[FFmpeg-devel] [PATCH 2/2] aacdec: correctly skip padding at the start of frames and during seeking

[Lynne]

Presently, our AAC decoder didn't skip any samples, unless the side data
contained skip information.

This uses the newly added ff_decoder_skip_samples function to skip samples
on two levels: base, and additional.
Base is the inherent decoder delay. Most containers already specify it,
hence it is overwritten in that case.
Additional is any extra samples that have to be skipped, in AAC's case,
it's the delay introduced by SBR.

Patch attached.


>From 82120460459cf90c330ff11fe0ccf4b954736b5c Mon Sep 17 00:00:00 2001
From: Lynne <d...@lynne.ee>
Date: Mon, 30 Oct 2023 05:40:22 +0100
Subject: [PATCH 2/2] aacdec: correctly skip padding at the start of frames and
 during seeking

Presently, our AAC decoder didn't skip any samples, unless the side data
contained skip information.

This uses the newly added ff_decoder_skip_samples function to skip samples
on two levels: base, and additional.
Base is the inherent decoder delay. Most containers already specify it,
hence it is overwritten in that case.
Additional is any extra samples that have to be skipped, in AAC's case,
it's the delay introduced by SBR.
---
 libavcodec/aac.h             |  1 +
 libavcodec/aacdec_template.c | 24 +++++++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/libavcodec/aac.h b/libavcodec/aac.h
index 285d3b7482..06ae2222cf 100644
--- a/libavcodec/aac.h
+++ b/libavcodec/aac.h
@@ -298,6 +298,7 @@ struct AACContext {
     AVCodecContext *avctx;
     AVFrame *frame;
 
+    int skip_samples;
     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 954399f86b..ab3037c0d5 100644
--- a/libavcodec/aacdec_template.c
+++ b/libavcodec/aacdec_template.c
@@ -532,6 +532,8 @@ static void flush(AVCodecContext *avctx)
             }
         }
     }
+
+    ac->skip_samples = 1;
 }
 
 /**
@@ -1251,6 +1253,7 @@ static av_cold int aac_decode_init(AVCodecContext *avctx)
         return AVERROR(ENOMEM);
     }
 
+    ac->skip_samples = 1;
     ac->random_state = 0x1f2e3d4c;
 
 #define MDCT_INIT(s, fn, len, sval)                                            \
@@ -2419,7 +2422,7 @@ static int decode_dynamic_range(DynamicRangeControl *che_drc,
 
 static int decode_fill(AACContext *ac, GetBitContext *gb, int len) {
     uint8_t buf[256];
-    int i, major, minor;
+    int i;
 
     if (len < 13+7*8)
         goto unknown;
@@ -2433,10 +2436,6 @@ static int decode_fill(AACContext *ac, GetBitContext *gb, int len) {
     if (ac->avctx->debug & FF_DEBUG_PICT_INFO)
         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;
-    }
-
 unknown:
     skip_bits_long(gb, len);
 
@@ -3403,6 +3402,21 @@ static int aac_decode_frame(AVCodecContext *avctx, AVFrame *frame,
         if (buf[buf_offset])
             break;
 
+    if (ac->skip_samples) {
+        int additional = 0;
+
+        if (ac->oc[1].m4ac.sbr)
+            additional = 3010;
+
+        err = ff_decode_skip_samples(avctx, frame,
+                                     ac->oc[1].m4ac.frame_length_short ? 960 : 1024,
+                                     additional);
+        if (err < 0)
+            return err;
+
+        ac->skip_samples = 0;
+    }
+
     return buf_size > buf_offset ? buf_consumed : buf_size;
 }
 
-- 
2.42.0


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