аЯрЁБс>ўџ 68ўџџџ5џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџьЅС7 №ПkbjbjUU "&7|7|kџџџџџџl^^^^^^^Тrrrr ~Тт2ІІІІІІІІacccccc$ 4 f‡^ІІІІІ‡Ш^^ІІœШШШІ"^І^ІaШІaШШаŽ%h^^aІš аЏЬЯЛФТАrШaВ0тФš Шš aШr. "^^^^йMPEG-4 ISO-14496, Coding of Moving Pictures and Audio Version 1 approved in 1999. Intended for very low bit rate systems; e.g. low quality video via modems. Originally optimized for three ranges: Less than 64 Kbits / sec 64 to 384 Kbits / sec 0.384 to 4 Mbits / sec Subsequent versions allow rates up to 38.4 Mbits / sec and 1.2 Gbits / sec. MPEG-4 is object-oriented and represents a scene as a collection of objects. Scenes are described using Binary Format for Scenes (BIFS), a language based on Virtual Reality Modeling Language (VRML). Unlike MPEG 1 and 2, MPEG-4 is designed for interactivity. Error resilience is a design consideration. Thus it is suitable for error-prone devices such as cell phones. Objects Different parts of the final scene can be coded and transmitted separately as video objects and audio objects. Example: In a soccer game, code the ball and the background separately. Broadcast the background as free TV, but make people pay to see the game with the ball. Text subtitles present a problem to MPEG 1 and 2. They would be compressed using DCT, which does not do well on the sharp edges present in letters. In MPEG-4, several text streams can be transmitted along with the video. The text can be added in by the decoder. Thus the user might select English, Spanish, Chinese or no subtitles. MPEG-4 allows specification of font, color and position for text. MPEG-4 streams allow Encoding multiple objects with different techniques. Objects can be photographic images, computer graphics or text. Different presentations can be selected from the same bitstream. Separation of objects opens up the possibility for interaction. The movie can become a game. Bandwidth can be used efficiently: the foreground can be updated more frequently than the background. A video object consists of Texture (the image) Shape (the alpha channel) The background is just another object with a rectangular shape. MPEG-4 Video Hierarchy: A video object plane (VOP) is a complete spatial representation of an object at one instant in time. It contains both texture and shape. If the object was filmed, the texture comes from a camera. Group of video object planes (GOV). Similar to GOP in earlier MPEGS. VOPs can correspond to I, P and B frames. Video object layer (VOL) is a sequence of GOVs. It supports scaling in space or time. The video object (VO) level includes all VOLs associated with an object. The video session (VS) is the top level that includes all objects. Shapes The shape of an object is an alpha channel mask. If you use a binary mask for a shape, you are likely to get aliasing around the edges. It is better to use anti-aliasing where gray levels are used at the edges of the mask. Simple rectangular masks may be used for sprites or picture-in-picture. The size of a mask must be a multiple of 16; set it for the maximum extent of the object. The macroblocks of a shape are divided into All pixels totally transparent All pixels totally opaque Mixed transparency Coding is trivial for the first two. The other is coded with motion compensated DCT. Textures Previous MPEGs used a subtraction filter to predict the DC value of macroblocks. MPEG-4 uses a more sophisticated technique, similar to the Paeth filter. In addition, MPEG-4 makes a prediction of the first row or column of the AC coefficients based on the macroblock used to predict the DC. The zigzag order for encoding the AC is also varied depending on whether the predictor is from above or to the left. In MPEG 1 and 2, the Huffman codes used for run/category can only be read in the forward direction. Thus if there is a transmission error, everything in the block after the error is lost. In MPEG-4, codes can be read in either direction. If there is an error, go to the end of the macroblock and decode the data backwards until reaching the error. Texture pixels that lie in totally transparent parts of the shape can be set to an arbitrary value, since they are invisible. The value that they are set to is based on the average value of their visible neighbors in the block. The values for the padding are set so that the DCT coefficients have minimum energy. Sprites A sprite is a rectangualr video object. It is usually too large to be displayed at once and used for background. In MPEG-4, it is possible to transmit the sprite once, and then send information on how to crop it or warp it. To speed up start time, it is possible to transmit the sprite in sections. Thus only the initially visible part need be sent. If the camera pans to one side, then the next required section can be transmitted. Another opotion is to progressively encode the sprite; start with low resolution background and improve on it as bandwidth becomes available. Sprites are always I-encoded, since they do not change frame to frame. MPEG-4 supports animation. The motion of a 3-D mesh is specified. Motion prediction is used to tell where each node goes. A static texture can be mapped onto the mesh at the decoder level. Wavelet based compression is used for the texture, which permits it to work well over a range of scales. Binary Format for Scenes (BIFS) BIFS is based on VRML (virtual reality modeling language). It specifies how the decoder brings together separate objects to create a full scene. Objects are formed from component parts in a hierarchical structure. All interactive devices must produce BIFS commands. Since the final scene is rendered by the decoder, the positions and velocities of objects may have come from a film or from a player using a joystick. Studio profiles allow image sizes of 4000 x 2000 pixels with up to 1.2 Gbits / sec. These may be in either YUV or RGB and do not require chromatic compression. Up to 12 bits per color component is supported. Since objects are kept separate, the original studio does not have to do the compositing; that can be left to the decoder. 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