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.
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