[FFmpeg-devel] [PATCH] libavcodec/flacenc: add backward-compatible 32 bit-per-sample capability

[Lynne]

19 Dec 2021, 22:41 by mvanb1 at gmail.com:

> Op zo 19 dec. 2021 om 22:11 schreef Lynne <dev at lynne.ee>:
>
>> What happens if there's an overflow and the prediction coefficients
>> are lowered? Is there a loss of bits? What about if it gives up?
>>
>
> The result remains lossless. If the prediction coefficients are
> lowered, the residual of the prediction increases. This means the file
> gets a little bigger, but the data is still all there.
>
> If it gives up on searching for suitable LPC coefficients, it uses a
> verbatim subframe instead of an LPC subframe. That means all data is
> stored as unencoded PCM, with no form of prediction whatsoever. This
> results in (usually a large) increase in storage space needed, but
> this is still lossless.
>

That's reasonable.

Some code style issues:

> +        for (i = 0; i < s->frame.blocksize * s->channels; i++) {
> +            AV_WL32(tmp + 4*i, samples0[i]);
> +        }

No brackets here.

> +                    if(!ff_flacdsp_lpc_encode_c_32_overflow_detect(res, smp, n, opt_order,
> +                                                                   lpc_try, shift[opt_order-1]))
> +                        continue;

Put a space after the if.

> +                    if(!ff_flacdsp_lpc_encode_c_32_overflow_detect(res, smp, n, i+1,
> +                                                                   coefs[i], shift[i]))
> +                        continue;

And here, and in 2 other places.

> +        if (pmax > 0) {
> +            double scale;
> +            if(lpc_reduction_tries >= 2)

Space after if.

> +                return 0;
> +            lpc_reduction_tries++;
> +            scale = (double)INT32_MAX/pmax;
> +            for (int i = 0; i < order; i++)
> +                coefs[i] *= scale;
> +        }

Could you replace this with a fixed-point implementation as well?
Even if it's outside of the audio path, we already have one issue
in the flac code where using a double in assembly is causing a
test to fail. I still have to rewrite that from inline assembly to
proper external asm.