[FFmpeg-devel] [PATCH] avcodec/aacenc: add strict bit rate control mode

[Lynne]

Apr 24, 2023, 11:27 by jrwu at chromium.org:

> From: Jeremy Wu <jrwu at google.com>
>
> In certain use cases, controlling the maximum frame size is critical. An
> example is when transmitting AAC packets over Bluetooth A2DP.
>
> While the spec allows the packets be fragmented (though UNRECOMMENDED),
> in practice most headsets do not recognize nor reassemble such packets.
>
> In this patch, we add a new mode to specify that the configured bit rate
> should be followed strictly up to frame level.
>
> Signed-off-by: Jeremy Wu <jrwu at chromium.org>
> ---
>  doc/APIchanges       |  3 +++
>  libavcodec/aacenc.c  | 11 +++++++++++
>  libavcodec/avcodec.h |  4 ++++
>  libavcodec/version.h |  2 +-
>  4 files changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/doc/APIchanges b/doc/APIchanges
> index 0b609e3d3b..e730a7e126 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -2,6 +2,9 @@ The last version increases of all libraries were on 2023-02-09
>
>  2023-04-10 - xxxxxxxxxx - lavu 58.6.100 - frame.h
>  av_frame_get_plane_buffer() now accepts const AVFrame*.
>  
> diff --git a/libavcodec/aacenc.c b/libavcodec/aacenc.c
> index ed036209e9..daf5538056 100644
> --- a/libavcodec/aacenc.c
> +++ b/libavcodec/aacenc.c
> @@ -1106,6 +1106,17 @@ static int aac_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
>  too_many_bits = FFMIN(too_many_bits, 6144 * s->channels - 3);
>  too_few_bits = FFMIN(FFMAX(rate_bits - rate_bits/4, target_bits), too_many_bits);
>  
> +        if (avctx->flags & AV_CODEC_FLAG_STRICT_BITRATE) {
>

Use avctx->bit_rate_tolerance instead. By default, it's set to ~400ish kbps.
Just detect if it's set to zero to enable this code path. You can set the
variable via both the command line and the API.


> +            if (rate_bits < frame_bits) {
> +                /* temporarily degrade quality and repeat until frame fits */
> +                s->lambda *= 0.75f;
> +                continue;
> +            }
> +            /* reset lambda when solution is found */
> +            s->lambda = avctx->global_quality > 0 ? avctx->global_quality : 120;
> +            break;
> +        }
>

That's a heavy handed approach. A better way would be to use the
rate_bits/frame_bits, multiply lambda by the ratio, and use that lambda as
the starting point. That way, you'd need less reencodes to satisfy the condition.

By the way, do you have any specific use for the encoder?
I am in the process of rewriting it, and I'd like to know where it's useful,
apart from the obvious streaming use-case.