Data for each picture consists of a picture header followed
by data for Group of Blocks, eventually followed by an end-of-sequence
code and stuffing bits. The structure is shown in Figure 5.
PSBI is only present if indicated by CPM. TRB and
DBQUANT are only present if PTYPE indicates "PB-frame".Combinations
of PSPARE and PEI may not be present. EOS may not be present, while ESTUF
may be present only if EOS is present. Picture headers for dropped pictures
are not transmitted.
PSC is a word of 22 bits. Its value is 0000 0000 0000
0000 1 00000. All picture start codes shall be byte aligned. This shall
be achieved by inserting PSTUF before the start code such that the first
bit of the start code is the first (most significant) bit of a byte.
Picture Start Code (PSC)|
It is formed by incrementing its value in the previously
transmitted picture header by one plus the number of non-transmitted pictures
(at 29.97 Hz) since the previously transmitted one. The arithmetic is performed
with only the eight LSBs. In the optional PB-frames mode, TR only addresses
P-pictures; for the temporal reference for B-pictures.
Temporal Reference (TR)|
Information about the complete picture:
Type Information (PTYPE)|
Bit 1: Always "1" in order to avoid start
Bit 2: Always "0" for distinction with Recommendation
Bit 3: Split screen indicator, "0" off, "1"
Bit 4: Document camera indicator, "0" off,
Bit 5: Freeze Picture Release, "0" off, "1"
Bits 6-8: Source Format, "000" forbidden,
"001" sub-QCIF, "010" QCIF,
"011" CIF, "100" 4CIF, "101" 16CIF, "110" reserved, "111" reserved.
Bit 9: Picture Coding Type, "0" INTRA (I-picture),
"1" INTER (P-picture).
Bit 10: Optional Unrestricted Motion Vector
mode, "0" off, "1" on.
Bit 11: Optional Syntax-based Arithmetic
Coding mode, "0" off, "1" on.
Bit 12: Optional Advanced Prediction mode,
"0" off, "1" on.
Bit 13: Optional PB-frames mode, "0" normal
I- or P-picture, "1" PB-frame.
Split screen indicator is a signal that indicates
that the upper and lower half of the decoded picture could be displayed
side by side. This bit has no direct effect on the encoding or decoding
of the picture.
Freeze Picture Release is a signal from an encoder
which responds to a request for packet retransmission (if not acknowledged)
or fast update request (Reference also Annex C of Recommendation H.263)
and allows a decoder to exit from its freeze picture mode and display decoded
picture in the normal manner.
If bits 6-8 indicate a different source format than
in the previous picture header, the current picture shall be an I-picture.
Bits 10-13 refer to optional modes that are only used after negotiation
between encoder and decoder (see also the Annexes D, E, F and G of Recommendation
H.263, respectively). If bit 9 is set to "0", bit 13 shall be set to "0"
A fixed length codeword of 5 bits which indicates the
quantizer QUANT to be used for the picture until updated by any subsequent
GQUANT or DQUANT. The codewords are the natural binary representations
of the values of QUANT which, being half the stepsizes, range from 1 to
Quantizer Information (PQUANT)|
A codeword of 1 bit that signals the use of the optional
Continuous Presence Multipoint mode (CPM); "0" is off, "1" is on.
Continuous Presence Multipoint (CPM)|
A fixed length codeword of 2 bits that is only present
if Continuous Presence Multipoint mode is indicated by CPM. The codewords
are the natural binary representation of the sub-bit stream number for
the picture header and all following information until the next Picture
or GOB start code.
Picture Sub-Bit stream Indicator (PSBI)|
TRB is present if PTYPE indicates "PB-frame" and indicates
the number of non-transmitted pictures (at 29.97 Hz) since the last P-
or I-picture and before the B-picture. The codeword is the natural binary
representation of the number of non-transmitted pictures plus one. The
maximum number of non-transmitted pictures is 6.
Temporal Reference for B-picture (TRB)|
DBQUANT is present if PTYPE indicates "PB-frame". In
the decoding process a quantization parameter QUANT is obtained for each
macroblock. With PB-frames QUANT is used for the P-block, while for the
B-block a different quantization parameter BQUANT is used. QUANT ranges
from 1 to 31. DBQUANT indicates the relation between QUANT and BQUANT as
defined follow. In this table, "/" means division by truncation. BQUANT
ranges from 1 to 31; if the value for BQUANT resulting from the following
table is greater than 31, it is clipped to 31.
Quantization information for B-pictures (DBQUANT)|
A bit which when set to "1" signals the presence of
the following optional data field.
Extra Insertion Information (PEI)|
If PEI is set to "1", then 9 bits follow consisting
of 8 bits of data (PSPARE) and then another PEI bit to indicate if a further
9 bits follow and so on. Encoders shall not insert PSPARE until specified
by the ITU. Decoders shall be designed to discard PSPARE if PEI is set
to 1. This will allow the ITU to specify future backward compatible additions
in PSPARE. If PSPARE is followed by PEI = 0, PSPARE = xx000000 is prohibited
in order to avoid start code emulation.
Spare Information (PSPARE)|
A codeword of variable length consisting of less than
8 zero-bits. Encoders may insert this codeword directly before an EOS codeword.
If ESTUF is present, the last bit of ESTUF shall be the last (least significant)
bit of a byte, so that the start of the EOS codeword is byte aligned. Decoders
shall be designed to discard ESTUF.
A codeword of 22 bits. Its value is 0000 0000 0000 0000
1 11111. It is up to the encoder to insert this codeword or not. EOS may
be byte aligned.
End Of Sequence (EOS)|
A codeword of variable length consisting of less than
8 zero-bits. Encoders shall insert this codeword for byte alignment of
the next PSC. The last bit of PSTUF shall be the last (least significant)
bit of a byte, so that the video bit stream including PSTUF is a multiple
of 8 bits from the first bit in the H.263 bit stream. Decoders shall be
designed to discard PSTUF.
If for some reason the encoder stops encoding pictures
for a certain time-period and resumes encoding later, PSTUF shall be transmitted
before the encoder stops, to prevent that the last up to 7 bits of the
previous picture are not sent until the coder resumes coding.