First image processing program in Computer Vision - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of the first image processing program?

To load an image, apply a simple operation like converting it to grayscale, and display the result.

beginner

Why do we convert a color image to grayscale in image processing?

Because grayscale images have only one channel, making processing simpler and faster while preserving important details.

beginner

Which Python library is commonly used for basic image processing tasks like reading and showing images?

OpenCV (cv2) is widely used for reading, processing, and displaying images easily.

beginner

What does the function cv2.imread() do?

It reads an image file from disk and loads it into memory as an array of pixel values.

beginner

How do you display an image window using OpenCV?

Use cv2.imshow() to open a window showing the image, and cv2.waitKey() to keep it open until a key is pressed.

What is the first step in a basic image processing program?

A Load the image from a file

B Convert the image to grayscale

C Display the image

D Save the image

The first step is to load the image so you can work with it.

Which OpenCV function converts a color image to grayscale?

A cv2.imwrite()

B cv2.imread()

C cv2.imshow()

D cv2.cvtColor()

cv2.cvtColor() changes the color space, such as from color to grayscale.

What does cv2.waitKey(0) do?

A Saves the image

B Waits indefinitely for a key press

C Closes all image windows

D Loads an image

cv2.waitKey(0) pauses the program until you press any key.

Why is grayscale image processing simpler than color image processing?

A Because grayscale has only one channel

B Because grayscale images are bigger

C Because color images have fewer pixels

D Because grayscale images are always black

Grayscale images have one channel, so fewer data to process.

Which library is commonly used for the first image processing program in Python?

A TensorFlow

B NumPy

C OpenCV

D Matplotlib

OpenCV is the go-to library for basic image processing tasks.

Describe the steps to write your first image processing program using OpenCV.

Think about loading, processing, showing, and closing the image.

Explain why converting an image to grayscale is useful in early image processing.

Consider how color channels affect complexity.

Practice

1. What does the OpenCV function imread do in an image processing program?

easy

A. It displays an image on the screen.

B. It reads an image file and loads it into the program.

C. It converts an image from color to grayscale.

D. It saves an image to a file.

Solution

1. Step 1: Understand the purpose of imread

   The function imread is used to load an image from a file into the program's memory.

2. Step 2: Differentiate from other functions

   Functions like imshow display images, and cvtColor changes image colors, so they do not read files.

3. Final Answer:

   It reads an image file and loads it into the program. -> Option B

4. Quick Check:

   imread = load image [OK]

Hint: imread always loads images from files [OK]

Common Mistakes:

• Confusing imread with imshow
• Thinking imread changes image colors
• Assuming imread saves images

2. Which of the following is the correct syntax to display an image stored in variable img using OpenCV?

easy

A. cv2.display(img)

B. cv2.showimage(img)

C. cv2.show('Window', img)

D. cv2.imshow('Window', img)

Solution

1. Step 1: Recall the OpenCV display function

   The correct function to show an image is cv2.imshow, which takes a window name and the image variable.

2. Step 2: Check the syntax of options

   Only cv2.imshow('Window', img) uses cv2.imshow with correct parameters: a string window name and the image.

3. Final Answer:

   cv2.imshow('Window', img) -> Option D

4. Quick Check:

   imshow = show image [OK]

Hint: imshow needs a window name and image [OK]

Common Mistakes:

• Using non-existent functions like display or showimage
• Forgetting the window name argument
• Swapping argument order

3. What will be the output of this code snippet?

import cv2
img = cv2.imread('photo.jpg')
print(img.shape)

medium

A. It prints the image pixel values.

B. It raises an error because shape is not valid.

C. It prints the dimensions of the image as (height, width, channels).

D. It prints the file size of 'photo.jpg'.

Solution

1. Step 1: Understand what img.shape returns

   In OpenCV, img.shape gives the dimensions of the image as a tuple: (height, width, number of color channels).

2. Step 2: Differentiate from other outputs

   It does not print pixel values or file size, and shape is a valid attribute for images loaded by imread.

3. Final Answer:

   It prints the dimensions of the image as (height, width, channels). -> Option C

4. Quick Check:

   img.shape = image size [OK]

Hint: shape shows image size and channels [OK]

Common Mistakes:

• Expecting pixel data instead of shape
• Thinking shape is a method, not attribute
• Confusing file size with image dimensions

4. Identify the error in this code snippet:

import cv2
img = cv2.imread('image.png')
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
cv2.imshow('Gray Image')
cv2.waitKey(0)
cv2.destroyAllWindows()

medium

A. Missing the image argument in cv2.imshow function.

B. cv2.cvtColor cannot convert color images.

C. cv2.waitKey requires an argument of 1, not 0.

D. cv2.destroyAllWindows should be called before imshow.

Solution

1. Step 1: Check the usage of cv2.imshow

   The function cv2.imshow requires two arguments: a window name and the image to display. Here, the image argument is missing.

2. Step 2: Verify other function calls

   cv2.cvtColor correctly converts color images, waitKey(0) waits indefinitely, and destroyAllWindows is correctly placed after showing images.

3. Final Answer:

   Missing the image argument in cv2.imshow function. -> Option A

4. Quick Check:

   imshow needs image argument [OK]

Hint: imshow always needs image to show [OK]

Common Mistakes:

• Forgetting the image argument in imshow
• Misunderstanding waitKey argument
• Calling destroyAllWindows too early

5. You want to write a program that reads an image, converts it to grayscale, and then saves the grayscale image. Which sequence of OpenCV functions is correct?

hard

A. cv2.imread() -> cv2.cvtColor() -> cv2.imwrite()

B. cv2.imshow() -> cv2.cvtColor() -> cv2.imwrite()

C. cv2.imread() -> cv2.imshow() -> cv2.cvtColor()

D. cv2.imwrite() -> cv2.imread() -> cv2.cvtColor()

Solution

1. Step 1: Understand the task steps

   The program must first read the image, then convert it to grayscale, and finally save the new image.

2. Step 2: Match functions to steps

   cv2.imread() reads the image, cv2.cvtColor() converts color spaces, and cv2.imwrite() saves the image to a file.

3. Final Answer:

   cv2.imread() -> cv2.cvtColor() -> cv2.imwrite() -> Option A

4. Quick Check:

   Read -> Convert -> Save = imread, cvtColor, imwrite [OK]

Hint: Read first, convert second, save last [OK]

Common Mistakes:

• Trying to save before reading
• Showing image before converting
• Mixing order of functions