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Rectangular-shaped video (called a texture) is coded in much the same way as it is in MPEG-2, except that the dimensions of the rectangle can be arbitrarily chosen. The way arbitrary-shaped video objects are coded makes use of an alpha channel. The actual video object is defined over a rectangular area, called a mask. The mask is sized to accommodate the greatest horizontal and vertical extent of the video, to the nearest 16 pixel multiple. The alpha channel denotes whether or not the underlying video pixels are visible. Shapes may be coded as binary or gray-scale data. Binary shapes are the simplest, with the alpha data merely controlling whether a pixel is visible or transparent. Unfortunately, this leads to edge effects that violate the Nyquist sampling theorem, and can lead to unwanted visible edge aliasing artifacts. Grayscale shapes, in contrast, permit a smooth, bandwidth-controlled transition between object and background. This alpha blending technique makes arbitraryshaped video objects appear more realistic. Binary shapes find application for simpler objects, where edge aliasing is not an issue. Burning a rectangular picture into a video session scene would be an example. Coding of binary alpha blocks is trivial for wholly transparent or opaque pixels. However, edge pixels representing the boundary of the shape are coded using techniques derived from IBM s patented arithmetic coding, an entropy encoding technique, which, unlike Huffman coding, self-
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adjusts its codeword choices according to the statistics of the input entropy that it keeps. Context-Based Arithmetic Encoding. The algorithm used to encode alpha channel bits with partial transparency is called contextbased arithmetic encoding (CAE). MPEG-4 extends the algorithm to include the use of motion estimation. As with regular arithmetic encoding, coding is based on a continuously updated probability estimate for each incoming symbol. In the basic intraCAE, the probability estimate is computed from ten pixels, above and to the left of the pixel being encoded. For interCAE, where motion estimation and prediction are used, pixels from the current video object plane (VOP) and some from the reference VOP (an I-VOP or P-VOP) form the basis for the probability estimate. Grayscale shapes are represented in a similar way to luminance signals. The pixels are usually quantized to 8-bit resolution, with 0 representing total transparency and 255 representing total opacity. The image plane, as with all MPEG video, is divided into macroblocks. Macroblocks falling entirely outside the boundary of the object are marked as all zero (i.e., fully transparent) and those wholly within the boundary of the object marked as all 255 (i.e., fully opaque). Macroblocks that contain any part of the shape s boundary are coded in a similar way to the underlying video texture, using motion-compensated DCT coding.
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Texture Coding
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Texture coding is analogous to conventional moving image coding in MPEG-2. The coding method builds upon earlier work from the JPEG and MPEG standards. In all MPEG-4 profiles, except for the studio profiles, 4:2:0 YUV video representations are used to describe textures. Although a rectangular area represents the extent of the video object, however, only those macroblocks in the image made visible by the shape signal are actually encoded. In other words, texture coding is performed only for those macroblocks wholly or partially within the boundary of the arbitrary shape. I-VOPs are coded in much the same way as MPEG-2 I-frames, except efficiency has been improved by using a more efficient predictor. In MPEG-4, the predictor measures horizontal and vertical brightness gradients and predicts the dc coefficient value from the block above or the block to the left, in the direction of the lesser gradient.
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Appendix E
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We know from our discussion of MPEG-2 that correlation helps in compression, because correlation identifies image redundancy. Image areas of similar texture have similar ac coefficients, after DCT. The most significant coefficients are the lower order ones, which contain the greatest proportion of the image s energy. These are the non-zero coefficients in the first row and/or column of the coefficient matrix for the DCT-coded macroblock and these coefficients are quantized least aggressively, since quantization errors in these coefficients are the most visible. In MPEG-4, the ac coefficients of either the first row or column are predicted from those of the block immediately above, the block to the immediate left, or the block diagonally above-left. We also described, in our discussion of MPEG-2, how coefficients are scanned (zigzag or alternate) and variable length encoded. MPEG-4 improves upon both scanning and variable length coding. The method chosen for coefficient readout is determined by the dc prediction. When there is no dc prediction, the MPEG-2 zigzag scanning technique is used. If the dc coefficient was predicted from the block to the left, alternatevertical scanning is used (a scanning system biased toward reading out the vertical coefficients first). Unsurprisingly, if the dc coefficient was predicted from the block above, alternate-horizontal scanning is used and the horizontal coefficients are read out first. To improve the efficiency of variable length encoding, two different VLC tables are provided in MPEG-4. The quantization level determines the choice of table. The VLC codes are reversible, for error resilience. If there is an error in the bit stream, decoding can continue up to the error. Data after the error may be decoded by starting at the end of the block and decoding the VLC codes in reverse until the error is reached.
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