[Mp4-tech] Parameters for Selection of reference frames in JM

[Tourapis, Alexis]

You can ignore that code for your application. You need to create your own process for this. That is a special example where reordering was useful. The idea there was to constrain the case of normal intra pictures to restrict future coded pictures not to go beyond them as references.. to make this more clear assume the following:
I0P1P2P3P4I5P6P7P8....
If I are not IDR and you use 4 references, after you encoder I5, P6 will have available for reference P2,P3,P4, and I5. However, for random access that would not be good since if you access I5 without decoding anything else before it the other references will not be available and P6 and everything else will get corrupted. The process of open gop basically forces P6 not to use any reference beyond I5 ensuring that decoding will be successful. Note that things get more complicated when you start using B coded pictures and especially if you use B coded pictures that are referenced. In that scenario you may need to do reordering but also mmco operations to keep the most appropriate references in your buffer. This does not affect you in any way. What you need to do is not replace that function but create a new function that given the new parameter that you introduce will perform reordering according to frame number parity. Basically see that code as an example, not something you should modify.
Thanks,
Alexis
-----Original Message-----
From: Yiting LIAO [mailto:liao umail.ucsb.edu]
Sent: Thursday, January 24, 2024 12:50 PM
To: Tourapis, Alexis
Cc: mp4-tech lists.mpegif.org
Subject: Re: [Mp4-tech] Parameters for Selection of reference frames in JM
Hello Alexis,
Thanks for the answer. I have another question:
poc_ref_pic_reorder is run only when EnableOpenGOP is set to 1. What does open GOPs mean? Can I just enable open GOPs and write the function you talked about to replace the original "poc_ref_pic_reorder" function and get the results? Thanks a lot. 
Yiting
Tourapis, Alexis wrote:
> Unfortunately there is no such option in the JM. What you need to do, apart from setting the 2nd picture to be coded as Intra, is to introduce reordering commands in the software that will basically select for coding for an odd positioned picture only previously coded odd positioned pictures, and similarly for an even positioned picture select again previously coded even ones. You can see an example of reordering that is based on POC distances in function slice.c (poc_ref_pic_reorder). What you need to do is write a similar function that orders references not on poc distance but in this scenario on whether poc (or frame number to be coded) is odd or even followed then by poc distance.
>
> I hope that helps.
>
> Alexis
>
> -----Original Message-----
> From: mp4-tech-bounces lists.mpegif.org 
> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Yiting LIAO
> Sent: Wednesday, January 23, 2024 6:06 PM
> To: mp4-tech lists.mpegif.org
> Subject: [Mp4-tech] Parameters for Selection of reference frames in JM
>
> Hello All,
>
> Is there a parameter that can be used to select reference frames used in the encoder. For example, odd frame only uses odd frames as reference frames and even frame only uses even frame as reference frames. If so, will the second frame in the video sequence be encoded into I-frame automatically? If not, which part should I modify to implement this function?
>
> Thanks,
> Yiting
>
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