Overview of the High Efficiency Video Coding(HEVC) Standard之五

V. 档次，层，和级
Profiles, Tiers, and Levels

A. 档次，级， 和层的概念 
Profile, Level, and Tier Concepts

Profiles, tiers, and levels specify conformance points for implementing the standard 
in an interoperable way across various applications that have similar functional 
requirements. A profile defines a set of coding tools or algorithms that can be used
in generating a conforming bitstream, whereas a level places constraints on certain 
key parameters of the bitstream, corresponding to decoder processing load and memory 
capabilities. 
为了使标准在跨平台的应用环境(它们有相同的功能需求)中有相同的效果，档次，层， 和组指定了
它们之间的一致性。
档次 定义了一个编码工具或算法集合，它用来生成符合要求的码流;
级   对码流的一些关键参数进行了限制，对应要解码的处理负载和存储开销。


Level restrictions are established in terms of maximum samplerate, maximum picture size, 
maximum bit rate, minimum compression ratio and capacities of the DPB, and the coded
picture buffer (CPB) that holds compressed data prior to its decoding for data flow 
management purposes. In the design of HEVC, it was determined that some applications 
existed that had requirements that differed only in terms of maximum bit rate and CPB 
capacities. To resolve this issue, two tiers were specified for some levels—a Main 
Tier for most applications and a High Tier for use in the most demanding applications.
级  通过以下参数来进行限制,包括有:
. 最大采样率(也就是帧率)，
. 最大图像尺寸，
. 最大比特率，
. 最小压缩率和DPB能力，
. 编码图像buffer(CPB),为了数据流的管理，它先于解码之前存储了码流数据;
在HEVC的设计中，有些应用环境只对最大比特率和CPB容量有不同的要求，
为了解决这个问题，两个层被指定成现一个级:
. 主层用于绝大多数的场景，
. 高层用于特定的场景

A decoder conforming to a certain tier and level is required to be capable of decoding 
all bitstreams that conform to the same tier or the lower tier of that level or any 
level below it. Decoders conforming to a specific profile must support all features 
in that profile. Encoders are not required to make use of any particular set of features 
supported in a profile, but are required to produce conforming bitstreams, 
i.e., bitstreams that obey the specified constraints that enable them to be decoded
by conforming decoders.
符合某个层和级的解码器要求能解码符合相同层和低层，以及它们包含的同级和低级的所有码流。
解码器要符合某个特定的档次，必须支持这个档次的所有功能 。
编码器不要求支持某个档次的所有功能。但它要求生成符合要求的码流，
例如，码流要遵守特定的限制以使符合要求的解码器能解码。

B. HEVC的档次和级的定义
The HEVC Profile and Level Definitions

Only three profiles targetting different application requirements, called the Main, 
Main 10, and Main Still Picture profiles, are foreseen to be finalized by January 2013. 
Minimizing the number of profiles provides a maximum amount of interoperability between 
devices, and is further justified by the fact that traditionally separate services, 
such as broadcast, mobile, streaming, are converging to the point where most
devices should become usable to support all of them. The three drafted profiles consist 
of the coding tools and highlayer syntax described in the earlier sections of this paper,
while imposing the following restrictions.
HEVC在2013年1月结案时只对不同的应用环境要求定义了三个档次，分别是:
. Main,
. Main 10, 
. Main Still Picture
档次的最小序号提供了设备间的最大互通性，它将更多的判断留给了传统的设备区分，如
广播，移动设备，直播等都支持。
草案中的三个档次对编码工具和更高级的语法限制如下:

1) Only 4:2:0 chroma sampling is supported.
   只支持4:2:0色度样本.

2) When an encoder encodes a picture using multiple tiles,
   it cannot also use wavefront parallel processing, and
   each tile must be at least 256 luma samples wide and
   64 luma samples tall.
   当编码器使用多个瓦片编码一帧图像时，它不能使用波前并行处理，
   并且每个瓦片至少要有256个亮度像素宽了64个亮度像素高。

3) In the Main and Main Still Picture profiles, only a video
   precision of 8 b per sample is supported, while the Main
   10 profile supports up to 10 b per sample.
   Main和 Main Still Picture档次中，只支持8比特每像素数度;
   Main 10档次支持10比特每像素精度。

4) In the Main Still Picture profile, the entire bitstream must
   contain only one coded picture (and thus interpicture
   prediction is not supported).
   Main Still Picture档次中，整个比特流只包含一个编码图像;

TABLE V
Level Limits for the Main Profile

​
Currently, the definition of 13 levels is planned to be included in the first version of 
the standard as shown in Table V, ranging from levels that support only relatively small 
picture sizes such as a luma picture size of 176×144 (sometimes called a quarter common 
intermediate format) to picture sizes as large as 7680×4320 (often called 8k×4k). The 
picture width and height are each required to be less than or equal to  √8 · MaxLumaPS, 
where MaxLumaPS is the maximum luma picture size as shown in Table V (to avoid the problems 
for decoders that could be involved with extreme picture shapes).
当前，标准的第一版只支持13级的定义，如表中所示;
图像尺寸的范围从176x144到7680×4320 (often called 8k×4k).
图像尺寸的宽度和高度均要求小于或等于
   √8 · MaxLumaPS.
   MaxLumaPS是表中的最大亮度图像尺寸

There are two tiers supported for eight of these levels (level 4 and higher). The CPB 
capacity is equal to the maximum bit rate times 1 s for all levels except level 1,
which has a (higher) CPB capacity of 350 000 b. The specified maximum DPB capacity in 
each level is six pictures when operating at the maximum picture size supported by the 
level (including both the current picture and all other pictures that are retained in 
the decoder at any point in time for reference or output purposes). When operating with 
a smaller picture size than the maximum size supported by the level, the DPB picture 
storage capacity can increase to as many as 16 pictures (depending on the particular 
selected picture size). Level specific constraints are also specified for the maximum 
number of tiles used horizontally and vertically within each picture and the maximum 
number of tiles used per second.
级4之上的8个级两个瓦片都越支持;
CPB容量等于除级1之外的所有级的最大比特率，级1的CPB容量为350 000 比特。
. 当对级中支持的最大图像尺寸操作时，
    每个级特定的最大的DPB容量6帧图像,
    这是为了当前图像和所有要用于参数的图像都留在buffer中。
. 当对级中支持的小于最大图像尺寸的图像操作时，
    DPB的图像存储容量可以增加到16帧图像。
级的限制对每秒中每个图像的最大数量的瓦片的水平和垂直都起作用。