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

[Tourapis, Alexis]

Dear Kaustubh,
I would suggest checking the function updateQuantizationParameter() and carefully read the code for the case that:
if(img->BasicUnit!=img->Frame_Total_Number_MB)
Correct me if I am wrong (I never use this RC myself while the truth is that have been slow in using/updating the JM lately) but you will observe that for I slices the QP remains unchanged (i.e. m_Qc=MyInitialQp), while for B slices the QP depends on the QPs of the last two "reference" pictures (I have not tested this but my guess is that would be the last QP used in those pictures). Note that this rate control obviously does not support hierarchical coding structures, picture level RDO decisions (these are based on multiple encodes and the software does not handle this correctly for RC), and interlace type of encodings. I would strongly caution anyone myself in using this particular rate control especially for evaluation purposes. It does not adapt its statistics very well (and does that quite slowly), can create severe artifacts (do not be misled by RD curves who tend to average results and hide quality variations), but also (if I recall correctly) has issues with high motion sequences. In any case, this is just my personal opinion. I would mainly see the RC in the software as an educational module than a practical one.
Best regards,
Alexis
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From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of kaustubh.patankar vsnl.net
Sent: Tuesday, May 09, 2024 1:10 AM
To: Gary Sullivan
Cc: James (Jianpeng) Dong; mp4-tech lists.mpegif.org; mp4-tech-bounces lists.mpegif.org; Tourapis, Alexis
Subject: Re: RE: [Mp4-tech] Why I or B frame can have one fixed QP?
But I'm under the strong impression that the current JM rate control algorithm, although it does have some shortcomings, does have the ability to change QP within an I or B picture -doesn't it?  (One of these days we need to improve that rate control.)
Best Regards,
-Gary Sullivan
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	From: Tourapis, Alexis [mailto:Alexis.Tourapis dolby.net] 
	Sent: Sunday, May 07, 2024 11:17 AM
	To: Gary Sullivan; James (Jianpeng) Dong; mp4-tech-bounces lists.mpegif.org; mp4-tech lists.mpegif.org
	Subject: RE: [Mp4-tech] Why I or B frame can have one fixed QP?
	Dear James,
	There is no quantization limitation in H.264/AVC. I am guessing your observation relates to the current Rate Control algorithm supported by the JM software. This should only be seen as an example design.
	Best regards,
	Alexis 
  _____  
	From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Gary Sullivan
	Sent: Saturday, May 06, 2024 8:31 PM
	To: James (Jianpeng) Dong; mp4-tech-bounces lists.mpegif.org; mp4-tech lists.mpegif.org
	Subject: RE: [Mp4-tech] Why I or B frame can have one fixed QP?
	I don't think it's true that an I or B frame can only have one fixed QP value in the JM reference software.  No, there is no requirement of fixed QP in the standard.  QP can change (from any value to any other value) on a macroblock (16x16) basis in the standard.
	Best Regards,
	Gary Sullivan
  _____  
		From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of James (Jianpeng) Dong
		Sent: Friday, May 05, 2024 4:43 PM
		To: mp4-tech-bounces lists.mpegif.org; mp4-tech lists.mpegif.org
		Cc: jianpengdong yahoo.com
		Subject: [Mp4-tech] Why I or B frame can have one fixed QP?
		Dear experts, 
		Just wondering why I or B frame can only have one fixed QP in H.264 JM reference software? Is a fixed QP required by H.264/AVC standard? 
		Thanks,
		James
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