On 2021-11-25 12:56 am, Michael Niedermayer wrote:
On Wed, Nov 24, 2021 at 10:58:00AM +0530, Gyan Doshi wrote:
On 2021-11-24 01:16 am, Michael Niedermayer wrote:
On Tue, Nov 23, 2021 at 06:41:06PM +0530, Gyan Doshi wrote:
Very high stts sample deltas may occasionally be intended but usually
they are written in error or used to store a negative value for dts correction
when treated as signed 32-bit integers.
This option lets the user set an upper limit, beyond which the delta
is clamped to 1. Negative values of under 1 second are used to adjust
dts.
Unit is the track time scale. Default is INT_MAX which maintains current
handling.
---
doc/demuxers.texi | 6 ++++++
libavformat/isom.h | 1 +
libavformat/mov.c | 14 +++++++++-----
3 files changed, 16 insertions(+), 5 deletions(-)
diff --git a/doc/demuxers.texi b/doc/demuxers.texi
index cab8a7072c..15078b9b1b 100644
--- a/doc/demuxers.texi
+++ b/doc/demuxers.texi
@@ -713,6 +713,12 @@ specify.
@item decryption_key
16-byte key, in hex, to decrypt files encrypted using ISO Common Encryption
(CENC/AES-128 CTR; ISO/IEC 23001-7).
+
+@item max_stts_delta
+The sample offsets stored in a track's stts box are 32-bit unsigned integers.
However, very large values usually indicate
+a value written by error or a storage of a small negative value as a way to
correct accumulated DTS delay.
+Range is 0 to UINT_MAX. Default is INT_MAX.
+
@end table
@subsection Audible AAX
diff --git a/libavformat/isom.h b/libavformat/isom.h
index ef8f19b18c..625dea8421 100644
--- a/libavformat/isom.h
+++ b/libavformat/isom.h
@@ -305,6 +305,7 @@ typedef struct MOVContext {
int32_t movie_display_matrix[3][3]; ///< display matrix from mvhd
int have_read_mfra_size;
uint32_t mfra_size;
+ uint32_t max_stts_delta;
} MOVContext;
int ff_mp4_read_descr_len(AVIOContext *pb);
diff --git a/libavformat/mov.c b/libavformat/mov.c
index 451cb78bbf..bbda07ac42 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -3965,14 +3965,17 @@ static void mov_build_index(MOVContext *mov, AVStream
*st)
current_offset += sample_size;
stream_size += sample_size;
- /* A negative sample duration is invalid based on the spec,
- * but some samples need it to correct the DTS. */
- if (sc->stts_data[stts_index].duration < 0) {
+ /* STTS sample offsets are uint32 but some files store it as
int32
+ * with negative values used to correct DTS delays.
+ There may be abnormally large values as well. */
+ if (sc->stts_data[stts_index].duration > mov->max_stts_delta) {
+ // assume high delta is a negative correction if less than
1 second
+ int32_t delta_magnitude = *((int32_t
*)&sc->stts_data[stts_index].duration);
av_log(mov->fc, AV_LOG_WARNING,
- "Invalid SampleDelta %d in STTS, at %d st:%d\n",
+ "Correcting too large SampleDelta %u in STTS, at %d
st:%d.\n",
sc->stts_data[stts_index].duration, stts_index,
st->index);
- dts_correction += sc->stts_data[stts_index].duration - 1;
+ dts_correction += (delta_magnitude < 0 && FFABS(delta_magnitude)
< sc->time_scale ? delta_magnitude - 1 : 0);
sc->stts_data[stts_index].duration = 1;
}
current_dts += sc->stts_data[stts_index].duration;
@@ -8566,6 +8569,7 @@ static const AVOption mov_options[] = {
{ "decryption_key", "The media decryption key (hex)",
OFFSET(decryption_key), AV_OPT_TYPE_BINARY, .flags = AV_OPT_FLAG_DECODING_PARAM },
{ "enable_drefs", "Enable external track support.",
OFFSET(enable_drefs), AV_OPT_TYPE_BOOL,
{.i64 = 0}, 0, 1, FLAGS },
+ { "max_stts_delta", "treat offsets above this value as invalid",
OFFSET(max_stts_delta), AV_OPT_TYPE_INT, {.i64 = INT_MAX}, 0, UINT_MAX, .flags =
AV_OPT_FLAG_DECODING_PARAM },
{ NULL },
};
The stts is used in other places, the value parsed as unsigned will cause
problems there too
I see stts duration used in 3 places in mov.c
mov_read_stts(), which my earlier patch changed.
mov_build_index(), which this patch changes,
mov_read_trak() where frame rate is populated for CFR streams and is called
very shortly after mov_build_index().
I don't think that can break for non-malformed files as the only duration
entry in the stream is not likely to be > INT_MAX
In any case, after this patch, with default option value, it will see the
same values as earlier.
If iam not mistaken there is code that adds the values up to initilize some
duration. this works with 64bit so the small negative values interpreted as
unsigned will result in a wrong result i would expect
that may very well not be the only issue
Yeah, I see the stream duration being set in mov_read_stts().
Interestingly, after 153639cb9cf, the unadjusted deltas were being
added, which would have included all negative deltas, small or large.
153639cb9cf shifted the original negative delta adjustment code from
mov_read_stts to mov_build_index. Was that necessary?
the cast is also fragile as it will break when someone tries to change it
to int64
In what circumstances would that happen?
when someone tries to change the code.
having both signed and unsigned 32bit in a 32bit element screams
Originally, negative values never left mov_read_stts. Maybe that's best.
Anyway, FATE passes, so it may only affect edge cases if it does. But I
will walk through the code to check.
[...]
also please select the default so that it works with all real world files
No single value can all work for all files. Hence the option.
but what i wrote was about all real world files.
Sure you can make a file that fails but so far the files i have seen and
the ones you told me about all would work with 49% of all possibly defaults
why do you pick a default that doesnt work with them and then argue about
that ?
Lets try to fix the code not win the argument please
I picked the default to keep the old behaviour. 99% of files I see have
all valid offsets.
Among the rest, large values are mostly errors. Very few are intended.
What default do you suggest?
Regards,
Gyan
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