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avformat/mp3enc: keep trailing padding spanning several packets

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The trailing padding is read from the AV_PKT_DATA_SKIP_SAMPLES side data of
every packet, overwriting the previous value, so only the last packet was
ever accounted for. A single packet holds at most one frame, which caps the
padding that can be written at 1152 + 528 + 1 samples.

LAME regularly reports more than that: gapless/gapless.mp3 carries 1984 and
comes out of a stream copy with 1681, decoding to 303 samples more than the
file it was copied from.

Accumulate instead, and add the decoder delay once the total is known.

Fixes: https://trac.ffmpeg.org/ticket/9755
This commit is contained in:
Romain Beauxis
2026-08-02 13:17:48 -05:00
committed by Romain Beauxis
parent 63d0237033
commit 23fccd657c
3 changed files with 14 additions and 2 deletions
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8
libavformat/mp3enc.c
View File

@@ -126,7 +126,7 @@ typedef struct MP3Context {
int initial_bitrate;
int has_variable_bitrate;
int delay;
int padding;
int64_t padding;
/* index of the audio stream */
int audio_stream_idx;
@@ -367,7 +367,9 @@ static int mp3_write_audio_packet(AVFormatContext *s, AVPacket *pkt)
AV_PKT_DATA_SKIP_SAMPLES,
&side_data_size);
if (side_data && side_data_size >= 10) {
mp3->padding = FFMAX((int64_t)AV_RL32(side_data + 4) + 528 + 1, 0);
uint32_t discard_padding = AV_RL32(side_data + 4);
/* Padding longer than a frame is spread over several packets. */
mp3->padding = discard_padding ? mp3->padding + discard_padding : 0;
if (!mp3->delay)
mp3->delay = FFMAX((int64_t)AV_RL32(side_data) - 528 - 1, 0);
} else {
@@ -449,6 +451,8 @@ static void mp3_update_xing(AVFormatContext *s)
}
/* write encoder delay/padding */
if (mp3->padding)
mp3->padding += 528 + 1;
if (mp3->delay >= 1 << 12) {
mp3->delay = (1 << 12) - 1;
av_log(s, AV_LOG_WARNING, "Too many samples of initial padding.\n");

5
tests/fate/gapless.mak
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@@ -4,6 +4,11 @@ fate-gapless-mp3: CMD = gapless $(TARGET_SAMPLES)/gapless/gapless.mp3 "-c:a mp3"
FATE_GAPLESSINFO_PROBE-$(CONFIG_MP3_DEMUXER) += fate-gapless-mp3-side-data
fate-gapless-mp3-side-data: CMD = ffprobe_demux $(TARGET_SAMPLES)/gapless/gapless.mp3
# The trailing padding of this sample spans two packets. The duration of the
# remuxed file has to stay the one fate-gapless-mp3-side-data reports.
FATE_GAPLESSENC_PROBE-$(call ALLYES, MP3_DEMUXER MP3_MUXER NULL_MUXER) += fate-gapless-mp3-remux
fate-gapless-mp3-remux: CMD = transcode mp3 $(TARGET_SAMPLES)/gapless/gapless.mp3 mp3 "-c copy" "-c copy" "-of compact -show_entries stream=start_pts,duration_ts" "" "" "" null
FATE_GAPLESS-$(call DEMDEC, MP3, MP3, ARESAMPLE_FILTER WAV_MUXER) += fate-audiomatch-square-mp3
fate-audiomatch-square-mp3: CMD = audio_match $(TARGET_SAMPLES)/audiomatch/square3.mp3 $(SAMPLES)/audiomatch/square3.wav

3
tests/ref/fate/gapless-mp3-remux Normal file
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@@ -0,0 +1,3 @@
4458dffaa3fd294d708f05965e1a3186 *tests/data/fate/gapless-mp3-remux.mp3
249138 tests/data/fate/gapless-mp3-remux.mp3
stream|start_pts=353600|duration_ts=218521600