Both AVC/H.264 and HEVC/H.265 supports quality (SNR) and spatial scalability.
The Quality (SNR) layering means that an encoder employs coarse quantization (high QPs) for the base layer, and successively finer quantization (smaller QPs) for the enhancement layers and as a result the quality is successively improved . from lower layers to higher ones.
The Quality Scalability enables a network node to drop an enhancement layer(s) if there a network congestion is sensed or reported. Notice that removal of enhanced level(s) is a trivial operation (e.g. for AVC/H.264 each NAL with a quality_id above a specified value is discarded).
Spatial scalability is similar to the quality one – each layer is encoded with progressively increasing resolution.
Example Quality (SNR) Scalability:
Example Spatial Scalability:
Note: the SNR scalability consumes less cycles than the spatial one, since downscaling and upscaling are not used in the SNR mode.
The problem is about the decoder availability to support scalable video due to complexity. On a decoder’s side the process of decoding of scalable video tends to be time consuming (especially spatial scalability due to upsampling), required much more cycles to decode two-layered stream than decoding same quality non-scalable stream.
It’s easy to keep tons of replicas of same sequence with different bitrates and resolutions as a remedy against network congestion and heterogeneity of end-user devices.
In some applications scalability is used to enhance of error resilience, if a bitstream error or a packet loss occurs in an enhancement layer a decoder can switch to a lower layer until next key frame received.
23+ years’ programming and theoretical experience in the computer science fields such as video compression, media streaming and artificial intelligence (co-author of several papers and patents).
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