  The guided filter is a technique for edge-aware image filtering. Because of
its nice visual quality, fast speed, and ease of implementation, the guided
filter has witnessed various applications in real products, such as image
editing apps in phones and stereo reconstruction, and has been included in
official MATLAB and OpenCV. In this note, we remind that the guided filter can
be simply sped up from O(N) time to O(N/s^2) time for a subsampling ratio s. In
a variety of applications, this leads to a speedup of >10x with almost no
visible degradation. We hope this acceleration will improve performance of
current applications and further popularize this filter. Code is released.
