Image Thresholding in Image Processing

Nikolaj Buhl
September 12, 2023
5 min read
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In digital image processing, thresholding is the simplest method of segmenting images. It plays a crucial role in image processing as it allows for the segmentation and extraction of important information from an image. By dividing an image into distinct regions based on pixel intensity or pixel value, thresholding helps distinguish objects or features of interest from the background. This technique is widely used in various applications such as object detection, image segmentation, and character recognition, enabling efficient analysis and interpretation of digital images. Additionally, image thresholding can enhance image quality by reducing noise and improving overall visual clarity. 

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Thresholding — Image Processing

The choice of thresholding technique is critical determination of the accuracy and effectiveness of image analysis. Different thresholding techniques have their own strengths and limitations. Selecting the appropriate technique depends on factors such as image complexity, noise levels, and the desired outcome. Therefore, it is essential to give careful consideration to the selection and to conduct experimentation to ensure optimal results in image processing tasks. 

In the article, we will cover the following:

  • What is Image Thresholding? 
  • Image Thresholding Techniques
  • Applications of Image Thresholding 
  • Practical Implementation and Considerations
  • Challenges with Image Thresholding
  • Future Developments in Image Thresholding
  • Image Thresholding: Key Takeaways

What is Image Thresholding?

Image thresholding involves dividing an image into two or more regions based on intensity levels, allowing for easy analysis and extraction of desired features. By setting a threshold value, pixels with intensities above or below the threshold can be classified accordingly This technique aids in tasks such as object detection, segmentation, and image enhancement. 

Image thresholding is a technique that simplifies a grayscale image into a binary image by classifying each pixel value as either black or white based on its intensity level or gray-level compared to the threshold value. This technique reduces the image to only two levels of intensity, making it easier to identify and isolate objects of interest. Binary image conversion allows for efficient processing and analysis of images, enabling various computer vision applications such as edge detection and pattern recognition. 

In imaging processing algorithms, the principle of pixel classification based on intensity threshold is widely used. By setting a specific threshold value, pixels with intensity levels above the threshold are classified as white, while those below the threshold are classified as black. This principle forms the foundation for various image enhancement techniques that help to extract important features from an image for further analysis. 

In data science and image processing, an entropy-based approach to image thresholding is used to optimize the process of segmenting specific types of image, often those with intricate textures or diverse patterns. By analyzing the entropy, which measures information randomness, this technique seeks to find the optimal threshold value that maximizes the information gained when converting the image into a binary form through thresholding. This approach is especially beneficial for images with complex backgrounds or varying lighting conditions. Through this technique, the binary thresholding process becomes finely tuned, resulting in more accurate segmentation and enhanced feature extraction, which is vital for applications in image analysis and computer vision tasks.

Image Thresholding Techniques

These are widely used in various fields such as medical imaging, computer vision, and remote sensing. These techniques are essential for accurate image processing and interpretation. They help to convert grayscale or color images into binary images, separating the foreground from the background, allowing for better segmentation and extraction of features from an image, which is crucial for various applications in computer vision and pattern recognition.

Global Thresholding

Global Thresholding is a widely used technique where a single threshold value is applied to an entire image. However, this technique  may not be suitable for images with varying lighting conditions or complex backgrounds. To overcome this limitation, adaptive thresholding techniques may be employed, which adjust the threshold value locally based on the characteristics of each pixel's neighborhood. These techniques are particularly useful in scenarios where there is significant variation in illumination across different regions of the image. 

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Thresholding-Based Image Segmentation

Simple thresholding is a basic technique that assigns a binary value to each pixel based on a global threshold value. It is effective when the image has consistent lighting conditions and a clear foreground-background separation. However, when images contain varying lighting conditions or complex backgrounds, adaptive thresholding techniques are more suitable. These techniques dynamically adjust the threshold value for each pixel based on its local neighborhood, allowing for better segmentation and accurate object detection. 

Otsu's Method for Automatic Threshold Determination is a widely used technique for automatically determining the optimal threshold value in image segmentation. It calculates the threshold by maximizing the between-class variance of pixel value, which effectively separates foreground and background regions. This method is particularly useful when dealing with images that have bimodal or multimodal intensity distributions, as it can accurately identify the threshold that best separates different objects or regions in the image. 

