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.