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

[James (Jianpeng) Dong]

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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