[FFmpeg-devel] [PATCH] aacdec: padding skip improvements

[Lynne]

For some reason, this was never set, which meant all **raw** AAC in ADTS
streams, except faac, had extra samples at the start.

Despite this being a standard MDCT-based codec with a frame size of 1024,
hence a delay of 1024 samples at the start, all major encoders, excluding
faac and FFmpeg, use 2048 samples of padding.

Because this is also such a mess, make it an option to allow for
users to configure the amount of padding.

The FFmpeg encoder will be modified to also output 2048 samples of padding
at the start, to make it in line with other encoders.

Yes, this leaves FATE pretty sad. Will be fixed by another patch once
the discussions have completed as to what should be the default.


>From dca29a894a60cdca1682fa6d0d3ee56c552b3d49 Mon Sep 17 00:00:00 2001
From: Lynne <d...@lynne.ee>
Date: Tue, 3 Oct 2023 05:57:50 +0200
Subject: [PATCH] aacdec: padding skip improvements

For some reason, this was never set, which meant all **raw** AAC in ADTS
streams, except faac, had extra samples at the start.

Despite this being a standard MDCT-based codec with a frame size of 1024,
hence a delay of 1024 samples at the start, all major encoders, excluding
faac and FFmpeg, use 2048 samples of padding.

The FFmpeg encoder will be modified to also output 2048 samples of padding
at the start, to make it in line with other encoders.

Yes, this leaves FATE pretty sad. Will fix it with the real version of the patch.
---
 libavcodec/aac.h             |  1 +
 libavcodec/aacdec_template.c | 20 +++++++++++++++-----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/libavcodec/aac.h b/libavcodec/aac.h
index 285d3b7482..79bbb3cce5 100644
--- a/libavcodec/aac.h
+++ b/libavcodec/aac.h
@@ -298,6 +298,7 @@ struct AACContext {
     AVCodecContext *avctx;
     AVFrame *frame;
 
+    int override_padding;
     int is_saved;                 ///< Set if elements have stored overlap from previous frame.
     DynamicRangeControl che_drc;
 
diff --git a/libavcodec/aacdec_template.c b/libavcodec/aacdec_template.c
index 51a4cb2b66..70c5c6ab4c 100644
--- a/libavcodec/aacdec_template.c
+++ b/libavcodec/aacdec_template.c
@@ -1273,6 +1273,9 @@ static av_cold int aac_decode_init(AVCodecContext *avctx)
     if (ret < 0)
         return ret;
 
+    /* Usually overridden by side data */
+    avctx->internal->skip_samples = ac->override_padding;
+
     return 0;
 }
 
@@ -2417,14 +2420,16 @@ static int decode_dynamic_range(DynamicRangeControl *che_drc,
     return n;
 }
 
-static int decode_fill(AACContext *ac, GetBitContext *gb, int len) {
-    uint8_t buf[256];
-    int i, major, minor;
+static int decode_fill(AACContext *ac, GetBitContext *gb, int len)
+{
+    uint8_t buf[256] = { 0 };
+    int i, major, minor, micro;
 
     if (len < 13+7*8)
         goto unknown;
 
-    get_bits(gb, 13); len -= 13;
+    get_bits(gb, 13);
+    len -= 13;
 
     for(i=0; i+1<sizeof(buf) && len>=8; i++, len-=8)
         buf[i] = get_bits(gb, 8);
@@ -2434,7 +2439,9 @@ static int decode_fill(AACContext *ac, GetBitContext *gb, int len) {
         av_log(ac->avctx, AV_LOG_DEBUG, "FILL:%s\n", buf);
 
     if (sscanf(buf, "libfaac %d.%d", &major, &minor) == 2){
-        ac->avctx->internal->skip_samples = 1024;
+        ac->avctx->internal->skip_samples -= 1024;
+    } else if ((sscanf(buf, "avc %d.%d.%d", &major, &minor, &micro) == 3)) {
+        ac->avctx->internal->skip_samples -= 1024;
     }
 
 unknown:
@@ -3471,6 +3478,9 @@ static const AVOption options[] = {
       { "coded",    "order in which the channels are coded in the bitstream",
         0, AV_OPT_TYPE_CONST, { .i64 = CHANNEL_ORDER_CODED }, .flags = AACDEC_FLAGS, "channel_order" },
 
+    { "padding", "Override the padding at the start of a stream.\n",
+        offsetof(AACContext, override_padding), AV_OPT_TYPE_INT, { .i64 = 2048 }, 1024, 8192, AACDEC_FLAGS },
+
     {NULL},
 };
 
-- 
2.42.0


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