Drawing with OpenCV Exercise

What you'll learn

• How to draw basic shapes (rectangles, circles, lines) using OpenCV
• How to add text to images with different fonts and styles
• How to control colors and thickness of drawings
• How to combine multiple drawing operations to create complex graphics

Introduction

This exercise will guide you through various drawing techniques using OpenCV in Python. You'll practice drawing shapes and text on images to create visual outputs.

Access the exercise materials here: Drive Link

Preparing Folders and Files :hammer_and_wrench:

Time needed: 5 minutes

Instructions: Follow these steps

1. Download the C15-DrawWitOpenCV-Template folder from Google Drive
2. Save it into your Program G folder
3. Extract the folder contents
4. Open the extracted folder in VSCode

:bulb: For students without VSCode: You can use any Python IDE or online Python editor. Just make sure you have OpenCV installed!

Read, Think, Code :computer:

Time needed: 20-25 minutes

Working in draw.py file:

Question One: Draw Rectangle

1. Locate the Question 1 section in your code
2. Use cv2.rectangle() function to draw a rectangle with these specifications:
   • Top left corner: (0, 0)
   • Bottom right corner: (250, 250)
   • Color: Blue (255, 0, 0) in BGR format
   • Thickness: 2 pixels

Question 2: Draw Circle

1. Find the Question 2 section
2. Use cv2.circle() function to draw a circle with:
   • Center point: Calculate the middle of the image using (width//2, height//2)
   • Radius: 100 pixels
   • Color: Green (0, 255, 0) in BGR format
   • Thickness: 4 pixels tip Hint: To find the middle of the image, use img.shape to get the dimensions, then divide by 2!

Question 3: Draw Line

1. Navigate to Question 3 section
2. Use cv2.line() function to draw a line with:
   • Starting point: (100, 0)
   • Ending point: Middle of the image (same calculation as Question 2)
   • Color: Red (0, 0, 255) in BGR format
   • Thickness: 4 pixels

Question 4: Write Text

1. Find Question 4 section
2. Use cv2.putText() function to add text with:
   • Text content: "Hello"
   • Starting position: (200, 50)
   • Font type: cv2.FONT_HERSHEY_COMPLEX
   • Font scale: 1.0
   • Color: Cyan (255, 255, 0) in BGR format
   • Thickness: 2 pixels

:bulb: Remember: OpenCV uses BGR color format, not RGB! So (255, 255, 0) gives you cyan, not yellow.

Expected Output

Your final image should look like this:

Try it Yourself :art:

Time needed: 15-20 minutes

Advanced Challenge

1. Create a new file called advanced.py
2. Copy the basic structure from draw.py
3. Create the following output by combining multiple shapes:

tip Challenge Hints:

• Look carefully at the image - what shapes do you see?
• Think about the order of drawing (what should be drawn first?)
• Experiment with different colors and thickness values
• You might need to draw multiple rectangles, circles, and lines!

Troubleshooting :wrench:

Common Issues and Solutions:

1. "cv2 module not found" error

   • Solution: Install OpenCV using pip install opencv-python

2. Image not displaying

   • Make sure you have cv2.imshow() and cv2.waitKey(0) at the end
   • Check that your image path is correct

3. Wrong colors appearing

   • Remember: OpenCV uses BGR format, not RGB
   • Blue = (255, 0, 0), Green = (0, 255, 0), Red = (0, 0, 255)

4. Shapes not appearing where expected

   • Double-check your coordinates
   • Remember: (0, 0) is the top-left corner
   • X increases going right, Y increases going down

Exercise Submission :emoji:

:warning: Warning Important: Submit your work!

Once you've completed both draw.py and advanced.py:

1. Take screenshots of both output images
2. Save your Python files
3. Submit everything through the form below

Submission Deadline: Check with your instructor

Submit your work here:

Click to Submit Exercise