Context adaptivity

where two 8× 8 blocks abut. Many MPEG-2 decoders include post-processing to mitigate the effects of blocking artifacts, but treatment after the fact (“out of the loop”) is invisible to the encoder.

H.264 standardizes an adaptive, in the loop deblocking filter. The filter adapts to picture content, such as edges (which typically cause the worst artifacts). Deblocking is standardized, and it takes place within the encoder’s prediction loop.

Buffer control

In MPEG-2, a video buffer verification (VBV) value is transmitted from the encoder to the decoder, to ensure that the decoder’s buffer tracks that of the encoder without underflowing or overflowing.

In H.264 it’s more complicated. Memory occupancy is tracked at both the encoder and the decoder for two hypothetical buffers: the coded picture buffer (CPB), representing pictures in the coded bitstream; and the decoded picture buffer (DPB), representing pictures after decoding.

Scalable video coding (SVC)

Scalable video coding – defined in Annex G of H.264 – allows conveyance of information structured in a hierarchical manner to allow portions of the bitstream to be extracted at lower bit rate than the complete sequence to enable decoding of pictures with multiple image structures (for sequences encoded with spatial scalability), pictures at multiple picture rates (for sequences encoded with temporal scalability), and/or pictures with multiple levels of image quality (for sequences encoded with SNR/quality scalability).

In a single bitstream, a decoder having limited computational resources can extract the base bitstream to decode a low-level representation. (No data rate advantage accrues in this case.)

Different layers can be separated into different bitstreams. All decoders access the base stream; more capable decoders can access enhancement streams. However, for some applications – like HTTP live streaming – it may be more efficient to encode a single program at several different rates, each in a self-

Table 48.6 AVC-Intra profile/level combinations are summarized.

contained stream, so that a decoder can simply access a single stream at a suitable rate.

Multiview video coding (MVC)

Multiview video coding – Annex H of H.264 – standardizes features to efficiently code two (or potentially more than two) pictures that are highly spatially correlated.

The common application is to code the left and right images of a stereo pair.

MVC adds two new profiles, multiview high profile (MHP) and stereo high profile (SHP). Video encoded with either of these profiles is backward compatible with H.264 high profile; the decoder sees just the base view. (In the case of SHP, this is typically the left eye.)

AVC-Intra

AVC-Intra is Panasonic’s notation for studio-quality H.264 using Hi10Intra profile. Video in 720p, 1080i, or 1080p format at various frame rates is represented in 10-bit Y’CBCR 4:2:2 components. In AVC-Intra 50, 720p, 1080i, and 1080p video is downsampled and compressed to 50 Mb/s. In AVC-Intra 100, 720p, 1080i, and 1080p video at native pixel count are compressed to 100 Mb/s. Table 48.6 summarizes.

Further reading

Richardson, Iain E.G. (2010), The H.264 Advanced Video Compression Standard (Chichester, U.K.: Wiley).

Sullivan, Gary J., Pankaj N. Topiwala, and Ajay Luthra

(2004), “The H.264/AVC advanced video coding standard: Overview and introduction to the fidelity range extensions,” in Proc. SPIE 5558: 454–474.

Sullivan, Gary J. and Thomas Wiegand (2005), “Video compression – From concepts to the H.264/AVC standard,” in Proc. IEEE 93 (1): 18–31.