[FFmpeg-devel] [PATCH] lavfi: add audio eval signal source

[Nicolas George]

Le duodi 22 vendémiaire, an CCXX, Stefano Sabatini a écrit :
> + at section aevalsrc

Cool idea, thanks.

> +Generate an audio signal generated by an expression.

The Department of Redundancy Department wants to thank you for having
generated such an awesome introduction line.

> +This source accepts in input an expression, which is evaluated and
> +used for generating a mono audio signal.

"to generate", I believe.

> + at table @option
> + at item E, PI, PHI
> +the corresponding mathematical approximated values for e
> +(euler number), pi (greek PI), PHI (golden ratio)

Shouldn't it be made part of the eval system once and for all?

> + at item n
> +the number of sample, starting from 0
> +
> + at item t
> +time of the sample expressed in second, starting from 0

The place of "the" is moving. I fond the second one easier to understand.

> +Generate a sin signal with frequence 4400 Hz:
> + at example
> +aevalsrc="sin(4400*t)"

sin(4400*t*2*PI), or it will be 700 Hz.

> +    int64_t chlayouts[] = { AV_CH_LAYOUT_MONO, -1 };

As Michael said, the possibility to generate several channels would be cool.
OTOH, it could be left to a separate amerge filter: (n channels, p channels)
-> n+p channels.

> +    eval->var_values[VAR_N] = eval->n;
> +
> +    /* evaluate expression for each single sample */
> +    for (i = 0; i < eval->nb_samples; i++) {
> +        eval->var_values[VAR_T] = eval->var_values[VAR_N] * (double)1/eval->sample_rate;
> +        *((double *) samplesref->data[0] + i) =
> +            av_expr_eval(eval->expr, eval->var_values, NULL);
> +        eval->var_values[VAR_N] = eval->n++;
> +    }

The computation for VAR_N is wrong by one except on the first round because
n++ is postincrementation, not preincrementation. I believe it could be more
clearly written as:

    for (i = 0; i < eval->nb_samples; i++) {
        eval->var_values[VAR_N] = eval->n;
        eval->var_values[VAR_T] = eval->var_values[VAR_N] * (double)1/eval->sample_rate;
        *((double *) samplesref->data[0] + i) =
            av_expr_eval(eval->expr, eval->var_values, NULL);
        eval->n++;
    }

with possibly the eval->n++ part of the third for clause.

Regards,

-- 
  Nicolas George
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