[Mp4-tech] [video] Error concealment - unreferenced memory error

[Adarsh Golikeri]

hi steve,
i've tried this same method before. the concept you are using is
correct (setting the ei flag to 1) but the place you are doing it is
wrong. instead of in function exit_picture, try doing it in the
function decode_one_slice. this was the code i used :
    // lose some MBs in i-frames
    loss = rand()%100;
	if (img->number>0 && img->type == 2 && img->current_mb_nr >= 11 &&
img->current_mb_nr <=22)
       { 
	   img->mb_data[img->current_mb_nr].ei_flag = 1;
       fprintf(stdout,"\nLosing MB %d of Picture
%d",img->current_mb_nr,img->number);
	   }
    // lose some MBs in i-frames
hope that helps,
adarsh
On Mon,  7 Feb 2024 16:09:30 +0000, Steve <S.T.C.Beesley lboro.ac.uk> wrote:
> Hi experts,
> 
> I have been working with JM93 to look at error concealment, specifically the
> way that the decoder tries to hide any missing data. In order to achieve
> this I have been adding something similar to:
>   img->mb_data[207].ei_flag = 1;
> in image.c after line 1206:
>   ...
>   //! mark the start of the first segment
>   if (!dec_picture->MbaffFrameFlag)
>   {
>     ercStartSegment(0, ercSegment, 0 , erc_errorVar);  // <-- 1206
>   ...
> 
> For the most part this method seems to work extremely well however under
> some circumstances the decoder is throwing an unreferenced memory error due
> to the Motion Vector reference slice being set to -1. One such example of
> this is in a cif version of mobile.yuv when ProfileIDC = 100 (High) is used
> at macroblock 207 (w=9, h=9), frame 18 (all frames up to 15 in the decode
> sequence work ok).
> 
> >From what I can tell there are only two lines that ever set the mv[2]
> variable to anything other than 0 and these are again in image.c:
>   pRegion->mv[2] = (dec_picture->ref_idx[idx][ii][jj]);
> or
>   pRegion->mv[2] = dec_picture->ref_idx[LIST_0][ii][jj];
> depending on the type of slice (non-B or B).
> 
> I'm wondering if anybody can shed any light onto why this variable would
> ever be set to -1, judging by the rest of the EC code it looks like it
> assumes that this variable is always going to contain a valid number as no
> checking appears to be done. One working theory I currently have is that it
> could be referencing a slice that has either not yet been decoded or is no
> longer in the decode buffer. Also can anybody suggest where to find the
> information to detect if the returned value is valid or not and what action
> should be taken in the cases where it is invalid?
> 
> If it is the case that I'm barking up the wrong tree here (which is more
> than possible) then can anyone suggest a better method of forcefully adding
> errors at the decoder?
> 
> Many thanks in advance,
> 
> Steve Beesley
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