H.264/AVC Layer Structure
H.264 的主要目標(biāo)：
1）高的視頻壓縮比，當(dāng)初提出的指標(biāo)是比H.263，MPEG-4，約為它們的2 倍，現(xiàn)在都已基本實(shí)現(xiàn)；
2）良好的網(wǎng)絡(luò)親和性，即可適用于各種傳輸網(wǎng)絡(luò)。
為此，H.264 的功能分為兩層，即視頻編碼層（VCL）和網(wǎng)絡(luò)提取層（NAL)。
VCL 數(shù)據(jù)即編碼處理的輸出，它表示被壓縮編碼后的視頻數(shù)據(jù)序列。
在VCL 數(shù)據(jù)傳輸或存儲(chǔ)之前，這些編碼的VCL 數(shù)據(jù)，先被映射或封裝進(jìn)NAL 單元中。
Video Coding Layer
A layer in H.264/AVC and HEVC that contains coded video data.
The coded video data in VCL layer will be wraped(包起來(lái)??) under NAL for transmission.
Network Abstraction Layer
The Network Abstraction Layer (NAL) is a part of the H.264/AVC and HEVC video coding standards.
The main goal of the NAL is the provision(供應(yīng)) of a "network-friendly" video representation addressing "conversational(雙向的)" (video telephony: 視頻電話) and "non conversational" (storage, broadcast, or streaming) applications.
NAL has achieved a significant improvement in application flexibility relative to prior video coding standards.
NAL units
The coded video data is organized into NAL units, each of which is effectively a packet that contains an integer number of bytes.
The first byte of each H.264/AVC NAL unit is a header byte that contains an indication of the type of data in the NAL unit.
For HEVC the header was extended to two bytes.
All the remaining bytes contain payload data of the type indicated by the header. The NAL unit structure definition specifies a generic format for use in both packet-oriented and bitstream-oriented transport systems, and a series of NAL units generated by an encoder is referred to as a NAL unit stream.
H.264中NAL unit的第一個(gè)字節(jié)表示數(shù)據(jù)的類型.
nal_unit_type:
- Coded slice : 1
- IDR : 5
- SEI : 6
- SPS : 7
- PPS : 8
每個(gè)NAL 單元包括一個(gè)原始字節(jié)序列負(fù)荷（RBSP）、一組對(duì)應(yīng)于視頻編碼數(shù)據(jù)的NAL 頭信息。 NAL 單元序列的結(jié)構(gòu)見(jiàn)圖:
NAL unit 序列的結(jié)構(gòu)
VCL and Non-VCL NAL Units
NAL units are classified into VCL and non-VCL NAL units.
- The VCL NAL units
The VCL NAL units contain the data that represents the values of the samples in the video pictures. - The non-VCL NAL units
The non-VCL NAL units contain any associated additional information such as parameter sets (important header data that can apply to a large number of VCL NAL units) and supplemental enhancement information (timing information and other supplemental data that may enhance usability of the decoded video signal but are not necessary for decoding the values of the samples in the video pictures).
包含圖像數(shù)據(jù)的unit屬于VCL NAL units.
SPS、PPS、和SEI 屬于Non-VCL NAL Units;
References:
https://en.wikipedia.org/wiki/Video_Coding_Layer
https://en.wikipedia.org/wiki/Network_Abstraction_Layer
http://iphome.hhi.de/wiegand/assets/pdfs/DIC_H264_07.pdf
新一代視頻壓縮編碼標(biāo)準(zhǔn) ——H.264/AVC 畢厚杰主編