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Otsu's method - Wikipedia

light-callout-cta “A nonparametric and unsupervised method of automatic threshold selection for picture segmentation. An optimal threshold is selected by the discriminant criterion, so as to maximize the separability of the resultant classes in gray levels. The procedure utilizies only the zeroth- and the first-order cumulative moments of the gray-level histogram.” - Nobuyuki Otsu  

Pros and Cons of Global Thresholding 

Gobal thresholding offers several advantages, including its simplicity and efficiency in determining a single threshold value for the entire image. It is particularly effective in scenarios where the foreground and background regions have distinct intensity distributions. However, global thresholding may not be suitable for images with complex intensity distributions or when there is significant variation in lighting conditions across the image. Additionally, it may not accurately segment objects or regions that have overlapping intensity values. 

Local (Adaptive) Thresholding 

Local thresholding addresses the limitations of global thresholding by considering smaller regions within the image. It calculates a threshold value for each region based on its local characteristics, such as mean or median intensity. This approach allows for better adaptability to varying lighting conditions and complex intensity distributions, resulting in more accurate segmentation of objects or regions with overlapping intensity values. However, local thresholding may require more computational resources and can be sensitive to noise or uneven illumination within the image, which can affect the overall performance of the segmentation algorithm.

Adaptive Thresholds for Different Image Regions are needed to overcome the challenges of variations in lighting conditions and contrast within an image. These adaptive thresholds help improve the accuracy and clarity of object or region detection. This approach involves dividing the image into smaller sub-regions and calculating a threshold value for each sub-region based on its local characteristics. By doing so, the algorithm can better account for these variations and mitigate the effects of noise or uneven illumination, as each sub-region is treated independently. 

light-callout-cta The simplest method to segment an image is thresholding. Using the thresholding method, segmentation of an image is done by fixing all pixels whose intensity values are more than the threshold to a foreground value.  

Mean and Gaussian Adaptive Thresholding 

Two commonly used methods in image processing are Mean and Gaussian Adaptive Thresholding. Mean adaptive thresholding calculates the threshold value for each sub-region by taking the average intensity of all pixels within that region. On the other hand, Gaussian adaptive thresholding uses a weighted average of pixel intensities, giving more importance to pixels closer to the center of the sub-region. These methods are effective in enhancing image quality and improving accuracy in tasks such as object detection or segmentation.  

Advantages over Global Thresholding 

Adaptive Thresholding has advantages over global thresholding. One advantage is that it can handle images with varying lighting conditions or uneven illumination. This is because adaptive thresholding calculates the threshold value locally, taking into account the specific characteristics of each sub-region. Additionally, adaptive thresholding can help preserve important details and fine textures in an image, as it adjusts the threshold value based on the local pixel intensities. 

Applications of Image Thresholding 

Image thresholding is a technique used in computer vision that has a variety of applications, including image segmentation, object detection, and character recognition. By separating objects from their background in an image, image thresholding makes it easier to analyze and extract relevant information. Optical character recognition (OCR) systems, for example, use image thresholding to distinguish between foreground (text) and background pixels in scanned documents, making them editable. Additionally, QR codes, which encode information within a grid of black and white squares, can be incorporated into images as a form of data representation and retrieval.

Real-world applications 

  • Object Detection: By setting a threshold value, objects can be separated from the background, allowing for more accurate and efficient object detection
  • Medical Images: Image thresholding can be used to segment different structures or abnormalities for diagnosis and analysis in medical imaging.
  • Quality Control: Image thresholding plays a crucial role in quality control processes, such as inspecting manufactured products for defects or ensuring consistency in color and texture of a color image.
  • Object Segmentation: Image thresholding is also commonly used in computer vision tasks such as object segmentation, where it helps to separate foreground objects from the background. This enables more accurate and efficient detection of objects within an image.
  • Noise Reduction: Thresholding can be utilized for noise reduction, as it can help to eliminate unwanted artifacts or disturbances in an image.
     
  • Edge Detection: Image thresholding aids in identifying and highlighting the boundaries between different objects or regions within an image with edge detection algorithms.

 A step by step guide to Image Segmentation in Computer Vision can be read here.

