Feature Extraction and Image Representation | CV

Feature Extraction:

Feature extraction is a critical step in computer vision that involves identifying and extracting relevant and distinctive information from images. Features are representations of specific patterns, structures, or characteristics that can be used to distinguish one image from another or describe important visual properties. Feature extraction techniques aim to capture meaningful information while reducing the dimensionality of the data. Features can be extracted from images at different levels of abstraction.

  1. Low-level features: These features are based on the pixel values in the image, such as the intensity, color, and texture of the pixels.
  2. Mid-level features: These features are based on the spatial relationships between pixels, such as edges, corners, and blobs.
  3. High-level features: These features are based on the semantics of the image, such as the presence of objects or scenes.

Here are some commonly used feature extraction methods:

Edge detection

Edge detection algorithms identify and localize sharp changes in pixel intensities, which typically correspond to object boundaries or significant transitions in the image. Techniques like the Canny edge detector or the Sobel operator are commonly used for edge detection.

Corner detection

Corner detection algorithms identify and localize corners or points in an image where two or more edges intersect. Corners are useful as they provide distinctive information about the image's local structure. The Harris corner detector and the Shi-Tomasi corner detector are popular corner detection algorithms.

Scale-invariant feature transform (SIFT)

SIFT is a widely used feature extraction technique that identifies key points in an image that are invariant to scale, rotation, and changes in viewpoint. SIFT computes descriptors that capture local image gradients and orientations, enabling robust feature matching and recognition.

Histogram of Oriented Gradients (HOG)

HOG computes histograms of local gradients in an image to capture information about object shapes and appearances. HOG features are commonly used for object detection tasks, especially in scenarios where object boundaries and shapes are essential cues.

Local Binary Patterns (LBP)

LBP is a texture descriptor that characterizes local patterns in an image by comparing pixel intensities with their neighboring pixels. LBP features are effective in capturing texture information and have applications in texture classification, facial recognition, and image retrieval.

Convolutional Neural Networks (CNN)

CNNs are deep learning models that automatically learn hierarchical features from images. CNNs consist of multiple layers of convolutional and pooling operations, enabling them to extract complex features at different spatial scales. CNNs have revolutionized feature extraction in computer vision and achieved state-of-the-art results in various tasks like image classification, object detection, and semantic segmentation.

The choice of features depends on the task at hand. For example, if the task is to classify objects in an image, then low-level features such as intensity and color may be sufficient. However, if the task is to identify objects in an image, then mid-level features such as edges and corners may be necessary.

Image Representation:

Image representation refers to the process of transforming an image into a suitable mathematical format that can be processed and analyzed by computer vision algorithms. It involves encoding the spatial and structural information of an image into a numerical representation. This can involve tasks such as:

  1. Converting the image into a numerical format: This can be done by representing each pixel in the image as a number.
  2. Normalizing the image: This can be done by scaling the pixel values so that they fall within a certain range.
  3. Feature extraction: This can be done by extracting features from the image that are relevant to the task at hand.

Different types of image representations are used based on the specific requirements of the application. Some common image representations include:

Pixel-based representation

In this representation, each pixel in the image is considered as a separate entity, and the image is represented as a grid of pixel values. Grayscale images can be represented as matrices, where each element represents the intensity value of a pixel. Color images can be represented using multiple matrices or tensors, one for each color channel (e.g., red, green, blue).

Histogram representation

In this representation, the distribution of pixel values in an image is captured using histograms. The histogram represents the frequency of occurrence of different pixel intensities. Histograms can be useful for analyzing global image properties, such as contrast, brightness, or color distribution.

Frequency domain representation

This representation utilizes the Fourier Transform to analyze the frequency content of an image. The image is transformed from the spatial domain to the frequency domain, where the information is represented in terms of frequency components. Frequency domain representations are particularly useful for tasks like image denoising or compression.

Structural representation

This representation captures the structural information of an image by detecting and representing key features or patterns. It involves identifying edges, corners, textures, or other distinctive structures in an image and representing them using descriptors or feature vectors.


Image representation and feature extraction are important steps in the computer vision pipeline. They can improve the accuracy and performance of computer vision algorithms by providing a way to represent images in a format that is easy to process and by extracting features that are relevant to the task at hand.