How does features extraction on images work?

Feature extraction in images involves identifying and isolating specific patterns or structures that are meaningful for tasks like object recognition or classification. The goal is to reduce the raw pixel data into a smaller set of representative values (features) that capture essential information. This process helps algorithms focus on relevant details while ignoring noise, making downstream tasks like machine learning more efficient and accurate. There are two primary approaches: traditional handcrafted methods and modern deep learning-based techniques.

Traditional methods rely on mathematical algorithms to detect low-level visual elements. For example, edge detection filters like Sobel or Canny identify abrupt changes in pixel intensity to outline object boundaries. Keypoint detectors like SIFT (Scale-Invariant Feature Transform) locate distinctive regions (e.g., corners or blobs) and describe them using gradient histograms, making them robust to rotation or scale changes. Histogram of Oriented Gradients (HOG) is another method that counts gradient directions in localized image regions, often used for pedestrian detection. These techniques require manual tuning and struggle with variations like lighting changes or complex textures. For instance, OpenCV’s SIFT_create() function can extract keypoints and descriptors, but developers must decide how to filter and match them for specific use cases.

Deep learning-based approaches, particularly Convolutional Neural Networks (CNNs), automate feature extraction by learning hierarchical patterns directly from data. Early CNN layers detect simple features like edges or color gradients, while deeper layers combine these into complex structures (e.g., shapes or object parts). For example, a pre-trained ResNet model might use its final convolutional layer outputs as a feature vector for an image. Frameworks like TensorFlow or PyTorch simplify this: developers can load a model, remove its classification head, and pass an image through the network to extract features. This method excels at handling variations in scale, rotation, and lighting because the model learns invariant representations during training. However, it requires large labeled datasets and computational resources. A practical implementation might involve using torchvision.models.resnet18(pretrained=True) and extracting features from the avgpool layer.