[FFmpeg-devel] [PATCH] lavu: support arbitrary-point FFT and all even (i)MDCT transforms

[James Almer]

On 1/12/2021 4:18 AM, Lynne wrote:
> This patch adds support for arbitrary-point FFTs and all even MDCT
> transforms.
> Odd MDCTs are not supported yet as they're based on the DCT-II and DCT-III
> and they're very niche.
> 
> With this we can now write tests.
> 
> Patch attached.

[...]

> @@ -575,11 +651,13 @@ int TX_NAME(ff_tx_init_mdct_fft)(AVTXContext *s, av_tx_fn *tx,
>                                   const void *scale, uint64_t flags)
>  {
>      const int is_mdct = ff_tx_type_is_mdct(type);
> -    int err, n = 1, m = 1, max_ptwo = 1 << (FF_ARRAY_ELEMS(fft_dispatch) - 1);
> +    int err, l, n = 1, m = 1, max_ptwo = 1 << (FF_ARRAY_ELEMS(fft_dispatch) - 1);
>  
>      if (is_mdct)
>          len >>= 1;
>  
> +    l = len;
> +
>  #define CHECK_FACTOR(DST, FACTOR, SRC)                                         \
>      if (DST == 1 && !(SRC % FACTOR)) {                                         \
>          DST = FACTOR;                                                          \
> @@ -601,12 +679,23 @@ int TX_NAME(ff_tx_init_mdct_fft)(AVTXContext *s, av_tx_fn *tx,
>      s->inv = inv;
>      s->type = type;
>  
> -    /* Filter out direct 3, 5 and 15 transforms, too niche */
> +    /* If we weren't able to split the length into factors we can handle,
> +     * resort to using the naive and slow FT. This also filters out
> +     * direct 3, 5 and 15 transforms as they're too niche. */
>      if (len > 1 || m == 1) {
> -        av_log(NULL, AV_LOG_ERROR, "Unsupported transform size: n = %i, "
> -               "m = %i, residual = %i!\n", n, m, len);
> -        return AVERROR(EINVAL);
> -    } else if (n > 1 && m > 1) { /* 2D transform case */
> +        if (is_mdct && (l & 1)) /* Odd (i)MDCTs are not supported yet */
> +            return AVERROR(ENOTSUP);

I think ENOTSUP is not portable, which is why we use ENOSYS to signal 
unimplemented/unsupported features.

> +        s->n = l;
> +        s->m = 1;
> +        *tx = naive_fft;
> +        if (is_mdct) {
> +            s->scale = *((SCALE_TYPE *)scale);
> +            *tx = inv ? naive_imdct : naive_mdct;
> +        }
> +        return 0;
> +    }
> +
> +    if (n > 1 && m > 1) { /* 2D transform case */
>          if ((err = ff_tx_gen_compound_mapping(s)))
>              return err;
>          if (!(s->tmp = av_malloc(n*m*sizeof(*s->tmp))))
> -- 
> 2.30.0.rc2
>