though they have only shifted by two places, clearly the content is meaningless to the decoder (the reader). As a consequence, video codecs tend to arrange the bitstream SO that if an error does occur its effects will be short-lived and can be recovered from at a later date. This is the structure of the coded bitstream, and this structure exists at the micro as well as the macro level. By providing structure of this kind, errors are prevented from propagating. Using the book example again, if the error occurs anywhere on a single page but is reset at the begin- ning of the next, the reader will have lost some understanding but will be able to regain the context when the alignment is restored. This restoration could occur in a piece of writing at the level of the chapter, page, paragraph, sentence, and finally word. In video coding it occurs at the group of pictures, at the individual picture, and finally at the slice level. In adopting this strategy when packet or bit loss occurs the degradation only continues until the next "alignment," where the spatial and temporal information is resynchronized. Types of Pictures Video compression can be categorized as spatial, temporal, and statistical. Statistical redun- dancy is used for both still- and moving-picture data reduction and is implied in most codecs. Coding relative to spatial information has another name: intraframe coding. Coding relative to temporal information has another name: interframe coding. As a consequence, a minimum of two types of frame can be identified in video codecs: intracoded frames or I frames, which are coded relative to themselves, and intercoded frames, or P frames (predicted), which are coded relative to other pictures. In the MPEG standard a further B-type picture is found, which is a predicted picture based on future as well as past information. Intracoded or I Pictures An intrapicture is independent from other pictures. In a hybrid DCT codec, no motion compensation is employed, and each macroblock is DCT-coded, quantized, and run-leng