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Thresholding

Practical Implementation and Considerations 

When implementing thresholding techniques, it is important to carefully select the appropriate threshold value based on the specific image and desired outcome. This can be achieved through experimentation or through the use of adaptive thresholding methods that automatically adjust the threshold based on local image characteristics. Furthermore, it is essential to consider the potential trade-off between noise reduction and preserving important details in the image, as aggressive thresholding may lead to the loss of valuable information. 

Steps for implementing thresholding algorithms (Python)

Here are step-by-step guides for implementing image thresholding algorithms using Python. You will implement the global thresholding and Otsu's thresholding, which are two commonly used thresholding techniques.

Implementing Image Thresholding Algorithms in Python

Global Thresholding

Let us review what we know so far, and for this you can use Google Colab to run the below code.

#Install the required library
!pip install opencv–python

#Get the image file
!wget -O /content/sunflower.jpg 'https://images.unsplash.com/photo-1503936380431-4a4ce0fc296c?ixlib=rb4.0.3&ixid=M3wxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8fA%3D%3D&auto=format&fit=crop&w=2970&q=80'

After acquiring the image file, right-click on it to copy the image path, and proceed to paste it into the designated section labeled "ADD YOUR FILE PATH HERE." If you are using an alternative IDE, you can alternatively input the path on your local system.

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import cv2
from google.colab.patches import cv2_imshow
# Read the image#image = cv2.imread('ADD YOUR FILE PATH HERE', cv2.IMREAD_GRAYSCALE)

image = cv2.imread('/content/sunflower.jpg', cv2.IMREAD_GRAYSCALE)

# Apply global thresholding

_, binary_image = cv2.threshold(image, 127, 255, cv2.THRESH_BINARY)

# Display the results

cv2_imshow(image)

cv2_imshow(binary_image)

cv2.waitKey(0)

cv2.destroyAllWindows()

Output:

Grayscale Image

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Binary Image

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Otsu's Thresholding

import cv2
from google.colab.patches import cv2_imshow
# Read the image
image = cv2.imread('/content/sunflower.jpg', cv2.IMREAD_GRAYSCALE)

# Define the desired width and height for the resized image
desired_width = 640  # Change this to your desired width
desired_height = 480  # Change this to your desired height

# Resize the image to the desired size
resized_image = cv2.resize(image, (desired_width, desired_height))

# Apply Otsu's thresholding
_, binary_image = cv2.threshold(resized_image, 0, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)

# Display the results
cv2_imshow(resized_image)
cv2_imshow(binary_image)
cv2.waitKey(0)
cv2.destroyAllWindows()

Output:

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Otsu’s Thresholding Image

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The code above applies Otsu's thresholding to the image and displays the original image and binary image or thresholded image.

Remember to replace `'image.jpg'` with the actual path of your image file. These examples demonstrate the basic implementation of global thresholding and Otsu's thresholding in both Python. You can further customize these codes to suit your specific image processing needs, including pre-processing steps, visualization enhancements, and additional algorithm parameters.

Global Thresholding Value in Python using Otsu’s Method

import cv2
import numpy as np
from google.colab.patches import cv2_imshow

# Read the image in grayscale
image = cv2.imread('/content/sunflower.jpg', cv2.IMREAD_GRAYSCALE)

# Define the desired width and height for the resized image
desired_width = 640  # Change this to your desired width
desired_height = 480  # Change this to your desired height

# Resize the image to the desired size
resized_image = cv2.resize(image, (desired_width, desired_height))

# Calculate global threshold using Otsu's method
_, global_thresholded = cv2.threshold(resized_image, 0, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)

# Calculate Otsu's threshold value directly
otsu_threshold_value = cv2.threshold(resized_image, 0, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)[0]

# Display the results
cv2_imshow(global_thresholded)
print("Global Threshold Value:", otsu_threshold_value)

cv2.waitKey(0)
cv2.destroyAllWindows()

Output:

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Global Threshold Value: 168.0

This code will display the original image and the image after global thresholding using Otsu's method, along with the threshold value determined by Otsu's algorithm.

Pre-processing and post-processing impact 

Pre-processing and post-processing techniques play a crucial role in achieving accurate and meaningful results in image thresholding. Employing a range of techniques before and after thresholding can significantly enhance the accuracy of segmentation and the usability of the final binary image. Pre-processing techniques such as noise reduction, image enhancement, and morphological operations before thresholding, can improve segmentation results. Similarly, post-processing techniques like connected component analysis and contour smoothing can further refine the binary image and remove any artifacts or imperfections. 

