[Mp4-tech] [H.264] Using B-Frames & Reducing Bitrate Problem

[Tourapis, Alexis]

Dear Ertugrul,
In order to take full advantage of B slices in AVC you would need to use at least 2 reference frames. By using NumberReferenceFrames=1 only 1 reference is available in your system and it is used for both list0 and list1. As you can understand that would severely reduce the efficacy of coding B slices. 
Obviously also the performance of using B slices strongly relates to the sequence you are encoding. For example, sequences with very high/chaotic motion may not benefit as much from using B slices. Note that also the distance between P slices increases which makes the motion estimation process a bit more difficult for those frames. Bitrate target may also affect considerably the efficacy of using B slices.
Finally, note that the JM uses additional concepts (i.e. lagrangian multipliers, thresholding, rounding etc) which behave differently for different slice types and you should not expect to see the same PSNR for these different slice types. Of course you can try to adjust these if that is your target.
Best regards,
Alexis
________________________________
From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Ertugrul Dogan (Design Engineer - VESTEK)
Sent: Thursday, December 28, 2023 7:15 AM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] [H.264] Using B-Frames & Reducing Bitrate Problem
Hi all,
I was experimenting with JM 11.0 software. I encoded the famous Foreman sequence with Qp = 28, GOP = IBPBP... with Intra period of 10. The Qp was the same for I, B and P frames. Also NumberReferenceFrames =  1.
I used the same configuration for IPPP... for the same sequence. In the end IPPPP.... GOP configuration resulted in a better bitrate. 
Isn't this weird ? Doesn't B frames supposed to reduce the bitrate with the same PSNR value ?
Thanks,
e.dogan
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