Drawing on images (lines, rectangles, circles, text) in Computer Vision - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to draw a blue line on the image.

Computer Vision

import cv2
import numpy as np

img = np.zeros((200, 200, 3), dtype=np.uint8)
cv2.line(img, (10, 10), (190, 10), [1], 3)
cv2.imwrite('line.png', img)

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

A(0, 0, 255)

B(255, 0, 0)

C(255, 255, 0)

D(0, 255, 0)

Attempts:

3 left

💡 Hint

Common Mistakes

Using RGB color instead of BGR.

Confusing blue with red color values.

OpenCV uses BGR color format. Blue is (255, 0, 0), which is option B. Option B (0, 0, 255) is red.

2fill in blank

medium

Complete the code to draw a filled green rectangle on the image.

Computer Vision

import cv2
import numpy as np

img = np.zeros((200, 200, 3), dtype=np.uint8)
cv2.rectangle(img, (50, 50), (150, 150), [1], -1)
cv2.imwrite('rectangle.png', img)

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

A(0, 255, 255)

B(0, 0, 255)

C(255, 0, 0)

D(0, 255, 0)

Attempts:

3 left

💡 Hint

Common Mistakes

Using RGB color instead of BGR.

Using positive thickness instead of -1 for filled shapes.

The color green in OpenCV BGR format is (0, 255, 0). The thickness -1 fills the rectangle.

3fill in blank

hard

Fix the error in the code to draw a red circle on the image.

Computer Vision

import cv2
import numpy as np

img = np.zeros((200, 200, 3), dtype=np.uint8)
cv2.circle(img, (100, 100), 50, [1], 5)
cv2.imwrite('circle.png', img)

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

A(0, 0, 255)

B(255, 0, 0)

C(0, 255, 0)

D(255, 255, 255)

Attempts:

3 left

💡 Hint

Common Mistakes

Using RGB color instead of BGR.

Using wrong color tuple causing unexpected colors.

The color red in OpenCV BGR format is (0, 0, 255). The code uses this color to draw the circle.

4fill in blank

hard

Fill both blanks to draw white text on the image at position (50, 180).

Computer Vision

import cv2
import numpy as np

img = np.zeros((200, 200, 3), dtype=np.uint8)
cv2.putText(img, 'Hello', (50, 180), [1], 1, [2], 2)
cv2.imwrite('text.png', img)

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

Acv2.FONT_HERSHEY_SIMPLEX

B(255, 255, 255)

C(0, 0, 0)

Dcv2.LINE_AA

Attempts:

3 left

💡 Hint

Common Mistakes

Using black color instead of white for text.

Using invalid font constants.

The font cv2.FONT_HERSHEY_SIMPLEX is a common readable font. The color white in BGR is (255, 255, 255).

5fill in blank

hard

Fill all three blanks to draw a green circle with thickness 3 and anti-aliased line.

Computer Vision

import cv2
import numpy as np

img = np.zeros((300, 300, 3), dtype=np.uint8)
cv2.circle(img, (150, 150), 75, [1], [2], [3])
cv2.imwrite('circle_aa.png', img)

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

A(0, 255, 0)

B3

Ccv2.LINE_AA

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong color format.

Using thickness 5 instead of 3.

Omitting anti-aliasing flag.

The green color is (0, 255, 0). Thickness is 3 pixels. cv2.LINE_AA enables smooth anti-aliased edges.

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