Techniques and tools for deriving chroma motion vectors for macroblocks of interlaced forward-predicted fields are described. For example, a video encoder or decoder determines a prevailing polarity among luma motion vectors for a macroblock. The encoder or decoder then determines a chroma motion vector for the macroblock based at least in part upon one or more of the luma motion vectors having the prevailing polarity.

Techniques and tools for encoding and decoding predicted images in interlaced video are described. For example, a video encoder or decoder computes a motion vector predictor for a motion vector for a portion (e.g., a block or macroblock) of an interlaced P-field, including selecting between using a same polarity or opposite polarity motion vector predictor for the portion. The encoder/decoder processes the motion vector based at least in part on the motion vector predictor computed for the motio...

Techniques and tools for signaling the number of reference fields for an interlaced forward-predicted field are described. For example, a video decoder processes a first signal indicating whether an interlaced forward-predicted field has one or two reference fields for motion compensation. If the first signal indicates the interlaced forward-predicted field has one reference field, the decoder processes a second signal identifying the one reference field from among the two reference fields. On t...

For interlaced B-fields or interlaced B-frames, an encoder/decoder uses 4MV coding. For example, 4MV is used in one-direction prediction modes (forward or backward modes), but not in other available prediction modes (e.g., direct, interpolated). Using 4MV allows more accurate motion compensation for interlaced B-fields and interlaced B-frames; limiting 4MV to forward and backward modes reduces coding overhead and avoids decoding complexity associated with combining 4MV with modes such as direct ...

Techniques and tools for extended range variable length coding and decoding of differential motion vector information are described. For example, a video decoder determines whether or not to use an extended range variable length decoding mode. When the extended range variable length decoding mode is used, the decoder decodes a differential motion vector in the extended range variable length decoding mode. Otherwise, the decoder decodes the differential motion vector in a default range variable l...

Techniques and tools for sub-block transform coding are described. For example, a video encoder adaptively switches between 8.times.8, 8.times.4, and 4.times.8 DCTs when encoding 8.times.8 prediction residual blocks; a corresponding video decoder switches between 8.times.8, 8.times.4, and 4.times.8 inverse DCTs during decoding. The video encoder may determine the transform sizes as well as switching levels (e.g., frame, macroblock, or block) in a closed loop evaluation of the different transform...

Techniques and tools for escape mode code resizing are described. For example, a video decoder receives encoded information (e.g., runs, levels) for transform coefficients of blocks. For at least some of the encoded information, the decoder decodes in an escape mode for which codes have sizes signaled on a sub-frame basis (e.g., on a per-interlaced field basis in a video frame, or on a per-slice basis in a video frame). A video encoder performs corresponding encoding and signaling.

In one aspect, an encoder/decoder selects a bitplane mode from a group of plural available bitplane modes, and processes a bitplane according to the selected bitplane mode, wherein the bitplane indicates AC prediction status information for plural macroblocks of a video picture. In another aspect, an encoder encodes a bitplane that indicates AC prediction status information for plural macroblocks of a video picture and signals the encoded bitplane. In another aspect, a decoder receives an encode...

Techniques and tools for using motion vector block patterns in video encoding and decoding are described. In general, a motion vector block pattern signals the presence or absence of motion vector data for a macroblock with multiple motion vectors. For example, a video decoder decodes variable length codes that represent motion vector block patterns. Each motion vector block pattern has one bit per corresponding luminance motion vector of a macroblock with multiple luminance motion vectors, wher...

With adaptive multiple quantization, a video or other digital media codec can adaptively select among multiple quantizers to apply to transform coefficients based on content or bit rate constraints, so as to improve quality through rate-distortion optimization. The switch in quantizers can be signaled at the sequence level or frame level of the bitstream syntax, or can be implicitly specified in the syntax.