[FFmpeg-cvslog] movenc: Get rid of frag_start
Hu Weiwen
git at videolan.org
Wed Aug 18 13:15:48 EEST 2021
ffmpeg | branch: master | Hu Weiwen <sehuww at mail.scut.edu.cn> | Thu Aug 12 12:42:12 2021 +0800| [4cdab8d022844dddcac3c4c5c849fd207bc73dea] | committer: Martin Storsjö
movenc: Get rid of frag_start
"frag_start" is redundant, and every occurance can be replaced with cluster[0].dts - start_dts
The proof of no behaviour changes: (All line number below is based on commit bff7d662d728)
"frag_start" is read at 4 place (with all possible call stacks):
mov_write_packet
...
mov_flush_fragment
mov_write_moof_tag
mov_write_moof_tag_internal
mov_write_traf_tag
mov_write_tfxd_tag (#1)
mov_write_tfdt_tag (#2)
mov_add_tfra_entries (#3)
mov_write_sidx_tags
mov_write_sidx_tag (#4)
mov_write_trailer
mov_auto_flush_fragment
mov_flush_fragment
... (#1 #2 #3 #4)
mov_write_sidx_tags
mov_write_sidx_tag (#4)
shift_data
compute_sidx_size
get_sidx_size
mov_write_sidx_tags
mov_write_sidx_tag (#4)
All read happens in "mov_write_trailer" and "mov_write_moof_tag". So we need to prove no behaviour change in these two
functions.
Condition 1: for every track that have "trk->entry == 0", trk->frag_start == trk->track_duration.
Condition 2: for every track that have "trk->entry > 0", trk->frag_start == trk->cluster[0].dts - trk->start_dts.
Definition 1: "Before flush" means just before the invocation of "mov_flush_fragment", except for the auto-flush case in
"mov_write_single_packet", which means before L5934.
Lemma 1: If Condition 1 & 2 is true before flush, Condition 1 & 2 is still true after "mov_flush_fragment" returns.
Proof:
No update to the tracks that have "trk->entry == 0" before flushing, so we only consider tracks that have "trk->entry > 0":
Case 1: !moov_written and moov will be written in this iteration
trk->entry = 0 L5366
trk->frag_start == trk->cluster[0].dts - trk->start_dts Lemma condition
trk->frag_start += trk->start_dts + trk->track_duration - trk->cluster[0].dts; L5363
So trk->entry == 0 && trk->frag_start == trk->track_duration
Case 2: !moov_written and moov will NOT be written in this iteration
nothing changed
Case 3: moov_written
trk->entry = 0 L5445
trk->frag_start == trk->cluster[0].dts - trk->start_dts Lemma condition
trk->frag_start += trk->start_dts + trk->track_duration - trk->cluster[0].dts; L5444
So trk->entry == 0 && trk->frag_start == trk->track_duration
Note that trk->track_duration may be updated for the tracks that have "trk->entry > 0" (mov_write_moov_tag will
update track_duration of "tmcd" track, but it must have 1 entry). But in all case, trk->frag_start is also updated
to consider the new value.
Lemma 2: If Condition 1 & 2 is true before "ff_mov_write_packet" invocation, Condition 1 & 2 is still true after it returns.
Proof:
Only the track corresponding to the pkt is updated, and no update to relevant variables if trk->entry > 0 before invocation.
So we only need to prove "trk->frag_start == trk->cluster[0].dts - trk->start_dts" after trk->entry increase from 0 to 1.
Case 1: trk->start_dts == AV_NOPTS_VALUE
Case 1.1: trk->frag_discont && use_editlist
trk->cluster[0].dts = pkt->dts at L5741
trk->frag_start = pkt->pts at L5785
trk->start_dts = pkt->dts - pkt->pts at L5786
So trk->frag_start == trk->cluster[0].dts - trk->start_dts
Case 1.2: trk->frag_discont && !use_editlist
trk->cluster[0].dts = pkt->dts at L5741
trk->frag_start = pkt->dts at L5790
trk->start_dts = 0 at L5791
So trk->frag_start == trk->cluster[0].dts - trk->start_dts
Case 1.3: !trk->frag_discont
trk->cluster[0].dts = pkt->dts at L5741
trk->frag_start = 0 init
trk->start_dts = pkt->dts at L5779
So trk->frag_start == trk->cluster[0].dts - trk->start_dts
Case 2: trk->start_dts != AV_NOPTS_VALUE
Case 2.1: trk->frag_discont
trk->cluster[0].dts = pkt->dts at L5741
trk->frag_start = pkt->dts - trk->start_dts at L5763
So trk->frag_start == trk->cluster[0].dts - trk->start_dts
Case 2.2: !trk->frag_discont
trk->cluster[0].dts = trk->start_dts + trk->track_duration at L5749
trk->track_duration == trk->frag_start Lemma condition
So trk->frag_start == trk->cluster[0].dts - trk->start_dts
Lemma 3: Condition 1 & 2 is true in all case before and after "ff_mov_write_packet" invocation, before flush and after
"mov_flush_fragment" returns.
Proof: All updates to relevant variable happen either in "ff_mov_write_packet", or during flush. And Condition 1 & 2
is true initially. So with lemma 1 & 2, we can prove this use induction.
Noticed that all read of "frag_start" only happen in "trk->entry > 0" branch. Now we need to prove Condition 2 is true
before each read.
