2Video Coding and Video Quality
2.1 Introduction
Video coding or video compression bridges the gap between displayable or uncompressed video and practical methods of storing or transmitting data. Digital video is made up of a series of frames or pictures, each of which contains hundreds of thousands or millions of picture elements or pixels. Storing or transmitting video in its original, uncompressed form can require an impractically large amount of storage capacity or bandwidth. A video encoder compresses video into a smaller and more compact form known as coded video, suitable for storage or transmission. A video decoder reverses this process, extracting displayable video from coded video (see Figure 2.1). The complementary pair of an encoder and a decoder is a video codec.
In this chapter, I will introduce each of the following aspects of a video codec, before covering them in further detail throughout the book:
- Inputs and outputs: The data inputs and outputs for a video codec. The input to an encoder and the output of a decoder are uncompressed or raw video data, i.e. video that can be displayed. The output of an encoder and the input to a decoder are an encoded, compressed bitstream that is suitable for storage or transmission.
- Data structures : A video codec processes data in a hierarchy from a complete video sequence, e.g. a video scene or programme, through groups of coded frames and individual coded frames, down to basic coding units (Coding Tree Units [CTUs], Macroblocks ...
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