Let's delve deeper into how pre-processing and post-processing impact image thresholding

Pre-processing Impact

  • Noise reduction techniques like Gaussian smoothing or median filtering techniques help suppress noise while preserving important edges and details.
  • Contrast Enhancement of an image before thresholding can lead to better separation between object and background intensities. Histogram equalization or adaptive histogram equalization techniques lead to better separation between object and background intensities. Basically the image histogram will be greatly affected if the image is thresholded as shown in the figure below.

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Histogram transformations

  • Illumination Correction is nothing but background subtraction or morphological operations normalize illumination across the image, especially in cases where lighting conditions are non-uniform or uneven.
  • Edge detection techniques can be applied as a pre-processing step to identify significant edges in the image. This can assist in defining regions of interest and guide the thresholding process, especially when the boundaries between objects and background are not well-defined.
  • Image Smoothing can be done using smoothing filters like Gaussian blur or mean filtering can reduce fine details and minor variations in the image, simplifying the thresholding process and leading to more coherent segmentation results.

Post-processing Impact

  • Connected Component Analysis identifies and labels separate regions in the binary image, distinguishing individual objects and eliminating isolated noise pixels.
  • Morphological Operations like erosion and dilation fine-tune the binary image by removing small noise regions and filling in gaps between segmented objects.
  • Object Size Filtering removes small objects or regions that are unlikely to be relevant, especially when dealing with noise or artifacts that may have been segmented as objects during thresholding.
  • Smoothing Edges is achieved when smoothing filters applied to the binary image, and can result in cleaner and more natural-looking object boundaries.
  • Object Feature Extraction involves area, perimeter, centroid, and orientation, and can be used for further analysis or classification.
  • Object Merging and Splitting techniques can be applied to merge nearby objects or split overly large ones in cases where thresholding results in objects that are too fragmented or split.

Pre-processing and post-processing steps are integral to obtaining accurate and meaningful results in image thresholding. The selection of appropriate techniques and their parameters should be guided by the specific characteristics of the image and the goals of the analysis. By thoughtfully combining pre-processing and post-processing techniques, it is possible to transform raw images into segmented binary images that provide valuable insights for various applications.

Challenges with Image Thresholding 

There are several challenges with image thresholding. Some of the main challenges are determining an appropriate threshold value, handling noise and variations in lighting conditions, and dealing with complex image backgrounds. Furthermore, selecting the right pre-processing and post-processing techniques can be difficult,  as it requires a deep understanding of the image content and the desired outcome. Overcoming these challenges requires careful consideration and experimentation.. 

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The challenge of thresholding continuous antibody measures

light-callout-cta Some of the challenges of image thresholding include high computational cost, insufficient performance, lack of generalization and flexibility, lack of capacity to capture various image degradations, and many more.

Image thresholding presents distinct challenges when dealing with complex images and varying lighting conditions. These challenges can impact the accuracy of segmentation results and require careful consideration to achieve reliable outcomes. Let's delve into the specific challenges posed by complex images and varying lighting conditions:

Complex Images

  • Complex Intensity Distributions: Images with complex intensity distributions, such as multi-modal or non-uniform distributions, can make selecting an appropriate threshold value difficult. Traditional thresholding methods that assume a bi-modal distribution might struggle to accurately segment objects when intensity values are spread across multiple peaks.
  • Gradual Intensity Transitions: Objects with gradual intensity changes or subtle edges can be challenging to segment accurately. Traditional thresholding methods are designed to work best with well-defined edges, and they might lead to fragmented or imprecise segmentation when applied to images with gradual transitions.
  • Overlapping Objects: Objects that overlap or occlude each other in the image can cause difficulties for thresholding. In such cases, a single threshold might segment a merged object as multiple objects, or vice versa. This can lead to inaccurate object separation and hinder subsequent analysis.
  • Texture and Pattern Variability: Images with intricate textures or complex patterns can be tough to segment accurately. Traditional thresholding, which relies on intensity values alone, might not effectively capture the variations in textures, leading to under-segmentation or over-segmentation.
  • Partial Occlusion: When an object is only partially visible due to occlusion or truncation, thresholding methods can struggle to define the boundaries accurately. Incomplete segmentation can lead to errors in size, shape, and feature measurements.
  • Multiple Object Types: Images containing multiple types of objects with varying shapes, sizes, and intensities pose a challenge for uniform thresholding. Adapting the threshold value to cater to these diverse objects can be complex.

