Coding Video

9Coded Video Filtering

9.1 Introduction

Video filtering involves applying a digital filter to pixels in a video image to modify or enhance the image in some way. Filtering can be applied for a number of purposes. We will concentrate in particular on filtering that is applied within a video encoder and a video decoder, sometimes described as loop filtering or in‐loop filtering. The main purpose of an in‐loop filter is to improve the performance of the video codec itself, by improving the prediction source and therefore improving the performance of prediction. As Figure 9.1 illustrates, reference frames in an encoder can be filtered before creating a prediction for a block to be encoded. The decoder has to create an identical prediction, so the same filter is applied to reference frames in the decoder (see Figure 9.2).

Two examples of in‐loop filters used in video coding are deblocking filters and deringing filters. A deblocking filter attempts to reduce distortion at block boundaries. This is the characteristic blockiness, introduced by the lossy encoding process, that is familiar from over‐compressed videos and images. A deringing filter attempts to reduce visible ripples near strong edges in a video image.

In this chapter, we will look at these and other types of in‐loop filters, with a particular focus on how they are used in the H.265/HEVC standard.

In‐loop filters affect the visual appearance of the filtered image. Visual quality is inherently subjective, which means that ...