[FFmpeg-devel] [PATCH] adpcm: Reset the ssd back to zero more often

[Michael Niedermayer]

On Thu, Nov 18, 2010 at 04:01:31PM +0200, Martin Storsjo wrote:
> If using very large trellis sizes (e.g. -trellis 15), the frontier
> is so large that the difference between the best and the worst
> trellis node in the frontier is large enough to cause wraparound.
> 
> Resetting at (1<<20) is enough to avoid the issue at -trellis 16
> with the sample I'm testing, therefore doing the resets at (1<<18)
> to have some extra safety margin.
> 
> This doesn't incur any noticeable slowdown.
> ---
> I noticed this while testing the optimizations that Michael suggested
> (using a hash to find out which samples to skip), which made the
> encoder fast enough to actually be able to try out the maximum
> trellis levels.
> 
>  libavcodec/adpcm.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/libavcodec/adpcm.c b/libavcodec/adpcm.c
> index 56eb602..f6e3cb7 100644
> --- a/libavcodec/adpcm.c
> +++ b/libavcodec/adpcm.c
> @@ -444,7 +444,7 @@ static void adpcm_compress_trellis(AVCodecContext *avctx, const short *samples,
>          nodes_next = u;
>  
>          // prevent overflow
> -        if(nodes[0]->ssd > (1<<28)) {
> +        if(nodes[0]->ssd > (1<<18)) {
>              for(j=1; j<frontier && nodes[j]; j++)
>                  nodes[j]->ssd -= nodes[0]->ssd;
>              nodes[0]->ssd = 0;

what if this is always performed and the subtraction added into some existing
code.

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Let us carefully observe those good qualities wherein our enemies excel us
and endeavor to excel them, by avoiding what is faulty, and imitating what
is excellent in them. -- Plutarch
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