[FFmpeg-devel] [PATCH] avcodec/mlp*: improvements

[Paul B Mahol]

On Wed, Oct 25, 2023 at 8:39 PM Tomas Härdin <git at haerdin.se> wrote:

>
> >             if (c) {
> >                 e[0] = 1 << 14;
> >                 e[1] = 0 << 14;
> >                 e[2] = v[1];
> >                 e[3] = v[0];
> >             } else {
> >                 e[0] = v[0];
> >                 e[1] = v[1];
> >                 e[2] = 0 << 14;
> >                 e[3] = 1 << 14;
> >             }
> >
> >             if (invert2x2(e, d)) {
> >                 sum = UINT64_MAX;
> >                 goto next;
> >             }
> >
>
> You can make use of the properties of e to simplify calculating the
> inverse. The determinant is always v[0]<<14, so you can just do if
> (!v[0]) continue; and skip the determinant check altogether.
>

Even for real 2x2 matrix case? (Once one of rows is not 1, 0) ?
May added such cases later.


>
> >                 if (d[i] != av_clip_intp2(d[i], 15)) {
>
> d[i] < INT16_MIN || d[i] > INT16_MAX is more clear and probably faster
>
> > +                    lt = ((lm * e[0]) >> 14) + ((rm * e[1]) >> 14);
> > +                    rt = ((lm * e[2]) >> 14) + ((rm * e[3]) >> 14);
>
> Result is implementation-defined. Use division by (1<<14). Also add
> then divide. The intermediate result is 49 bits so fits easily in 64
> bits.
>
> You could also simplify this calculation by again making use of the
> properties of e.
>
> >                     if (c)
> >                         v += FFABS(rt);
> >                     else
> >                         v += FFABS(lt);
> >                     sum += v;
> >                     if (sum > best_sum)
> >                         goto next;
>
> Seems like this reduces to solving a linear program.
>
> >                     if ((((lt * d[0]) >> 14) + ((rt * d[1]) >> 14))
> > != lm) {
> >                         sum = UINT64_MAX;
> >                         goto next;
> >                     }
> >
> >                     if ((((lt * d[2]) >> 14) + ((rt * d[3]) >> 14))
> > != rm) {
> >                         sum = UINT64_MAX;
> >                         goto next;
> >                     }
>
> Looks like a massive hack. I'd prefer to formally verify that the
> arithmetic works out. Also again you can make use of the properties of
> e, or inv(e) as it were.
>
> /Tomas
>
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