Color transforms (brightness, contrast, hue) in Computer Vision - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to increase the brightness of an image by adding a value to all pixels.

Computer Vision

bright_img = img + [1]

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

A50

B0.5

C-50

D255

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value less than 1 like 0.5 which is too small for brightness increase

Using negative values which decrease brightness

Adding 50 to all pixel values increases brightness by a fixed amount.

2fill in blank

medium

Complete the code to adjust contrast by multiplying pixel values by a factor.

Computer Vision

contrast_img = img * [1]

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

A0

B0.5

C-1

D1.5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or negative values which remove or invert the image

Using values less than 1 which reduce contrast

Multiplying by 1.5 increases contrast by making bright pixels brighter and dark pixels darker.

3fill in blank

hard

Fix the error in the code to correctly convert an RGB image to HSV and adjust the hue.

Computer Vision

hsv_img = cv2.cvtColor(img, [1])
hsv_img[:, :, 0] = (hsv_img[:, :, 0] + 10) % 180

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

Acv2.COLOR_BGR2HSV

Bcv2.COLOR_HSV2BGR

Ccv2.COLOR_RGB2HSV

Dcv2.COLOR_BGR2GRAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using BGR to HSV conversion when image is RGB

Using HSV to BGR conversion which is backward

Use cv2.COLOR_RGB2HSV to convert from RGB to HSV color space before adjusting hue.

4fill in blank

hard

Fill both blanks to create a function that adjusts brightness and contrast of an image.

Computer Vision

def adjust_brightness_contrast(img, brightness, contrast):
    return img * [1] + [2]

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

Acontrast

Bbrightness

Cimg

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping brightness and contrast

Adding brightness before multiplying contrast

The formula for brightness and contrast adjustment is: new_img = img * contrast + brightness.

5fill in blank

hard

Fill all three blanks to adjust hue, saturation, and value in an HSV image.

Computer Vision

hsv_img[:, :, 0] = (hsv_img[:, :, 0] + [1]) % 180
hsv_img[:, :, 1] = hsv_img[:, :, 1] * [2]
hsv_img[:, :, 2] = hsv_img[:, :, 2] * [3]

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

A10

B1.2

C0.8

D180

Attempts:

3 left

💡 Hint

Common Mistakes

Using 180 instead of 10 for hue shift

Multiplying saturation or value by values greater than 1 incorrectly

Hue is shifted by 10 degrees (mod 180), saturation is increased by 20%, and value is decreased by 20%.

Practice

1. What does increasing the brightness of an image do to its pixel values?

easy

A. It makes pixel values larger, making the image lighter

B. It rotates the colors around the color wheel

C. It stretches the difference between pixel values

D. It converts the image to grayscale

Solution

1. Step 1: Understand brightness effect on pixels

   Brightness changes add or multiply pixel values to make them lighter or darker.

2. Step 2: Identify what increasing brightness means

   Increasing brightness means making pixel values larger, so the image looks lighter.

3. Final Answer:

   It makes pixel values larger, making the image lighter -> Option A

4. Quick Check:

   Brightness increase = lighter pixels [OK]

Hint: Brightness changes scale pixel values up or down [OK]

Common Mistakes:

• Confusing brightness with contrast
• Thinking brightness rotates colors
• Assuming brightness converts to grayscale

2. Which of the following is the correct way to increase contrast of an image using a simple formula?

easy

A. new_pixel = (old_pixel - 128) * 1.5 + 128

B. new_pixel = old_pixel + 50

C. new_pixel = old_pixel / 2

D. new_pixel = old_pixel * 0

Solution

1. Step 1: Recall contrast adjustment formula

   Contrast changes stretch pixel values around the midpoint (usually 128 for 8-bit images).

2. Step 2: Identify correct formula

   The formula (old_pixel - 128) * factor + 128 increases contrast by scaling differences from midpoint.

3. Final Answer:

   new_pixel = (old_pixel - 128) * 1.5 + 128 -> Option A

4. Quick Check:

   Contrast = scale around midpoint [OK]

Hint: Contrast scales difference from midpoint, not just add values [OK]

Common Mistakes:

• Adding a constant increases brightness, not contrast
• Dividing pixel values reduces brightness
• Multiplying by zero makes image black

3. Given this Python code snippet using OpenCV, what will be the effect on the image stored in img?

import cv2
import numpy as np
img = cv2.imread('image.jpg')
hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
hsv[:, :, 0] = (hsv[:, :, 0] + 30) % 180
img_shifted = cv2.cvtColor(hsv, cv2.COLOR_HSV2BGR)

medium

A. The image is converted to grayscale

B. The image contrast is increased by 30%

C. The image brightness is increased by 30 units

D. The image hue is shifted by 30 degrees on the color wheel

Solution

1. Step 1: Understand HSV color space and hue channel

   HSV separates color into Hue, Saturation, and Value. Hue channel controls color angle on the wheel.

2. Step 2: Analyze code effect on hue channel

   The code adds 30 to the hue channel and wraps around with modulo 180, shifting colors on the wheel.

3. Final Answer:

   The image hue is shifted by 30 degrees on the color wheel -> Option D

4. Quick Check:

   Hue shift = color rotation [OK]

Hint: Hue changes rotate colors in HSV space [OK]

Common Mistakes:

• Confusing hue shift with brightness or contrast
• Ignoring modulo wrap-around for hue
• Assuming color space conversion changes brightness

4. The following code attempts to increase image contrast but does not work as expected. What is the main error?

def increase_contrast(img):
    mean = img.mean()
    return (img - mean) * 1.2 + mean

result = increase_contrast(image)

medium

A. The image should be converted to grayscale first

B. The multiplication factor should be less than 1

C. The mean should be computed per channel, not overall

D. The code should add 1.2 instead of multiplying

Solution

1. Step 1: Understand contrast adjustment per channel

   Contrast should be adjusted per color channel to preserve color balance.

2. Step 2: Identify error in mean calculation

   The code calculates mean over entire image, mixing channels, causing wrong contrast scaling.

3. Final Answer:

   The mean should be computed per channel, not overall -> Option C

4. Quick Check:

   Contrast per channel = correct scaling [OK]

Hint: Calculate mean per channel for contrast adjustment [OK]

Common Mistakes:

• Using single mean for all channels
• Using factor less than 1 reduces contrast
• Adding constant changes brightness, not contrast

5. You want to apply a color transform pipeline to an image that first increases brightness by 20%, then shifts hue by 45 degrees, and finally increases contrast by 30%. Which order of operations is best to preserve natural colors?

hard

A. Increase contrast -> Shift hue -> Increase brightness

B. Increase brightness -> Increase contrast -> Shift hue

C. Shift hue -> Increase brightness -> Increase contrast

D. Shift hue -> Increase contrast -> Increase brightness

Solution

1. Step 1: Consider effect of brightness before contrast

   Increasing brightness first ensures pixel values are lifted before stretching differences with contrast.

2. Step 2: Apply hue shift last to avoid color distortion

   Hue shift should be last to rotate colors after brightness and contrast adjustments.

3. Step 3: Confirm order preserves natural colors

   Brightness -> Contrast -> Hue maintains natural look by adjusting lightness and contrast before color rotation.

4. Final Answer:

   Increase brightness -> Increase contrast -> Shift hue -> Option B

5. Quick Check:

   Brightness then contrast then hue = natural colors [OK]

Hint: Adjust brightness and contrast before hue shift [OK]

Common Mistakes:

• Shifting hue before brightness causes unnatural colors
• Increasing contrast before brightness can clip pixels
• Applying hue shift first distorts brightness and contrast