borderfollowing

Borderfollowing, also known as contour tracing, is a class of image processing algorithms that extract the boundary of connected foreground regions in binary or segmented images. The result is typically a polygonal chain or a chain-code that represents the contour of each object, enabling subsequent shape analysis, vectorization, or object recognition.

The standard approach begins by locating a starting boundary pixel, defined as a foreground pixel that has at least one background neighbor. The algorithm then follows the boundary by repeatedly examining the neighborhood of the current boundary pixel in a fixed order (often clockwise) to find the next boundary pixel. The search continues until the process returns to the initial pixel with the appropriate orientation, producing a closed contour. Inner boundaries or holes are handled as separate contours. Output can be a list of pixel coordinates or a chain code describing the direction of travel along the boundary.

Common border-following methods include Moore-Neighbor tracing (8-connected boundary following) and variants such as Suzuki and Abe’s border tracing, which are widely used for handling binary images with holes and for robust contour extraction. Variants differ in neighborhood definitions, starting conditions, and how they treat touches and corners. The techniques yield linear-time complexity with respect to the number of boundary pixels and are often accompanied by post-processing steps such as smoothing or polygonal approximation.

Applications of borderfollowing include optical character recognition, shape and feature extraction, vectorization of bitmap images, and quality control in manufacturing. Limitations include sensitivity to noise and fragmentation; preprocessing steps like noise removal, thinning, or morphological filtering are commonly applied to improve performance and accuracy.