otsu

Otsu is a city in Shiga Prefecture, Japan. It serves as the prefectural capital and lies on the southern shore of Lake Biwa, east of Kyoto. The city is part of the Kyoto metropolitan area and functions as a regional transportation hub with JR West's Biwako Line and multiple rail lines connecting to Osaka, Kyoto, and surrounding areas. Otsu has historical roots as the former capital of Ohmi Province and later as the center of Otsu Domain during the Edo period. In modern times it developed as an administrative, commercial, and educational center for Shiga Prefecture. The area is known for its lakefront scenery and for nearby religious sites on Mount Hiei, including Enryaku-ji, as well as for serving as a gateway to the broader Lake Biwa region.

Otsu's method is a technique in image processing for automatic image thresholding. Introduced by Nobuyuki Otsu in 1979, it determines an optimal threshold value by maximizing the between-class variance (equivalently minimizing the intra-class variance) of gray-level pixels when an image is separated into two classes: background and foreground. The method operates on the histogram of grayscale pixel intensities, iterating possible thresholds and selecting the one that yields the greatest separation of the two classes. It is widely used for segmentation in various applications and is often taught as a foundational algorithm in computer vision.