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.

Practice

1. Which OpenCV function is used to draw a rectangle on an image?

easy

A. cv2.line

B. cv2.rectangle

C. cv2.circle

D. cv2.putText

Solution

1. Step 1: Understand drawing functions in OpenCV

   OpenCV provides specific functions for different shapes: cv2.line for lines, cv2.circle for circles, cv2.rectangle for rectangles, and cv2.putText for text.

2. Step 2: Identify the function for rectangles

   The function named cv2.rectangle is designed to draw rectangles on images.

3. Final Answer:

   cv2.rectangle -> Option B

4. Quick Check:

   Rectangle drawing = cv2.rectangle [OK]

Hint: Rectangle drawing uses cv2.rectangle function [OK]

Common Mistakes:

• Confusing cv2.line with rectangle drawing
• Using cv2.circle for rectangles
• Trying to draw text with cv2.rectangle

2. Which parameter in cv2.putText controls the thickness of the text?

easy

A. thickness

B. fontScale

C. fontFace

D. color

Solution

1. Step 1: Review cv2.putText parameters

   The function cv2.putText has parameters: fontFace (font style), fontScale (size), color (text color), and thickness (line thickness of text).

2. Step 2: Identify thickness parameter

   The thickness parameter controls how bold or thick the text lines appear.

3. Final Answer:

   thickness -> Option A

4. Quick Check:

   Text thickness = thickness parameter [OK]

Hint: Thickness of text is set by 'thickness' parameter [OK]

Common Mistakes:

• Confusing fontScale with thickness
• Using color to control thickness
• Mistaking fontFace for thickness

3. What will be the color of the line drawn by this code snippet?

cv2.line(img, (10, 10), (100, 10), (0, 0, 255), 2)

medium

A. Red

B. Green

C. Blue

D. Black

Solution

1. Step 1: Understand BGR color format in OpenCV

   OpenCV uses BGR order for colors, so (0, 0, 255) means Blue=0, Green=0, Red=255.

2. Step 2: Identify the color from the tuple

   Since only the last value (Red) is 255, the line color will be bright red.

3. Final Answer:

   Red -> Option A

4. Quick Check:

   BGR (0,0,255) = Red [OK]

Hint: Remember OpenCV colors are BGR, last value 255 means Red [OK]

Common Mistakes:

• Assuming (0,0,255) is blue (RGB confusion)
• Mixing up color order
• Ignoring OpenCV's BGR format

4. Identify the error in this code that tries to draw a circle:

cv2.circle(img, (50, 50), -10, (255, 0, 0), 3)

medium

A. Center coordinates must be floats

B. Color tuple is wrong format

C. Thickness cannot be 3

D. Negative radius is invalid

Solution

1. Step 1: Check circle parameters

   The radius parameter must be a positive integer representing the circle size.

2. Step 2: Identify invalid radius

   The radius given is -10, which is invalid and will cause an error.

3. Final Answer:

   Negative radius is invalid -> Option D

4. Quick Check:

   Radius must be positive integer [OK]

Hint: Radius must be positive; negative values cause errors [OK]

Common Mistakes:

• Using negative radius values
• Thinking thickness 3 is invalid
• Assuming center coordinates must be floats

5. You want to draw a blue rectangle with thickness 4 and label it "Object" in white text above it. Which code snippet correctly does this?

img = cv2.imread('image.jpg')
start = (30, 30)
end = (150, 150)
# Options below

hard

A. cv2.rectangle(img, start, end, (255, 0, 0), 2)
cv2.putText(img, 'Object', (30, 20), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 255, 255), 4)

B. cv2.rectangle(img, start, end, (0, 0, 255), 4)
cv2.putText(img, 'Object', (30, 20), cv2.FONT_HERSHEY_SIMPLEX, 0.7, (0, 0, 0), 2)

C. cv2.rectangle(img, start, end, (255, 0, 0), 4)
cv2.putText(img, 'Object', (30, 20), cv2.FONT_HERSHEY_SIMPLEX, 0.7, (255, 255, 255), 2)

D. cv2.rectangle(img, start, end, (0, 255, 0), 4)
cv2.putText(img, 'Object', (30, 20), cv2.FONT_HERSHEY_SIMPLEX, 0.7, (255, 255, 255), 2)

Solution

1. Step 1: Identify blue color in BGR

   Blue in BGR is (255, 0, 0), so rectangle color must be (255, 0, 0) with thickness 4.

2. Step 2: Check text color and position

   Text "Object" should be white (255, 255, 255) and placed above rectangle at (30, 20) with reasonable font scale and thickness.

3. Final Answer:

   Blue rectangle with thickness 4 and white "Object" text above -> Option C

4. Quick Check:

   Blue = (255,0,0), white text, thickness 4 [OK]

Hint: Blue is (255,0,0); white text is (255,255,255) [OK]

Common Mistakes:

• Mixing up BGR color order
• Using wrong thickness values
• Placing text inside rectangle instead of above