The earliest DCT-based video coding standard is the ITU-T H.261 standard. This algorithm assumes one of two formats, CIF and QCIF. A block diagram of the H.261 video coder is shown in Figure 18.10. The basic idea is simple. An input image is divided into blocks of 8×8 pixels. For a given 8×8 block, we subtract the prediction generated using the previous frame. (If there is no previous frame or the previous frame is very different from the current frame, the prediction might be zero.) The difference between the block being encoded and the prediction is transformed using a DCT. The transform coefficients are quantized and the quantization label encoded using a variable-length code. In the following discussion, we will take a more detailed look at the various components of the compression algorithm.
