2 Rec. ITU-T H.265 (04/2013)
This is the first version of this Specification. Additional versions are anticipated.
0.5 Profiles, tiers and levels
This Recommendation | International Standard is designed to be generic in the sense that it serves a wide range of
applications, bit rates, resolutions, qualities, and services. Applications should cover, among other things, digital storage
media, television broadcasting and real-time communications. In the course of creating this Specification, various
requirements from typical applications have been considered, necessary algorithmic elements have been developed, and
these have been integrated into a single syntax. Hence, this Specification will facilitate video data interchange among
different applications.
Considering the practicality of implementing the full syntax of this Specification, however, a limited number of subsets
of the syntax are also stipulated by means of "profiles", "tiers", and "levels". These and other related terms are formally
defined in clause 3.
A "profile" is a subset of the entire bitstream syntax that is specified in this Recommendation | International Standard.
Within the bounds imposed by the syntax of a given profile it is still possible to require a very large variation in the
performance of encoders and decoders depending upon the values taken by syntax elements in the bitstream such as the
specified size of the decoded pictures. In many applications, it is currently neither practical nor economic to implement
a decoder capable of dealing with all hypothetical uses of the syntax within a particular profile.
In order to deal with this problem, "tiers" and "levels" are specified within each profile. A level of a tier is a specified
set of constraints imposed on values of the syntax elements in the bitstream. These constraints may be simple limits on
values. Alternatively they may take the form of constraints on arithmetic combinations of values (e.g., picture width
multiplied by picture height multiplied by number of pictures decoded per second). A level specified for a lower tier is
more constrained than a level specified for a higher tier.
Coded video content conforming to this Recommendation | International Standard uses a common syntax. In order to
achieve a subset of the complete syntax, flags, parameters, and other syntax elements are included in the bitstream that
signal the presence or absence of syntactic elements that occur later in the bitstream.
0.6 Overview of the design characteristics
The coded representation specified in the syntax is designed to enable a high compression capability for a desired image
or video quality. The algorithm is typically not lossless, as the exact source sample values are typically not preserved
through the encoding and decoding processes. A number of techniques may be used to achieve highly efficient
compression. Encoding algorithms (not specified in this Recommendation | International Standard) may select between
inter and intra coding for block-shaped regions of each picture. Inter coding uses motion vectors for block-based inter
prediction to exploit temporal statistical dependencies between different pictures. Intra coding uses various spatial
prediction modes to exploit spatial statistical dependencies in the source signal for a single picture. Motion vectors and
intra prediction modes may be specified for a variety of block sizes in the picture. The prediction residual may then be
further compressed using a transform to remove spatial correlation inside the transform block before it is quantized,
producing a possibly irreversible process that typically discards less important visual information while forming a close
approximation to the source samples. Finally, the motion vectors or intra prediction modes may also be further
compressed using a variety of prediction mechanisms, and, after prediction, are combined with the quantized transform
coefficient information and encoded using arithmetic coding.
0.7 How to read this Specification
It is suggested that the reader starts with clause 1 (Scope) and moves on to clause 3 (Definitions). Clause 6 should be
read for the geometrical relationship of the source, input, and output of the decoder. Clause 7 (Syntax and semantics)
specifies the order to parse syntax elements from the bitstream. See clauses 7.1–7.3 for syntactical order and see
clause 7.4 for semantics; e.g., the scope, restrictions, and conditions that are imposed on the syntax elements. The actual
parsing for most syntax elements is specified in clause 9 (Parsing process). Clause 10 (Sub-bitstream extraction process)
specifies the sub-bitstream extraction process. Finally, clause 8 (Decoding process) specifies how the syntax elements
are mapped into decoded samples. Throughout reading this Specification, the reader should refer to clauses 2
(Normative references), 4 (Abbreviations), and 5 (Conventions) as needed. Annexes A through E also form an integral
part of this Recommendation | International Standard.
Annex A specifies profiles each being tailored to certain application domains, and defines the so-called tiers and levels
of the profiles. Annex B specifies syntax and semantics of a byte stream format for delivery of coded video as an
ordered stream of bytes. Annex C specifies the hypothetical reference decoder, bitstream conformance, decoder
conformance, and the use of the hypothetical reference decoder to check bitstream and decoder conformance. Annex D
specifies syntax and semantics for supplemental enhancement information message payloads. Annex E specifies syntax
and semantics of the video usability information parameters of the sequence parameter set.