Drawing on images (lines, rectangles, circles, text) in Computer Vision - ML Experiment: Train & Evaluate

Experiment - Drawing on images (lines, rectangles, circles, text)

Problem:You want to learn how to draw basic shapes and text on images using computer vision tools. Currently, you can load and display images but do not know how to add drawings like lines, rectangles, circles, or text on them.

Current Metrics:No drawings are present on images; only original images are shown.

Issue:Lack of knowledge on how to use drawing functions to annotate images for visualization or data augmentation.

Your Task

Learn to draw a blue line, a green rectangle, a red circle, and add white text on a sample image. The output image should clearly show all these drawings.

Use OpenCV (cv2) library in Python.

Drawings must be visible and correctly positioned.

Use simple, clear colors and thickness for shapes and text.

Hint 1

Hint 2

Hint 3

Hint 4

Hint 5

Solution

Computer Vision

import cv2
import numpy as np

# Create a blank black image
image = np.zeros((400, 600, 3), dtype=np.uint8)

# Draw a blue line from top-left to bottom-right
cv2.line(image, (50, 50), (550, 350), (255, 0, 0), thickness=3)

# Draw a green rectangle
cv2.rectangle(image, (100, 100), (300, 250), (0, 255, 0), thickness=4)

# Draw a red circle
cv2.circle(image, (400, 300), 50, (0, 0, 255), thickness=5)

# Add white text
cv2.putText(image, 'Hello OpenCV', (150, 380), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 255, 255), 2)

# Show the image
cv2.imshow('Drawing on Image', image)
cv2.waitKey(0)
cv2.destroyAllWindows()

Created a blank image to draw on.

Added a blue line using cv2.line().

Added a green rectangle using cv2.rectangle().

Added a red circle using cv2.circle().

Added white text using cv2.putText().

Displayed the final image with all drawings.

Results Interpretation

Before: Only a blank black image was shown with no drawings.

After: The image now contains a blue line, green rectangle, red circle, and white text, demonstrating how to annotate images.

Drawing shapes and text on images is essential for visualization and annotation in computer vision. OpenCV provides simple functions to add these elements clearly and efficiently.

Bonus Experiment

Try drawing multiple shapes with different colors and thicknesses on a real photo loaded from disk.

💡 Hint

Load an image using cv2.imread(), then apply drawing functions at different positions. Experiment with font styles and sizes for text.

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