[FFmpeg-devel] [PATCH v2] avcodec/aacenc: add strict bit rate control option

[Jeremy Wu]

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 to be fragmented (but UNRECOMMENDED),
in practice most headsets do not recognize nor reassemble such packets.

In this patch, we allow setting `bit_rate_tolerance` to 0 to indicate
that the specified bit rate should be treated as an upper bound up to
frame level.

Signed-off-by: Jeremy Wu <jrwu at chromium.org>
---
 libavcodec/aacenc.c        | 12 ++++++++++++
 libavcodec/options_table.h |  2 +-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/libavcodec/aacenc.c b/libavcodec/aacenc.c
index ed036209e9..f48f057022 100644
--- a/libavcodec/aacenc.c
+++ b/libavcodec/aacenc.c
@@ -1106,6 +1106,18 @@ 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);
 
+        /* When strict bit-rate control is demanded */
+        if (avctx->bit_rate_tolerance == 0) {
+            if (rate_bits < frame_bits) {
+                float ratio = ((float)rate_bits) / frame_bits;
+                s->lambda *= FFMIN(0.9f, ratio);
+                continue;
+            }
+            /* reset lambda when solution is found */
+            s->lambda = avctx->global_quality > 0 ? avctx->global_quality : 120;
+            break;
+        }
+
         /* When using ABR, be strict (but only for increasing) */
         too_few_bits = too_few_bits - too_few_bits/8;
         too_many_bits = too_many_bits + too_many_bits/2;
diff --git a/libavcodec/options_table.h b/libavcodec/options_table.h
index f331ce2861..716787aa59 100644
--- a/libavcodec/options_table.h
+++ b/libavcodec/options_table.h
@@ -50,7 +50,7 @@ static const AVOption avcodec_options[] = {
 {"bt", "Set video bitrate tolerance (in bits/s). In 1-pass mode, bitrate tolerance specifies how far "
        "ratecontrol is willing to deviate from the target average bitrate value. This is not related "
        "to minimum/maximum bitrate. Lowering tolerance too much has an adverse effect on quality.",
-       OFFSET(bit_rate_tolerance), AV_OPT_TYPE_INT, {.i64 = AV_CODEC_DEFAULT_BITRATE*20 }, 1, INT_MAX, V|E},
+       OFFSET(bit_rate_tolerance), AV_OPT_TYPE_INT, {.i64 = AV_CODEC_DEFAULT_BITRATE*20 }, 0, INT_MAX, A|V|E},
 {"flags", NULL, OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = DEFAULT }, 0, UINT_MAX, V|A|S|E|D, "flags"},
 {"unaligned", "allow decoders to produce unaligned output", 0, AV_OPT_TYPE_CONST, { .i64 = AV_CODEC_FLAG_UNALIGNED }, INT_MIN, INT_MAX, V | D, "flags" },
 {"mv4", "use four motion vectors per macroblock (MPEG-4)", 0, AV_OPT_TYPE_CONST, {.i64 = AV_CODEC_FLAG_4MV }, INT_MIN, INT_MAX, V|E, "flags"},
-- 
2.40.0.634.g4ca3ef3211-goog