[Mp4-tech][video] Choice of POC mode

[Gary Sullivan]

Angel Zhang et al,
Your question (about POC) is sort of a frequently-asked question for us.
Here is a basic description of picture order count (POC) and the three
types of POC counting that are supported in the standard:
The basic concept of POC is to provide a counter that specifies the
relative order of the pictures in the bitstream in output order (which
may differ from the relative order in which the coded pictures appear in
the data of the bitstream, which is referred to as the decoding order).
The relative order of the pictures is indicated in POC, rather than the
timing of the pictures.  This allows systems that carry the video
bitstream to control the exact timing of the processing and output of
the video bitstream without affecting the decoding process for the
values of the samples in the luma and chroma sample arrays of the
pictures.  In some cases, the values of the samples in the luma and
chroma sample arrays will depend on POC values.  However, the values of
the samples in the luma and chroma sample arrays will never depend on
the timing of the pictures.
There are three modes of POC operation:
In POC type 0, each slice header contains a simple fixed-length counter
syntax element (pic_order_cnt_lsb) that provides the LSBs of the current
POC.  The MSBs of the current POC are calculated by the decoder by
tracking modulus wrapping in the LSBs.
In POC type 1, each slice header contains one or two
variable-length-encoded syntax elements that provide the difference to
apply to a prediction of the current POC to compute the actual current
POC.  This POC type provides the encoder with the ability to encode the
POC values using significantly fewer bits per slice than what would
otherwise be needed when using POC type 0 in cases where the encoder
will usually be using a repetitive pattern of POC behavior.
In POC type 2, no data is carried in the slice header to compute the
current POC.  When POC type 2 is in use, the output order of the
pictures in the bitstream will be the same as the order in which the
coded pictures appear in the data of the bitstream.  This POC type
eliminates the need for the encoder to send any syntax data in the slice
header for POC derivation.  However, it provides no flexibility to allow
the output order of the pictures in the bitstream to differ from their
decoding order.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: AngelZhang innosis.com.cn
+> Sent: Thursday, August 04, 2023 12:42 AM
+> Subject: [Mp4-tech] Choice of POC mode
+> 
+> hi,experts
+> there are three POC mode choice in the H.264 reference 
+> encoder, but i don't
+> know what's the feature and difference between them.
+> so please give me some suggestions or hints. or give me some 
+> materials as
+> well!
+> 
+> 
+> best regars!
+> angel zhang