ArUco marker landing in Drone Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Detecting ArUco Marker IDs

What will be the output of this code snippet that detects ArUco markers and prints their IDs?

Drone Programming

import cv2
import cv2.aruco as aruco

# Load a sample image with ArUco markers
image = cv2.imread('markers.jpg')

# Convert to grayscale
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)

# Define dictionary and parameters
aruco_dict = aruco.Dictionary_get(aruco.DICT_4X4_50)
parameters = aruco.DetectorParameters_create()

# Detect markers
corners, ids, rejectedImgPoints = aruco.detectMarkers(gray, aruco_dict, parameters=parameters)

print(ids)

A [[23], [7], [15]]

B [23, 7, 15]

C SyntaxError

D None

💡 Hint

Remember that the ids returned by detectMarkers is a numpy array of shape (N,1).

The detectMarkers function returns ids as a numpy array with shape (N,1), so printing ids shows a list of lists like [[23], [7], [15]].

🧠 Conceptual

intermediate

Understanding Pose Estimation Output

After detecting an ArUco marker, the pose estimation function returns rotation and translation vectors. What do these vectors represent?

A Both vectors show the marker's position in GPS coordinates

B Rotation vector shows camera's orientation; translation vector shows camera's position relative to the marker

C They represent the marker's color and size

D Rotation vector shows marker's orientation; translation vector shows marker's position relative to the camera

💡 Hint

Think about how the camera sees the marker in 3D space.

The rotation vector describes how the marker is rotated relative to the camera, and the translation vector describes where the marker is located relative to the camera.

🔧 Debug

advanced

Fixing Marker Detection Failure

This code fails to detect any ArUco markers in a live video feed. What is the most likely cause?

Drone Programming

import cv2
import cv2.aruco as aruco

cap = cv2.VideoCapture(0)

while True:
    ret, frame = cap.read()
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    aruco_dict = aruco.Dictionary_get(aruco.DICT_6X6_250)
    parameters = aruco.DetectorParameters_create()
    corners, ids, _ = aruco.detectMarkers(gray, aruco_dict, parameters=parameters)
    if ids is not None:
        print('Markers detected:', ids)
    else:
        print('No markers detected')
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

A The dictionary used (DICT_6X6_250) does not match the markers in the scene

B The video capture device is not opened properly

C The grayscale conversion is incorrect

D The waitKey function is missing

💡 Hint

Check if the marker dictionary matches the actual markers you want to detect.

If the dictionary used in detectMarkers does not match the actual marker type in the scene, no markers will be detected.

📝 Syntax

advanced

Correcting Pose Estimation Syntax

Which option correctly calls the pose estimation function for an ArUco marker?

Drone Programming

rvec, tvec, _ = aruco.estimatePoseSingleMarkers(corners, markerLength, cameraMatrix, distCoeffs)

A rvec, tvec = aruco.estimatePoseSingleMarkers(corners, markerLength, cameraMatrix, distCoeffs)

B rvec, tvec, _ = aruco.estimatePoseSingleMarkers(corners, markerLength, cameraMatrix, distCoeffs)

C rvec, tvec, _ = aruco.estimatePoseSingleMarkers(corners, markerLength, distCoeffs, cameraMatrix)

D rvec, tvec, _ = aruco.estimatePoseSingleMarkers(markerLength, corners, cameraMatrix, distCoeffs)

💡 Hint

Check the order and number of returned values from the function.

The function returns three values: rotation vectors, translation vectors, and object points. The order of parameters must be corners, markerLength, cameraMatrix, distCoeffs.

🚀 Application

expert

Calculating Drone Landing Coordinates

Given the translation vector tvec = [[0.5, -0.2, 1.5]] meters from the camera to the ArUco marker, and the drone's current GPS coordinates (lat: 40.0, lon: -74.0), which option best describes how to calculate the drone's GPS coordinates to land on the marker?

A Add tvec values directly to latitude and longitude values

B Use only the z component of tvec to adjust altitude, ignore x and y

C Convert tvec from camera frame to world frame, then convert meters offset to GPS offset and add to current GPS coordinates

D Ignore tvec and land at current GPS coordinates

💡 Hint

Think about coordinate frames and unit conversions between meters and GPS degrees.

The translation vector is relative to the camera frame; it must be transformed to the world frame and converted from meters to GPS degrees before adjusting the drone's GPS position.

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