Because no update to variables relevant to Condition 2 between "before flush" and "mov_write_moof_tag" invocation, we
can conclude Condition 2 is true before every invocation of "mov_write_moof_tag". No behaviour change in
"mov_write_moof_tag" is proved.
In "mov_write_trailer", No update to relevant variables after the last flush and before the invocation of
"mov_write_sidx_tag". So no behaviour change to "mov_write_trailer" is proved.
Q.E.D.
Signed-off-by: Hu Weiwen <sehuww at mail.scut.edu.cn>
Signed-off-by: Martin Storsjö <martin at martin.st>
> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=4cdab8d022844dddcac3c4c5c849fd207bc73dea
---
libavformat/movenc.c | 24 ++++--------------------
libavformat/movenc.h | 1 -
2 files changed, 4 insertions(+), 21 deletions(-)
diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index a460cd9ada..18410c70fa 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -4483,8 +4483,7 @@ static int mov_write_tfxd_tag(AVIOContext *pb, MOVTrack *track)
avio_write(pb, uuid, sizeof(uuid));
avio_w8(pb, 1);
avio_wb24(pb, 0);
- avio_wb64(pb, track->start_dts + track->frag_start +
- track->cluster[0].cts);
+ avio_wb64(pb, track->cluster[0].dts + track->cluster[0].cts);
avio_wb64(pb, track->end_pts -
(track->cluster[0].dts + track->cluster[0].cts));
@@ -4563,8 +4562,7 @@ static int mov_add_tfra_entries(AVIOContext *pb, MOVMuxContext *mov, int tracks,
info->size = size;
// Try to recreate the original pts for the first packet
// from the fields we have stored
- info->time = track->start_dts + track->frag_start +
- track->cluster[0].cts;
+ info->time = track->cluster[0].dts + track->cluster[0].cts;
info->duration = track->end_pts -
(track->cluster[0].dts + track->cluster[0].cts);
// If the pts is less than zero, we will have trimmed
@@ -4602,7 +4600,7 @@ static int mov_write_tfdt_tag(AVIOContext *pb, MOVTrack *track)
ffio_wfourcc(pb, "tfdt");
avio_w8(pb, 1); /* version */
avio_wb24(pb, 0);
- avio_wb64(pb, track->frag_start);
+ avio_wb64(pb, track->cluster[0].dts - track->start_dts);
return update_size(pb, pos);
}
@@ -4679,8 +4677,7 @@ static int mov_write_sidx_tag(AVIOContext *pb,
if (track->entry) {
entries = 1;
- presentation_time = track->start_dts + track->frag_start +
- track->cluster[0].cts;
+ presentation_time = track->cluster[0].dts + track->cluster[0].cts;
duration = track->end_pts -
(track->cluster[0].dts + track->cluster[0].cts);
starts_with_SAP = track->cluster[0].flags & MOV_SYNC_SAMPLE;
@@ -5359,10 +5356,6 @@ static int mov_flush_fragment(AVFormatContext *s, int force)
mov->moov_written = 1;
mov->mdat_size = 0;
for (i = 0; i < mov->nb_streams; i++) {
- if (mov->tracks[i].entry)
- mov->tracks[i].frag_start += mov->tracks[i].start_dts +
- mov->tracks[i].track_duration -
- mov->tracks[i].cluster[0].dts;
mov->tracks[i].entry = 0;
mov->tracks[i].end_reliable = 0;
}
@@ -5416,11 +5409,7 @@ static int mov_flush_fragment(AVFormatContext *s, int force)
MOVTrack *track = &mov->tracks[i];
int buf_size, write_moof = 1, moof_tracks = -1;
uint8_t *buf;
- int64_t duration = 0;
- if (track->entry)
- duration = track->start_dts + track->track_duration -
- track->cluster[0].dts;
if (mov->flags & FF_MOV_FLAG_SEPARATE_MOOF) {
if (!track->entry)
continue;
@@ -5440,8 +5429,6 @@ static int mov_flush_fragment(AVFormatContext *s, int force)
ffio_wfourcc(s->pb, "mdat");
}
- if (track->entry)
- track->frag_start += duration;
track->entry = 0;
track->entries_flushed = 0;
track->end_reliable = 0;
@@ -5760,7 +5747,6 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
/* New fragment, but discontinuous from previous fragments.
* Pretend the duration sum of the earlier fragments is
* pkt->dts - trk->start_dts. */
- trk->frag_start = pkt->dts - trk->start_dts;
trk->end_pts = AV_NOPTS_VALUE;
trk->frag_discont = 0;
}
@@ -5782,12 +5768,10 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
/* Pretend the whole stream started at pts=0, with earlier fragments
* already written. If the stream started at pts=0, the duration sum
* of earlier fragments would have been pkt->pts. */
- trk->frag_start = pkt->pts;
trk->start_dts = pkt->dts - pkt->pts;
} else {
/* Pretend the whole stream started at dts=0, with earlier fragments
* already written, with a duration summing up to pkt->dts. */
- trk->frag_start = pkt->dts;
trk->start_dts = 0;
}
trk->frag_discont = 0;
diff --git a/libavformat/movenc.h b/libavformat/movenc.h
index af1ea0bce6..daeaad1cc6 100644
--- a/libavformat/movenc.h
+++ b/libavformat/movenc.h
@@ -138,7 +138,6 @@ typedef struct MOVTrack {
AVIOContext *mdat_buf;
int64_t data_offset;
- int64_t frag_start;
int frag_discont;
int entries_flushed;
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