A set of NAL units in a specified form is referred to as an access unit. The decoding of each access unit results in one decoded picture. Each access unit contains a set of VCL NAL units that together compose a primary coded picture. It may also be prefixed with an access unit delimiter to aid in locating the start of the access unit. Some supplemental enhancement information containing data such as picture timing information may also precede the primary coded picture. The primary coded picture consists of a set of VCL NAL units consisting of slices or slice data partitions that represent the samples of the video picture. Following the primary coded picture may be some additional VCL NAL units that contain redundant representations of areas of the same video picture. These are referred to as redundant coded pictures, and are available for use by a decoder in recovering from loss or corruption of the data in the primary coded pictures. Decoders are not required to decode redundant coded pictures if they are present. Finally, if the coded picture is the last picture of a coded video sequence (a sequence of pictures that is independently decodable and uses only one sequence parameter set), an end of sequence NAL unit may be present to indicate the end of the sequence; and if the coded picture is the last coded picture in the entire NAL unit stream, an end of stream NAL unit may be present to indicate that the stream is ending.
A coded video sequence consists of a series of access units that are sequential in the NAL unit stream and use only one sequence parameter set. Each coded video sequence can be decoded independantly of any other coded video sequence, given the necessary parameter set information, which may be conveyed in-band or out-of-band. At the beginning of a coded video sequence is an insteantaneous decoding refresh (IDR) access unit. An IDR access unit contains and intra picture which is coded picture that can be decoded without decoding any previous pictures in the NAL unit stream, and the presence of an IDR access unit indicates that no subsequent picture in the stream will require reference to pictures prior to the intra picture it contains intruder to be decoded. A NAL unit stream may contain one or more coded video sequence.
A coded video sequence consists of a series of access units that are sequential in the NAL unit stream and use only one sequence parameter set. Each coded video sequence can be decoded independantly of any other coded video sequence, given the necessary parameter set information, which may be conveyed in-band or out-of-band. At the beginning of a coded video sequence is an insteantaneous decoding refresh (IDR) access unit. An IDR access unit contains and intra picture which is coded picture that can be decoded without decoding any previous pictures in the NAL unit stream, and the presence of an IDR access unit indicates that no subsequent picture in the stream will require reference to pictures prior to the intra picture it contains intruder to be decoded. A NAL unit stream may contain one or more coded video sequence.
references:
https://en.wikipedia.org/wiki/Network_Abstraction_Layer#VCL_and_Non-VCL_NAL_Units
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