Re: [FFmpeg-devel] [PATCH 27/35] avcodec/proresenc_anatoliy: remove TO_GOLOMB2()

[Stefano Sabatini <stefasab@gmail.com>]

On date Monday 2024-01-08 00:00:37 +0100, Clément Bœsch wrote:
> On Sun, Dec 24, 2023 at 12:43:32AM +0100, Stefano Sabatini wrote:
> > On date Monday 2023-12-11 02:35:28 +0100, Clément Bœsch wrote:
> > > A few cosmetics aside, this makes the function identical to the one with
> > > the same name in proresenc_kostya.
> > > ---
> > >  libavcodec/proresenc_anatoliy.c | 6 ++----
> > >  1 file changed, 2 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/libavcodec/proresenc_anatoliy.c b/libavcodec/proresenc_anatoliy.c
> > > index bdf7bface4..aed5c68b1b 100644
> > > --- a/libavcodec/proresenc_anatoliy.c
> > > +++ b/libavcodec/proresenc_anatoliy.c
> > > @@ -257,7 +257,6 @@ static void encode_vlc_codeword(PutBitContext *pb, unsigned codebook, int val)
> > >  
> > >  #define GET_SIGN(x)  ((x) >> 31)
> > >  #define MAKE_CODE(x) (((x) * 2) ^ GET_SIGN(x))
> > > -#define TO_GOLOMB2(val,sign) ((val)==0 ? 0 : ((val) << 1) + (sign))
> > >  
> > >  static av_always_inline int get_level(int val)
> > >  {
> > > @@ -271,7 +270,6 @@ static void encode_dcs(PutBitContext *pb, int16_t *blocks,
> > >  {
> > >      int i;
> > >      int codebook = 5, code, dc, prev_dc, delta, sign, new_sign;
> > > -    int diff_sign;
> > >  
> > >      prev_dc = (blocks[0] - 0x4000) / scale;
> > >      encode_vlc_codeword(pb, FIRST_DC_CB, MAKE_CODE(prev_dc));
> > > @@ -282,8 +280,8 @@ static void encode_dcs(PutBitContext *pb, int16_t *blocks,
> > >          dc        = (blocks[0] - 0x4000) / scale;
> > >          delta     = dc - prev_dc;
> > >          new_sign  = GET_SIGN(delta);
> > 
> > > -        diff_sign = new_sign ^ sign;
> > > -        code      = TO_GOLOMB2(get_level(delta), diff_sign);
> > > +        delta     = (delta ^ sign) - sign;
> > > +        code      = MAKE_CODE(delta);
> > 
> > These don't look equivalent,
> > 
> > MAKE_CODE((delta ^ sign) - sign) is equivalent to
> > TO_GOLOMB2(get_level(delta), sign)
> > 
> > not to
> > TO_GOLOMB2(get_level(delta), diff_sign)
> 
> OK so this one is a bit tricky.
> 
> Let's start from the specs, which states that the signed integer to symbol
> (code) mapping should be:
> 
>     2|n|    if n>=0
>     2|n|-1  if n<0
> 
> We also know that n>>31 is -1 if n < 0, and 0 if n>=0, which means the
> above condition can be simplified to:
> 
>     2|n| + (n>>31)
> 
> With prores_aw we have:
> 
>     s = -1 if different sign, 0 otherwise
>     2|n| + s
>  
> Because:
> - get_level() is an absolute function¹
> - the val==0 case doesn't matter because in this case s will also be 0
> 
> In prores_ks we have:
> 
>     n'=-n if different sign, n otherwise
>     (2n')^sign(n') <=> 2|n'|-(n'>>31)
> 
> So basically, aw does use the comparison with the previous delta and
> encodes it accordingly, while ks decides to swap the sign of n according
> to that previous delta, then encode it using its new sign.
> 
> I wouldn't mind a third pair of eyes on the matter, but these look
> equivalent to me.
> 
> Note that in practice I also tried to encode a bunch of frames from
> testsrc2 with and without the patch, and they are bit identical.

Thanks for the detailed explanation, I think it's safe to push
especially considering that there are no output changes.
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
https://ffmpeg.org/mailman/listinfo/ffmpeg-devel

To unsubscribe, visit link above, or email
ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".