Layered coding

Layered coding is a type of data compression for digital video or digital audio where the result of compressing the source video data is not just one compressed data stream, as in other types of compression, but multiple streams, called layers, allowing decompression even if some layers are missing.

Overview
With layered coding, multiple data streams or layers are created when compressing the original video stream. This is in contrast to other types of compression, where the result is typically a single data stream.

During decompression, all layers can be combined to recreate the original video stream. Additionally, the stream can be decoded even if some layers are missing (though usually a layer hierarchy has to be respected, with a base layer that must available). If layers are missing, the resulting stream will have reduced visual quality, but will still be usable.

Use cases
Layered coding is helpful when the same video stream needs to be available in different qualities, for example for adaptive bitrate streaming. Without layered coding, the source video stream must be encoded multiple times to obtain compressed streams with different qualities and bitrates. Layered coding allows only encoding a single time, because streams with different qualities can be obtained by discarding layers.

Related technologies
Layered coding is similar to multiple description coding in that both produce multiple compressed streams that can be combined. However, with multiple description coding the different streams are independent of each other, so any subset can be decoded, providing additional flexibility.

Scalable Video Coding is a video compression standard that makes use of layered coding.