ArUco marker landing in Drone Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the ArUco marker dictionary.

Drone Programming

import cv2
aruco_dict = cv2.aruco.Dictionary_get(cv2.aruco.[1])

Drag options to blanks, or click blank then click option'

ADICT_7X7_1000

BDICT_5X5_100

CDICT_6X6_250

DDICT_4X4_50

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a dictionary size that does not match the marker size.

The DICT_4X4_50 dictionary is commonly used for ArUco markers with 4x4 bits and 50 unique markers.

2fill in blank

medium

Complete the code to detect ArUco markers in the image.

Drone Programming

corners, ids, rejected = cv2.aruco.detectMarkers(image, [1])

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Aparameters

Bcv2.aruco.DetectorParameters_create()

Caruco_dict

Dcv2.aruco.Dictionary_get()

Attempts:

3 left

💡 Hint

Common Mistakes

Passing detector parameters instead of the dictionary.

The detectMarkers function requires the dictionary of markers to detect, which is aruco_dict.

3fill in blank

hard

Fix the error in the code to estimate the pose of the detected marker.

Drone Programming

rvec, tvec, _ = cv2.aruco.estimatePoseSingleMarkers(corners, marker_length, [1], camera_matrix, dist_coeffs)

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Adist_coeffs

Bcamera_matrix

Cparameters

Daruco_dict

Attempts:

3 left

💡 Hint

Common Mistakes

Passing distortion coefficients instead of camera matrix.

The third argument must be the camera matrix, which contains the camera's intrinsic parameters.

4fill in blank

hard

Fill both blanks to draw the detected marker's axis on the image.

Drone Programming

cv2.aruco.drawAxis(image, [1], [2], rvec[0], tvec[0], axis_length)

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Acamera_matrix

Bdist_coeffs

Caruco_dict

Dparameters

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of camera matrix and distortion coefficients.

The drawAxis function requires the camera matrix and distortion coefficients to correctly draw the axis.

5fill in blank

hard

Fill all three blanks to command the drone to land on the detected ArUco marker.

Drone Programming

if ids is not None:
    x, y, z = tvec[0][0]
    drone.move_to(x[1]0.1, y[2]0.1, z[3]0.1)
    if abs(x) < 0.05 and abs(y) < 0.05 and abs(z) < 0.1:
        drone.land()

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A-

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or multiplication which moves the drone away from the marker.

The drone should adjust its position by subtracting 0.1 to move closer to the marker coordinates.

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