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

[Paul B Mahol <onemda@gmail.com>]

On Wed, Oct 25, 2023 at 9:03 PM Tomas Härdin <git@haerdin.se> wrote:

> On Wed, 2023-10-25 at 21:00 +0200, Paul B Mahol wrote:
> > On Wed, Oct 25, 2023 at 8:39 PM Tomas Härdin <git@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.
>
> You can just work the math out on paper. Inverse of
>
>  1     0
>  v[1]  v[0]
>
> is
>
>  1           0
>  -v[1]/v[0]  1/v[0]
>
> not accounting for shifts.
>

But I want to add real 2x2 matrix with no 0 cell, with:

a, b
c, d

later. (even though gains are small, as encoded files use it rarely)


>
> Also RE: my other comments, you are right. I didn't take into account
> that MLP is lossless and that there may be off-by-one errors.
>
> And as I said on IRC you can formulate this as a least squares problem,
> then solve it using a linear system solve. This patch seems finds a
> solution that minimizes L1 rather than L2 though. Not sure what the
> implications of that are compressionwise. What happens if you replace
> FFABS() with a square for scoring?
>

It reduces size usually by less then 0.002 %


Linear system solver gives vectors to create equations for both channels at
same time?


>
> /Tomas
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