Drawing on images (lines, rectangles, circles, text) in Computer Vision - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of drawing shapes like lines, rectangles, and circles on images in computer vision?

Drawing shapes helps highlight or mark important parts of an image, like objects or areas of interest, making it easier to understand or analyze the image.

beginner

Which function is commonly used to draw a line on an image in OpenCV?

The function cv2.line() is used to draw a line by specifying the start and end points, color, and thickness.

beginner

How do you specify the position and size of a rectangle when drawing it on an image?

You provide the top-left corner and bottom-right corner coordinates of the rectangle to define its position and size.

beginner

What parameters are needed to draw text on an image?

You need the text string, position coordinates, font type, font scale (size), color, and thickness to draw text on an image.

beginner

Why is it important to choose contrasting colors when drawing on images?

Contrasting colors make the drawn shapes or text stand out clearly against the image background, improving visibility and understanding.

Which OpenCV function draws a circle on an image?

A cv2.circle()

B cv2.rectangle()

C cv2.line()

D cv2.putText()

cv2.circle() is used to draw circles by specifying the center, radius, color, and thickness.

What does the thickness parameter control when drawing shapes on images?

A The width of the shape's border

B The color of the shape

C The size of the shape

D The transparency of the shape

Thickness controls how wide or thick the border or line of the shape will be.

To draw text on an image, which parameter specifies the font style?

A fontScale

B thickness

C color

D fontFace

fontFace specifies the font style, such as cv2.FONT_HERSHEY_SIMPLEX.

Which coordinate system is used when drawing on images?

A Polar coordinates

B Cartesian coordinates with origin at top-left

C Cartesian coordinates with origin at center

D Spherical coordinates

Images use Cartesian coordinates with the origin (0,0) at the top-left corner.

What happens if you set thickness to -1 when drawing a rectangle in OpenCV?

A The rectangle is drawn with default thickness

B The rectangle border is invisible

C The rectangle is filled with color

D An error occurs

Setting thickness to -1 fills the rectangle with the specified color.

Explain how to draw a rectangle and add text on an image using OpenCV.

Think about the functions cv2.rectangle() and cv2.putText() and their key parameters.

Why is drawing shapes and text on images useful in computer vision projects?

Consider how marking parts of an image helps people or algorithms.

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