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

[James (Jianpeng) Dong]

Dear Gary and Alexis,
Thanks a lot.
My observation is the same as Alexis'. The JM reference software (the rate control part) does restrict the QP to be fixed (or, simply use the same QP value) for all the MBs within an I-frame/B-frame in a GOP, although the QP value for I-frame or B-frame may change on a frame basis. 
This may be due to the fact (as pointed out by Alexis) that the rate control in the JM reference software is only for the demo purpose. I am glad that there is no such restriction in the H.264 standard.
Regards,
James
Gary Sullivan <garysull windows.microsoft.com> wrote:     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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