This is a novel type of explicit image filter - guided filter. Derived from a local linear model, the guided filter
generates the filtering output by considering the content of a guidance
image, which can be the input image itself or another different image.

The guided filter can perform as an edge-preserving smoothing operator
like the popular bilateral filter, but has better behavior near the edges. It also has a theoretical connection with the matting Laplacian matrix, so is a more generic concept than a smoothing operator and can better utilize the structures in the guidance image. Moreover, the
guided filter has a fast and on-approximate linear-time algorithm, whose
computational complexity is independent of the filtering kernel size. The guided filter is both effective and efficient in a great variety of computer vision and computer graphics applications including
noise reduction, detail smoothing/enhancement, HDR compression, image matting/feathering, haze removal, and joint upsampling.

There are two files: 'guided_filter.m' contains the guided filter function; 'guided.m' is an example script demonstrating the use of the filter.