[Mp4-tech] GOP concept in H.264

[Tourapis, Alexis]

Dear Tummala,
You may wish to refer to JVT-Q042 with regards of how to use the JM reference software. You may download this from http://ftp3.itu.int/av-arch/jvt-site/2005_10_Nice/JVT-Q042-L.zip
The parameter "EnableOpenGOP" essentially tries to emulate GOP structures (Open vs Closed GOP) used in MPEG-2 and older standards. I.e. If you wish to encode a sequence while having a 15 frame intra period, this parameter enforces a special encoding mode in the JM where inter predicted slices after the intra coded frame are not allowed to consider as references frames prior to that intra coded frame. However, this excludes non reference frames prior to that frame (in MPEG-2 this definition was explicitly used for B frames).
NumberBFrames has to be used in combination with FrameSkip. i.e. if you wish to enter two B coded frames, then you need to set both parameters to 2. Please read JVTQ042 for more info. FrameSkip may sound a bit confusing, but this was the way this parameter was used from the early stages of the JM and therefore we did not try to alter its behavior.
You will definitely find more parameters that could be of use to you in JVT-Q042, including how to set up hierarchical encoding.
I can't comment on your results since I do not know how you are doing your simulations. Obviously at this point you are not using B slices, maybe you are enforcing too many intra frames, RDO usage etc. In any case, hopefully all your questions would be answered after reviewing that document.
Best regards,
Alexis
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From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of tummala kalyani
Sent: Tuesday, November 08, 2023 8:45 AM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] GOP concept in H.264
Hi All,
I would like to know about GOP concept in H.264. In JM encoder config file there is a parameter "support for open GOP". How can I use this? I tried to 1-enable and 0-disable options. But I don't find any difference. 
There is provision to set I frame distance and number of B frames. When I treid to set number of B frames more than 0 , it gives me an error message saying that  "Number of B frames should me more than frames skipped in the input". What is the use of Frames skipped parameter in the encoder.cfg file in JM?
I need bitrate vs psnr comparison graphs for H.264 codec and MPEG-2 codec. For that I am running JM and MPEG-2 reference softwares with foreman.yuv as input video. I found that for baseline and main profiles H.264 codec output file size  is double to MPEG-2 file size. I use this filesize to calculate bitrate( filesize * (frame rate / total frames)). But according to specs. H.264 codec bitrate should be half to that of MPEG-2. But I am getting in reverse. I am confused by this. I think I have not understood it properly. Can some one explain me this clearly?
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