

   Detect marker -> Calculate center -> Move drone to center -> Descend slowly -> Confirm landing -> Option D

4. Quick Check:

   Correct landing sequence = Detect marker -> Calculate center -> Move drone to center -> Descend slowly -> Confirm landing [OK]

Hint: Center first, then descend slowly for safe landing [OK]

Common Mistakes:

• Descending before centering marker
• Moving drone randomly without marker detection
• Descending too fast risking crash