Varying Lighting Conditions

  • Uneven Illumination: Images captured under uneven or non-uniform lighting conditions can result in inaccurate segmentation using global thresholding. Objects illuminated differently might not be accurately separated from the background, leading to segmentation errors.
  • Shadows and Highlights: Varying lighting conditions can create shadows and highlights, altering the perceived intensity values of objects. Shadows can cause objects to be under-segmented, while highlights can lead to over-segmentation.
  • Local Intensity Variations: In the presence of varying lighting, the assumption of consistent intensity values across an object might not hold true. Adaptive thresholding methods that consider local intensity characteristics are better suited to handle such scenarios.
  • Dynamic Scenes: Images captured in dynamic environments with changing lighting conditions, such as outdoor scenes or real-time video feeds, require continuous adjustment of threshold values to account for the evolving illumination. Static thresholding might result in poor segmentation.
  • Reflections and Glare: Reflective surfaces or glare can cause spikes in intensity values, complicating the thresholding process. These spikes can be misleading and result in the misclassification of pixels.

Addressing these challenges requires a combination of techniques, including adaptive thresholding methods, pre-processing steps, and post-processing refinements. Adaptive thresholding takes into account local intensity variations and is particularly effective in dealing with varying lighting conditions. Pre-processing steps, such as contrast enhancement and illumination normalization, can help mitigate the effects of uneven lighting. Post-processing techniques, like morphological operations and edge smoothing, can refine the segmentation results and eliminate artifacts.

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Image Thresholding in varying Lighting Conditions

Furthermore, the integration of machine learning techniques, like convolutional neural networks (CNNs), can enhance segmentation accuracy for complex images and varying lighting conditions. These approaches learn from data and can adapt to the intricacies of the image content.

Overall, understanding the unique challenges presented by complex images and varying lighting conditions and applying appropriate techniques is crucial for successful image thresholding in these scenarios. 

Future Developments in Image Thresholding 

Upcoming advancements in image processing include the integration of deep learning algorithms, which can further enhance segmentation accuracy by automatically learning and extracting features from complex images. Furthermore, advancements in hardware technology, such as the development of specialized processors for image processing tasks, may also contribute to faster and more efficient image thresholding in the future. 

The potential impact of emerging technologies in image thresholding is signficant. With the integration of deep learning algorithms, we can expect more accurate and precise segmentation results, leading to improved applications in fields like medical imaging, autonomous vehicles, and object recognition. Furthermore, advancements in hardware technology can significantly enhance the speed and efficiency of image thresholding algorithms, enabling real-time processing and analysis of large-scale image datasets

Image Thresholding: Key Takeaways

  • Crucial Role of Thresholding: Image thresholding is vital for segmenting images, extracting features, and enhancing image quality. It's used in object detection, segmentation, and character recognition, aiding efficient image analysis.
  • Technique Selection Importance: Choosing the right thresholding technique is crucial. Different methods have strengths and limitations, based on image complexity, noise, and goals. Careful consideration is essential for optimal results.
  • Binary Conversion: Image thresholding simplifies images by converting them to binary form (black and white). This simplification aids in isolating objects and features of interest.
  • Global and Adaptive Thresholding: Global thresholding is straightforward but not suitable for complex backgrounds. Adaptive thresholding adjusts locally, making it effective for varying lighting conditions.
  • Otsu's Method and Applications: Otsu's method automatically determines optimal thresholds, especially useful for complex images. Thresholding finds applications in object detection, segmentation, edge detection, and quality control.
  • Implementation and Challenges: Implementing thresholding involves selecting thresholds, pre-processing, and post-processing. Challenges include noise, lighting variations, complex backgrounds, and overlapping objects.

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Written by Nikolaj Buhl
Nikolaj is a Product Manager at Encord and a computer vision enthusiast. At Encord he oversees the development of Encord Active. Nikolaj holds a M.Sc. in Management from London Business School and Copenhagen Business School. In a previous life, he lived in China working at the Danish Embas... see more
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