[XviD-devel] about my B-frame decode code

[Christoph Lampert]

On Thu, 18 Jul 2002, CM wrote:

> Hi:
> 
>     I found my B-frame decode code is wrong, because the Divx not
> write vol_control_param head, so I'm cann't get the low_delay flage
> at this time my code will broken bitstream, and program will error!
>     I update my code at today (see decode.c), but I don't known
> where is the best way to fix that error and Divx how to?

I guess low_delay=0, if it is not specified. 

However, this won't work with broken encoders (like old xvid and
DivX), so ffmpeg does is in the following way: 

     /* detect buggy encoders which dont set the low_delay flag
(divx4/xvid/opendivx)*/
     // note we cannot detect divx5 without b-frames easyly (allthough its
buggy too)
     if(s->vo_type==0 && s->vol_control_parameters==0 &&
s->divx_version==0 && s->picture_number==0){
         printf("looks like this file was encoded with
(divx4/(old)xvid/opendivx) -> forcing low_delay flag\n");
         s->low_delay=1;
     }

So if no low_delay flag is present, it assumes that the decoder does not
know about low_delay and sets low_delay=1.

This may not always work, but what else should we do if the bytestream is
broken? We could add a "backwards compatability" field which sets 
low_delay to 1 (if it is not present).

We should concentrate on producing and decoding _correct_ bytestreams.

gruel 

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