What is descriptor in computer vision?

A descriptor in computer vision is a numerical representation of a region or feature in an image, designed to capture its unique characteristics in a way that algorithms can compare or match it to other features. Think of it as a compact “fingerprint” for a specific part of an image, like the edges of an object or a textured area. Descriptors are critical because raw pixel data is too noisy and variable for direct comparison—lighting changes, rotations, or scale differences can make the same feature look entirely different. By converting features into numerical vectors, descriptors enable tasks like object recognition, image stitching, or tracking by measuring similarity between features.

Descriptors are typically generated by analyzing the pixels around a keypoint (a point of interest detected by algorithms like SIFT, SURF, or ORB). For example, the SIFT descriptor calculates gradients (direction and magnitude of intensity changes) in small regions around the keypoint, then groups them into histograms to summarize the local pattern. ORB, a faster alternative, uses binary comparisons between pixel intensities to create a compact binary string. These methods aim to balance distinctiveness (uniqueness of the descriptor) and invariance (robustness to transformations like rotation or scaling). Modern approaches often prioritize efficiency—binary descriptors like FREAK or BRISK use pre-defined sampling patterns to speed up computation, making them suitable for real-time applications.

In practice, descriptors are used in applications like panoramic photo stitching (matching overlapping image regions), augmented reality (aligning virtual objects with real-world scenes), or robotics (navigation via visual landmarks). Developers often leverage libraries like OpenCV, which provide optimized implementations of descriptor algorithms. A key consideration is choosing the right descriptor for the task: SIFT offers high accuracy but is computationally heavy, while ORB sacrifices some robustness for speed. Understanding trade-offs between descriptor size, computation time, and matching accuracy is essential for building efficient computer vision systems.