This paper describes the definition and testing of a new type of median filter for images. The topological median filter implements some existing ideas and some new ideas on fuzzy connectedness to improve, over a conventional median filter, the extraction of edges in noise. The concept of alpha-connectivity is defined and used to create an algorithm for computing the degree of connectedness of a pixel to all the other pixels in an arbitrary neighborhood. The resulting connectivity map of the neighborhood effectively disconnects peaks in the neighborhood that are separated from the center pixel by a valley in the brightness topology. The median of the connectivity map is an estimate of the median of the peak or plateau to which the center pixel belongs. Unlike the conventional median filter, the topological median is relatively unaffected by disconnected features in the neighborhood of the center pixel. Four topological median filters are defined. Qualitative and statistical analyses of the four filters are presented. It is demonstrated that edge detection can be more accurate on topologically median filtered images than on conventionally median filtered images.

One of the fastest methods of localizing edges in images is based on small gradient kernels, such as Sobel, Prewitt, and Roberts. Although small gradient kernels provide a fast way of computing the gradients, they have little control over noise, edge location, and edge orientation. They are known to be only sensitive to step edges and fail to detect smooth boundaries. On the other hand, large kernels provide superior noise suppression characteristics, but they suffer from wide response area around edges. They cause edges of neighboring objects to merge due to their wide support. Problems associated with large gradient kernels prevent their widespread usage. This paper presents a fuzzy topology-based method to facilitate the use of larger gradient kernels. The new method effectively limits the response area around the edge and prevents neighboring objects to affect each other. Synthetic images are used to show the superior noise suppression properties and response characteristics to both step and ramp edges. Natural images are also used to assess the performance of the newly proposed topological gradient estimation qualitatively.

ÖZET : Görüntü işlemede kenar bulma, görüntülerde piksel değerlerinin değiştiği yerlerin bulunmasıdır. İlk yöntemler, doğrusal süzgeç ve yönlü türeve dayalı basit işleçlerdi. Türevin yaklaşık olarak hesaplanmasına dayanan bu yöntemlerin en önemli sorunlarından biri, küçük işleç genişlikleri nedeniyle, gürültüye karşı hassas olmalarıdır. Yakındaki nesnelerin kenarlarının da o yerdeki türeve etki etmesini engellemek için küçük genişlikle işleçlerin kullanımı yaygındır. Bu çalışmada, daha geniş işleç boyutlarının kullanılmasını sağlayan, gradyan işleçlerinden önce uygulanabilecek bulanık topolojiye dayalı bir yöntem önerilmektedir. Bu yöntem, adım kenarlarla birlikte yavaş değişim gösteren yokuş kenarlarda türevin etki alanını sınırlandırarak daha ince kenar çizgilerinin oluşmasını sağlamaktadır. Önerilen yöntemin uygulandığı sentetik ve doğal görüntüler üzerinde yapılan inceleme sonucu, gradyan işlecinin tepkisinin kenar dışına taşmasının engellendiği ve düz alanlarda gürültünün daha iyi bastırıldığı anlaşılmıştır. ANAHTAR KELİMELER : Görüntü İşleme, Kenar Bulma, Bulanık Görüntü Topolojisi, Sayısal Görüntü Topolojisi. TOPOLOGICAL GRADIENT OPERATORS FOR EDGE DETECTIONABSTRACT : Edge detection in image processing is the task of locating pixel value variations in images. First methods were directional derivative based linear filters. One of the most important problems of these methods that are based on computation of approximate derivative were their sensitivity to noise due to small kernel sizes. Small kernels are widely used to avoid the effect of nearby objects. In this work, we propose a fuzzy topology based method that allows the use of larger gradient kernels. This method produces thin gradient lines by limiting the support +area of gradient kernels for slowly varying ramplike edges. By applying the proposed method on synthetic and natural images, it is observed that it decreases the output area around the edge and suppresses noise on constant image areas.