Drawing on images (lines, rectangles, circles, text) in Computer Vision - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output image after drawing a red line?

Given the following code that draws a red line on a blank image, what will be the color of the pixel at position (50, 50)?

Computer Vision

import numpy as np
import cv2

img = np.zeros((100, 100, 3), dtype=np.uint8)
cv2.line(img, (0, 0), (99, 99), (0, 0, 255), 2)
pixel_color = img[50, 50].tolist()
print(pixel_color)

A [0, 0, 0]

B [255, 0, 0]

C [0, 255, 0]

D [0, 0, 255]

💡 Hint

Remember OpenCV uses BGR color format, not RGB.

The line is drawn from top-left to bottom-right with color (0,0,255) which is red in BGR. The pixel at (50,50) lies on this line, so its color is red.

❓ Model Choice

intermediate

Which OpenCV function draws a filled rectangle?

You want to draw a filled blue rectangle on an image. Which function and parameter combination is correct?

A cv2.rectangle(img, (10, 10), (50, 50), (255, 0, 0), thickness=-1)

B cv2.line(img, (10, 10), (50, 50), (255, 0, 0), thickness=-1)

C cv2.rectangle(img, (10, 10), (50, 50), (255, 0, 0), thickness=0)

D cv2.circle(img, (30, 30), 20, (255, 0, 0), thickness=0)

💡 Hint

In OpenCV, thickness=-1 fills the shape.

The cv2.rectangle function draws rectangles. Setting thickness=-1 fills the rectangle with the given color.

❓ Hyperparameter

advanced

Choosing thickness for drawing a circle

You want to draw a green circle with a radius of 30 pixels on an image. Which thickness value will draw only the circle's outline with a thickness of 5 pixels?

A thickness=-1

B thickness=5

C thickness=0

D thickness=None

💡 Hint

Thickness=-1 fills the circle, positive thickness draws outline.

A positive thickness value draws the outline with that thickness. thickness=5 draws a 5-pixel thick outline.

🔧 Debug

advanced

Why does this text not appear on the image?

Consider this code snippet: import numpy as np import cv2 img = np.zeros((100, 200, 3), dtype=np.uint8) cv2.putText(img, 'Hi', (50, 50), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 255, 255), 2) Why might the text not be visible when displaying the image?

Computer Vision

import numpy as np
import cv2

img = np.zeros((100, 200, 3), dtype=np.uint8)
cv2.putText(img, 'Hi', (50, 50), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 255, 255), 2)
# Display code omitted

A The image is not displayed or saved, so text is not visible.

B The text color is white but the image is black, so text is visible.

C The font scale is too small to see the text.

D The coordinates (50, 50) place the text outside the image boundaries.

💡 Hint

Check if the image is actually shown or saved after drawing.

The code draws text on the image correctly, but if the image is not displayed or saved, you won't see the text.

🧠 Conceptual

expert

Why use BGR color format in OpenCV instead of RGB?

OpenCV uses BGR color format by default instead of the more common RGB. What is the main reason for this choice?

A BGR uses less memory than RGB.

B BGR is easier to convert to grayscale than RGB.

C BGR matches the order used by most cameras and image file formats internally.

D BGR is the standard color format in all image processing libraries.

💡 Hint

Think about how images are stored and captured by hardware.

Many cameras and image formats store colors in BGR order, so OpenCV adopted this for efficiency and compatibility.