Why face analysis is a core CV application in Computer Vision - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to load an image using OpenCV for face analysis.

Computer Vision

import cv2
image = cv2.imread([1])

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

Aimage.jpg

B'face.jpg'

Ccv2.imread

Dface.jpg

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the filename

Passing a variable name instead of a string

The cv2.imread function requires the file path as a string, so the filename must be in quotes.

2fill in blank

medium

Complete the code to convert the image to grayscale, which is common in face analysis.

Computer Vision

gray_image = cv2.cvtColor(image, [1])

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Acv2.COLOR_BGR2RGB

Bcv2.COLOR_RGB2GRAY

Ccv2.COLOR_BGR2GRAY

Dcv2.COLOR_GRAY2BGR

Attempts:

3 left

💡 Hint

Common Mistakes

Using RGB instead of BGR

Using conversion codes for wrong color spaces

To convert a color image to grayscale, use cv2.COLOR_BGR2GRAY because OpenCV loads images in BGR format.

3fill in blank

hard

Fix the error in the code to detect faces using a Haar cascade classifier.

Computer Vision

face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
facedetected = face_cascade.[1](gray_image, scaleFactor=1.1, minNeighbors=5)

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AdetectMultiScale

BdetectFaces

CdetectFace

DdetectScale

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names

Typo in method name

The correct method to detect objects with Haar cascades in OpenCV is detectMultiScale.

4fill in blank

hard

Fill both blanks to draw rectangles around detected faces and display the image.

Computer Vision

for (x, y, w, h) in facedetected:
    cv2.rectangle(image, (x, y), ([1], [2]), (255, 0, 0), 2)
cv2.imshow('Faces', image)
cv2.waitKey(0)
cv2.destroyAllWindows()

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Ax + w

By + h

Cx - w

Dy - h

Attempts:

3 left

💡 Hint

Common Mistakes

Subtracting width or height instead of adding

Swapping x and y coordinates

The rectangle's bottom-right corner is at (x + w, y + h), which marks the face boundary.

5fill in blank

hard

Fill all three blanks to create a dictionary of face bounding boxes with keys as face IDs.

Computer Vision

faces_dict = {f'face_[1]': (x, y, [2], [3]) for [1], (x, y, w, h) in enumerate(facedetected)}

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

Ai

Bw

Ch

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names

Mixing up width and height

Use i from enumerate for keys, and w, h for width and height values.

Practice

1. Why is face analysis considered a core application in computer vision?

easy

A. Because it is used only for artistic photo filters

B. Because it helps computers recognize people and their emotions

C. Because it replaces all other computer vision tasks

D. Because it only detects objects like cars and trees

Solution

1. Step 1: Understand the purpose of face analysis

   Face analysis helps computers identify who is in an image and what emotions they show.

2. Step 2: Compare options to this purpose

   Only Because it helps computers recognize people and their emotions matches this purpose; others describe unrelated or incorrect uses.

3. Final Answer:

   Because it helps computers recognize people and their emotions -> Option B

4. Quick Check:

   Face analysis = Recognize people and emotions [OK]

Hint: Face analysis = recognizing faces and feelings [OK]

Common Mistakes:

• Thinking face analysis detects all objects
• Believing it replaces all CV tasks
• Assuming it is only for fun filters

2. Which of the following code snippets correctly detects a face using OpenCV in Python?

easy

A. face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')

B. face_cascade = cv2.load('face_detector.xml')

C. face_cascade = cv2.detectFace('haarcascade.xml')

D. face_cascade = cv2.faceDetector('default.xml')

Solution

1. Step 1: Recall correct OpenCV face detection syntax

   OpenCV uses cv2.CascadeClassifier with a valid XML file for face detection.

2. Step 2: Check each option's correctness

   Only face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml') uses the correct function and file name format; others use invalid methods.

3. Final Answer:

   face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml') -> Option A

4. Quick Check:

   Correct OpenCV face detection code = face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml') [OK]

Hint: OpenCV face detection uses CascadeClassifier with XML file [OK]

Common Mistakes:

• Using non-existent cv2 functions
• Wrong method names like detectFace
• Incorrect file names or extensions

3. What will the output be when running this Python code snippet for face detection?

import cv2
face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
img = cv2.imread('group_photo.jpg')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
faces = face_cascade.detectMultiScale(gray, 1.1, 4)
print(len(faces))

medium

A. An error because detectMultiScale is misspelled

B. The image itself printed as text

C. Always prints zero regardless of image

D. Number of faces detected in the image

Solution

1. Step 1: Understand detectMultiScale output

   detectMultiScale returns a list of rectangles for detected faces; len(faces) counts them.

2. Step 2: Analyze the print statement

   Printing len(faces) outputs the number of faces found in the image.

3. Final Answer:

   Number of faces detected in the image -> Option D

4. Quick Check:

   len(faces) = number of faces [OK]

Hint: detectMultiScale returns face count with len() [OK]

Common Mistakes:

• Confusing detectMultiScale spelling
• Expecting image data printed
• Assuming zero faces always

4. Identify the error in this face detection code snippet:

import cv2
face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
img = cv2.imread('photo.jpg')
gray = cv2.cvtColor(img, cv2.COLOR_RGB2GRAY)
faces = face_cascade.detectMultiScale(gray, 1.1, 4)
print(len(faces))

medium

A. Incorrect color conversion code; should use COLOR_BGR2GRAY

B. Missing import statement for numpy

C. detectMultiScale function is called with wrong parameters

D. The image file name is invalid

Solution

1. Step 1: Check color conversion code

   OpenCV reads images in BGR format, so conversion should be COLOR_BGR2GRAY, not COLOR_RGB2GRAY.

2. Step 2: Verify other parts

   Other parts like imports, function calls, and file name are correct or assumed valid.

3. Final Answer:

   Incorrect color conversion code; should use COLOR_BGR2GRAY -> Option A

4. Quick Check:

   Color conversion must match image format [OK]

Hint: Use COLOR_BGR2GRAY for OpenCV images [OK]

Common Mistakes:

• Using COLOR_RGB2GRAY instead of COLOR_BGR2GRAY
• Assuming missing numpy import causes error
• Thinking detectMultiScale parameters are wrong

5. You want to build a phone app that unlocks only when it recognizes your face. Which reason best explains why face analysis is essential for this app?

hard

A. It enables the app to detect all objects in the background

B. It helps the app to change the phone wallpaper automatically

C. It allows the app to detect and verify your unique facial features quickly

D. It makes the phone camera take selfies automatically

Solution

1. Step 1: Understand app requirement

   The app must recognize your face to unlock securely and quickly.

2. Step 2: Match face analysis role

   Face analysis identifies unique facial features to verify identity, enabling unlocking.

3. Final Answer:

   It allows the app to detect and verify your unique facial features quickly -> Option C

4. Quick Check:

   Face analysis = verify identity for unlocking [OK]

Hint: Face analysis verifies identity for secure unlocking [OK]

Common Mistakes:

• Confusing face analysis with background detection
• Thinking it changes wallpapers or takes selfies
• Ignoring the verification purpose