Side window guided filtering

This paper presents an improved version of the popular guided filtering (GF) technique. Although very effective, GF can suffer from artifacts such as edge blurring and halos. Through analyzing GF, we discover that traditional filtering practices of placing the center of the filtering window over the pixel being processed is one of the causes of these artifacts. This leads GF to perform linear combination and averaging operations on pixels on both sides of the edges thus causing edge blurring. Based on this discovery, we introduce the Side Window Guided Filtering (SWGF) technique. Treating each pixel as a potential edge, SWGF aligns the sides or corners of the filtering window with the pixels under processing. There are left, right, upper and bottom windows placed on one side of the pixel and northwest, northeast, southwest and southeast windows placed on one corner of the pixel. The one that closest approximates the current pixel is chosen as the output. We will present mathematical analysis to demonstrate the correctness of this method. SWGF is applied to a variety of image processing applications. Results demonstrate that under exactly the same parameter settings, SWGF consistently improves the edge preservation performances of GF and its other variations.