[Mp4-tech] About the definition of num_ref_frames

[Gary Sullivan]

Oops, I left out 3 important characters  "Min"
MaxDpbSize is equal to Min( 1024 * MaxDPB / ( PicWidthInMbs * FrameHeightInMbs * 384 ), 16 )
-Gary
________________________________
	From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Gary Sullivan
	Sent: Wednesday, April 20, 2024 5:49 PM
	To: 皇斌(Huang-Bin); mp4-tech lists.mpegif.org
	Subject: RE: [Mp4-tech] About the definition of num_ref_frames
	H.B. et al,
	Please note the definition of MaxDpbSize in Annex A of the current document:
	MaxDpbSize is equal to ( 1024 * MaxDPB / ( PicWidthInMbs * FrameHeightInMbs * 384 ), 16 ) 
	Therefore num_ref_frames cannot exceed 16.
	An encoder can keep a long-term reference picture in the DPB as long as it wants to.
	Best regards,
	Gary Sullivan
________________________________
		From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of ??(Huang-Bin)
		Sent: Wednesday, April 20, 2024 4:55 AM
		To: mp4-tech lists.mpegif.org
		Subject: [Mp4-tech] About the definition of num_ref_frames
		Hi all,
		I am confused about that the parameter in JM is different with 
		the definition of H.264 recommendation.
		In recommendation document JVT-G050(It seems to be a old version), I check some definitions in 7.4 as below:
		[=============
		num_ref_frames specifies the maximum total number of short-term and long-term reference frames, complementary reference field pairs, and non-paired reference fields used by the decoding process for inter prediction of any picture in the sequence. num_ref_frames also determines the size of the sliding window operation as specified in subclause 8.2.5.3. The value of num_ref_frames shall be in the range of 0 to 16, inclusive.
		max_long_term_frame_idx_plus1 minus 1 specifies the maximum value of long-term frame index allowed for long-term reference pictures (until receipt of another value of max_long_term_frame_idx_plus1). The value of max_long_term_frame_idx_plus1 shall be in the range of 0 to num_ref_frames, inclusive.
		=============]
		Here the text "The value of num_ref_frames shall be in the range of 0 to 16, inclusive"
		is obey the parameter in JM.
		However, I download the latest recommendation document from ITU web-site
		the definitions change as below:
		[=============
		num_ref_frames specifies the maximum number of short-term and long-term reference frames, complementary reference field pairs, and non-paired reference fields that may be used by the decoding process for inter prediction of any picture in the sequence. num_ref_frames also determines the size of the sliding window operation as specified in subclause 8.2.5.3. The value of num_ref_frames shall be in the range of 0 to MaxDpbSize (as specified in subclause A.3.1), inclusive.
		=============]
		It defines the maximum value of num_ref_frame is 0 to MaxDpbSize instead of 0 to 16.
		For example , if we use the profile with the level value equal to 5, and the picure format is CIF,
		we might have a huge amount DPB size to store many reference frames in CIF format. 
		At this situation, We can store more than '16' reference frames(including short-term and long-term) in the DPB.
		So, the questions are:
		Qn 1>
		Why does JM restrict the parameter 'NumberReferenceFrames' in the range 1-16 ?
		("NumberReferenceFrames =  5  # Number of previous frames used for inter motion search (1-16)")
		Can we set the parameter "NumberReferenceFrames" more than '16' for reference if the DPB size is enough ?(In JM)
		Qn 2>
		Can the long-term reference picture stay in the DPB as long as the encoder wants it to ?
		(Will the parameter'NumberReferenceFrames' restrict the logh-term reference index ?)
		Qn 3>
		Could anyone tell me that the meaning of the parameter in JM 'PList0Reference'  ?
		Best regards,
		H.B.
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