Contents Page

This section describes the output of the theoretical model of the decoding process that decodes bitstreams conforming to this specification.

The decoding process input is one or more coded video bitstreams (one for each of the layers). The video layers are generally multiplexed by the means of a system stream that also contains timing information.

The output of the decoding process is a series of fields or frames that are normally the input of a display process. The order in which fields or frames are output by the decoding process is called the display order, and may be different from the coded order (when B-pictures are used). The display process is responsible for the action of displaying the decoded fields or frames on a display device. If the display device cannot display at the frame rate indicated in the bitstream, the display process may perform frame rate conversion. This specification does not describe a theoretical model of display process nor the operation of the display process.

Since some of the syntax elements, such as progressive_frame, may be needed by the display process, in this theoretical model of the decoding process, all the syntactic elements that are decoded by the decoding process are output by the decoding process and may be accessed by the display process.

When the a progressive sequence is decoded (progressive_sequence is equal to 1), the luminance and chrominance samples of the reconstructed frames are output by decoding process in the form of progressive frames and the output rate is the frame rate. Figure 7-18 illustrates this in the case of chroma_format equals to 4:2:0.

The same reconstructed frame is output one time if repeat_first_field is equal to 0, and two or three consecutive times if repeat_first_field is equal to 1, depending on the value of top_field_first. Figure 7-19 illustrates this in the case of chroma_format equals to 4:2:0 and repeat_first_field equals 1.

When decoding an interlaced sequence (progressive_sequence is equal to 0), the luminance samples of the reconstructed frames are output by the decoding process in the form of interlaced fields at a rate that is twice the frame rate. Figure 7-20 illustrates this.

It is a requirement on the bitstream that the fields at the output of the decoding process shall always be alternately top and bottom (note that the very first field of a sequence may be either top or bottom).

If the reconstructed frame is interlaced (progressive_frame is equal to 0), the luminance samples and chrominance samples are output by the decoding process in the form of two consecutive fields. The first field output by the decoding process is the top field or the bottom field of the reconstructed frame, depending on the value of top_field_first.

Although all the samples of progressive frames represent the same instant in time, all the samples are not output at the same time by the decoding process when the sequence is interlaced.

If the reconstructed frame is progressive (progressive_frame is equal to 1), the luminance samples are output by the decoding process in the form of two or three consecutive fields, depending on the value of repeat_first_field.

NOTE - The information that these fields originate from the same progressive frame in the bitstream is conveyed to the display process.

All of the chrominance samples of the reconstructed progressive frame are output by the decoding process at the same time as the first field of luminance samples. This is illustrated in Figures 7-21 and 7-22.



Figure 7-21. progressive_sequence == 0 with 4:2:0 chrominance.



Figure 7-22. progressive_sequence == 0 with 4:2:2 or 4:4:4 chrominance.

8 Profiles and levels

Profiles and levels provide a means of defining subsets of the syntax and semantics of this Specification and thereby the decoder capabilities required to decode a particular bitstream. A profile is a defined sub-set of the entire bitstream syntax that is defined by this Specification. A level is a defined set of constraints imposed on parameters in the bitstream. Conformance tests will be carried out against defined profiles at defined levels.

The purpose of defining conformance points in the form of profiles and levels is to facilitate bitstream interchange among different applications. Implementers of this Specification are encouraged to produce decoders and bitstreams which correspond to those defined conformance regions. The discretely defined profiles and levels are the means of bitstream interchange between applications of this Specification.

In this clause the constrained parts of the defined profiles and levels are described. All syntactic elements and parameter values which are not explicitly constrained may take any of the possible values that are allowed by this Specification. In general, a decoder shall be deemed to be conformant to a given profile at a given level if it is able to properly decode all allowed values of all syntactic elements as specified by that profile at that level. One exception to this rule exists in the case of a Simple profile Main level decoder, which must also be able to decode Main profile, Low level bitstreams. A bitstream shall be deemed to be conformant if it does not exceed the allowed range of allowed values and does not include disallowed syntactic elements.

Attention is drawn to 5.4 which defines the convention for specifying a range of numbers. This is used throughout to specify the range of values and parameters.

The profile_and_level_indication in the sequence_extension indicates the profile and level to which the bitstream complies. The meaning of the bits in this parameter is defined in Table 8-1.