[FFmpeg-devel] [PATCH 2/3] bink: get rid of double precission float use

[Vitor Sessak]

On Tue, Feb 7, 2012 at 8:17 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Tue, Feb 07, 2012 at 08:06:19PM +0100, Vitor Sessak wrote:
>> On Mon, Feb 6, 2012 at 7:31 PM, Michael Niedermayer <michaelni at gmx.at>
>> wrote:
>> > Add a small table of integers and out commented code to generate it with
>> > the double precision code.
>>
>> Not against the idea, but note that this should not be needed for
>> bit-exactness: I checked that all the double values are far enough from
>> integers for this code to be bitexact across archs.
>
> do you think its better to leave the double prec code ?

No, not really. I like integer code, it's way more obvious if it is
bit-exact or not.

>> > Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
>> > ---
>> >  libavcodec/bink.c |   36 ++++++++++++++++++++++++------------
>> >  1 files changed, 24 insertions(+), 12 deletions(-)
>> >
>> > diff --git a/libavcodec/bink.c b/libavcodec/bink.c
>> > index ff8bc8a..c77c139 100644
>> > --- a/libavcodec/bink.c
>> > +++ b/libavcodec/bink.c
>> > @@ -1222,40 +1222,52 @@ static int decode_frame(AVCodecContext *avctx,
>> void *data, int *data_size, AVPac
>> >  static av_cold void binkb_calc_quant(void)
>> >  {
>> >     uint8_t inv_bink_scan[64];
>> > -    double s[64];
>> > +    static const int s[64]={
>> > +        1073741824,1489322693,1402911301,1262586814,1073741824,
>> 843633538, 581104888, 296244703,
>> > +
>>  1489322693,2065749918,1945893874,1751258219,1489322693,1170153332,
>> 806015634, 410903207,
>> > +
>>  1402911301,1945893874,1832991949,1649649171,1402911301,1102260336,
>> 759250125, 387062357,
>> > +        1262586814,1751258219,1649649171,1484645031,1262586814,
>> 992008094, 683307060, 348346918,
>> > +        1073741824,1489322693,1402911301,1262586814,1073741824,
>> 843633538, 581104888, 296244703,
>> > +         843633538,1170153332,1102260336, 992008094, 843633538,
>> 662838617, 456571181, 232757969,
>> > +         581104888, 806015634, 759250125, 683307060, 581104888,
>> 456571181, 314491699, 160326478,
>> > +         296244703, 410903207, 387062357, 348346918, 296244703,
>> 232757969, 160326478,  81733730,
>> > +    };
>> >     int i, j;
>> > -
>> > +#define C (1LL<<30)
>> > +#if 0
>> >     for (j = 0; j < 8; j++) {
>> >         for (i = 0; i < 8; i++) {
>> >             if (j && j != 4)
>> >                if (i && i != 4)
>>
>> > -                   s[j*8 + i] = cos(j * M_PI/16.0) * cos(i * M_PI/16.0)
>> * 2.0;
>> > +                   s[j*8 + i] = llrint(cos(j * M_PI/16.0) * cos(i *
>> M_PI/16.0) * 2.0 * C);
>>
>> Are you sure about this change? This will make the table different from the
>> reference decoder (and I bet also from the reference encoder), and thus
>> should change the output in a negative way.
>
> where does a table that is used outside the init function become
> different ?

Does the binkb_inter_quant table is the same with and without the
llrint()? I suppose your int code is bitexact to the llrint()
version...

-Vitor