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Computer Visionml~12 mins

Drawing on images (lines, rectangles, circles, text) in Computer Vision - Model Pipeline Trace

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Model Pipeline - Drawing on images (lines, rectangles, circles, text)

This pipeline shows how an image is taken and simple shapes and text are drawn on it. This helps in highlighting parts of the image or adding information visually.

Data Flow - 5 Stages
1Input Image
1 image x 400 pixels height x 600 pixels width x 3 color channels→Load a color image from disk→1 image x 400 x 600 x 3
A photo of a park with trees and sky
↓
2Draw Line
1 image x 400 x 600 x 3→Draw a blue line from point (50,50) to (550,50)→1 image x 400 x 600 x 3
Image now has a blue horizontal line near the top
↓
3Draw Rectangle
1 image x 400 x 600 x 3→Draw a green rectangle from (100,100) to (300,200)→1 image x 400 x 600 x 3
Image now has a green rectangle around a tree
↓
4Draw Circle
1 image x 400 x 600 x 3→Draw a red circle centered at (400,300) with radius 50→1 image x 400 x 600 x 3
Image now has a red circle highlighting a flower
↓
5Draw Text
1 image x 400 x 600 x 3→Write white text 'Park View' at position (200,350)→1 image x 400 x 600 x 3
Image now shows the label 'Park View' near the bottom
Training Trace - Epoch by Epoch
N/A
EpochLoss ↓Accuracy ↑Observation
1N/AN/ANo training involved; this is an image drawing pipeline
Prediction Trace - 5 Layers
Layer 1: Load Image
Layer 2: Draw Line
Layer 3: Draw Rectangle
Layer 4: Draw Circle
Layer 5: Draw Text
Model Quiz - 3 Questions
Test your understanding
What happens to the image size after drawing shapes?
AThe image size halves
BThe image size stays the same
CThe image size doubles
DThe image size becomes zero
Key Insight
Drawing on images changes pixel colors but does not alter image size or require training. It is a direct image editing step useful for highlighting or labeling parts of images.

Practice

(1/5)
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