[FFmpeg-devel] [PATCH] lavfi: port decimate libmpcodecs filter

[Nicolas George]

Le tridi 3 fructidor, an CCXX, Stefano Sabatini a écrit :
> > Maybe store it as an integer? That would help making the filter bit-exact.
> Please elaborate, also I'd like to keep compatibility with mp=decimate.

My suggestion would be to have "unsigned frac", with default value
1431655765 ((1<<32)/3) or 1417339208 ((1<<32)*0.33) and replace the
following code 

> +    int t = (w/16)*(h/16)*decimate->frac;

by:

int t = av_rescale((w/16)*(h/16), decimate->frac, 0x100000000);

In other words: frac_as_a_float = frac_as_an_unsigned / (float)(1 << 32),
but avoiding all floating-point arithmetic.

Since (w/16)*(h/16) should be way smaller than 1<<32 (if av_image_check_size
did its work, (w/16)*(h/16) < 1<<20), it will be accurate enough. And
without floating-point arithmetic, we can be sure the computation is
bit-exact across architectures and add FATE tests.

The /16 look very strange, though, as x and y are incremented by steps of 4.

> +        char c1, c2, c3, c4;
> +        int n = sscanf(args, "%d%c%d%c%d%c%f%c",
> +                       &decimate->max_drop_count, &c1,
> +                       &decimate->hi, &c2, &decimate->lo, &c3,
> +                       &decimate->frac, &c4);
> +        if (n != 1 &&
> +            (n != 3 || c1 != ':') &&
> +            (n != 5 || c1 != ':' || c2 != ':') &&
> +            (n != 7 || c1 != ':' || c2 != ':' || c3 != ':')) {
> +            av_log(ctx, AV_LOG_ERROR,
> +                   "Invalid syntax for argument '%s': "
> +                   "must be in the form 'max:hi:lo:frac'\n", args);
> +            return AVERROR(EINVAL);

This test look very clumsy, but I guess it comes from the original code.


No other remarks.

Regards,

-- 
  Nicolas George
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