Coding Video

6Inter Prediction

6.1 Introduction

Inter prediction is the process of creating a prediction block based on pixels in a different frame from the current video frame. We saw in Chapter 2 that for each block of pixels in the current video frame, it is often possible to find a similar block of pixels in a different video frame. Inter prediction creates a prediction block based on previously coded pixels in a different video frame. This could be the frame just before the current frame i.e. one frame in the past, the frame just after – one frame in the future, or an older previous frame or newer future frame. Figure 6.1 shows a few options for predicting a block in the current frame. We can use the most recent previous frame (1) as our prediction source, or we could use an older frame (2) as the prediction source. We can use a future frame (3) as the source of the prediction. We can make a prediction by combining data from two or more frames, for example, one in the past and one in the future (4).

As Figure 6.2 shows, the encoder decodes or reconstructs each coded frame and creates inter predictions from these previously coded reference pictures. The encoder sends the residual, which is the difference between the prediction and the actual frame data, together with instructions so that the decoder can create an identical prediction.

The decoder stores decoded frames and uses these as reference pictures. It uses the instructions sent by the encoder to create the same prediction as ...