[Mp4-tech] Why I or B frame can have one fixed QP?

[Tourapis, Alexis]

Dear James,
This has nothing to do with Rate Control but, by default, if field coding is selected to encode an entire frame, and if that frame was originally scheduled to use I slices, only the top field would be coded as I and the bottom will be coded as P. This enabled the encoder to exploit correlation that may exist between top and bottom field to improve performance. This consideration could be disabled with a flag that we added a considerable time ago named "IntraBottom". Furthermore, the software also allows you to test the use of B slices for encoding this bottom field as well. Although to some it might sound odd in fact, for some content this can provide further benefits due to biprediction considerations. 
Kind regards,
Alexis
  _____  
From: James (Jianpeng) Dong [mailto:jianpengdong yahoo.com] 
Sent: Wednesday, May 10, 2024 9:13 AM
To: �̼���; kaustubh.patankar vsnl.net
Cc: mp4-tech lists.mpegif.org; mp4-tech-bounces lists.mpegif.org; Tourapis, Alexis; Gary Sullivan
Subject: RE: Re: RE: [Mp4-tech] Why I or B frame can have one fixed QP?
Dear experts,
Here is some code snippet for basic unit rate contrl from ratectl.c from JM 10.2 (note that the comment for bottom field of I frame and the related code do not match, I believe this might be a bug?):
    /*top filed of I frame*/
    if(img->type==I_SLICE)
    {
      m_Qc=MyInitialQp;
      return m_Qc;
    }
    /*bottom field of I frame*/
    else if((img->type==P_SLICE)&&(img->IFLAG==1)&&(img->FieldControl==1))
    {
      m_Qc=MyInitialQp;
      return m_Qc;
    }
Note also MyInitialQp won't change util a new GOP arrives. From the above, it is apparent that the rate control in JM 10.2 uses a fixed QP for I SLICEs. The  parameters in the configuration file won't affect the I-frames:
########################################################################################
#Rate control
########################################################################################
RateControlEnable    =      1   # 0 Disable, 1 Enable
Bitrate              =  48000   # Bitrate(bps)
InitialQP            =     26   # Initial Quantization Parameter for the first I frame
                                # InitialQp depends on two values: Bits Per Picture,
                                # and the GOP length
BasicUnit            =      1   # Number of MBs in the basic unit
                                # should be a fractor of the total number 
                                # of MBs in a frame
ChannelType          =      0   # type of channel( 1=time varying channel; 0=Constant channel)
Regards,
James
"�̼���" <dv2 naver.com> wrote: 
	HI ~
	I think so.
	you can have control that configuration file.
	<-----Original Message----->
	> From: 
	> To: "Gary Sullivan" 
	> Cc: "James \(Jianpeng\) Dong" ,
	,
	,
	"Tourapis, Alexis" 
	> Sent: 2024-05-09 23:56
	> Subject: Re: RE: [Mp4-tech] Why I or B frame can have one fixed QP?
	Hi
	I think there are options to have Frame level and MB level rate control
	in JM code and it also works for I and B frames.
	regards
	Kaustubh
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	��ο� ��� ��ȭ�� ����, ���Ǻ�
	http://happybean.naver.com
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