Color transforms (brightness, contrast, hue) in Computer Vision

Introduction

Color transforms help change how an image looks by adjusting brightness, contrast, or hue. This makes images clearer or more interesting for computers to understand.

Making a dark photo brighter so details are easier to see.

Increasing contrast to highlight differences between objects in an image.

Changing the hue to correct colors or create artistic effects.

Preparing images for machine learning models to improve accuracy.

Augmenting data by varying colors to help models learn better.

Syntax

Computer Vision

brightness_image = original_image * brightness_factor
contrast_image = (original_image - mean) * contrast_factor + mean
hue_image = adjust_hue(original_image, hue_factor)

Brightness factor > 1 makes the image brighter; between 0 and 1 makes it darker.

Contrast adjustment usually centers around the mean pixel value.

Examples

This makes the image 20% brighter.

Computer Vision

brightness_image = original_image * 1.2

This increases contrast by 50%, assuming pixel values range 0-255.

Computer Vision

contrast_image = (original_image - 128) * 1.5 + 128

This shifts the hue slightly to change colors.

Computer Vision

hue_image = adjust_hue(original_image, 0.1)

Sample Model

This code creates a small 2x2 color image and applies brightness, contrast, and hue changes. It prints the original and transformed images as arrays.

Computer Vision

import cv2
import numpy as np

def adjust_brightness(image, factor):
    return np.clip(image * factor, 0, 255).astype(np.uint8)

def adjust_contrast(image, factor):
    mean = np.mean(image, axis=(0,1), keepdims=True)
    return np.clip((image - mean) * factor + mean, 0, 255).astype(np.uint8)

def adjust_hue(image, factor):
    hsv = cv2.cvtColor(image, cv2.COLOR_BGR2HSV).astype(np.float32)
    hsv[..., 0] = (hsv[..., 0] + factor * 180) % 180
    hsv = hsv.astype(np.uint8)
    return cv2.cvtColor(hsv, cv2.COLOR_HSV2BGR)

# Load sample image
image = np.full((2, 2, 3), [100, 150, 200], dtype=np.uint8)  # simple 2x2 image with color

bright = adjust_brightness(image, 1.5)
contrast = adjust_contrast(image, 1.5)
hue = adjust_hue(image, 0.1)

print("Original image:\n", image)
print("\nBrightened image:\n", bright)
print("\nContrast adjusted image:\n", contrast)
print("\nHue adjusted image:\n", hue)

Important Notes

Brightness and contrast changes can cause pixel values to go outside 0-255, so clipping is important.

Hue adjustment works best in HSV color space, which separates color from brightness.

Small images are used here for simplicity; real images are larger but use the same methods.

Summary

Brightness changes make images lighter or darker by scaling pixel values.

Contrast changes stretch or shrink differences between pixel values around their average.

Hue changes shift colors by rotating the color wheel in HSV space.

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