[FFmpeg-devel] [PATCH] avutil/timestamp: keep microsecond precision in av_ts_make_time_string

[Andreas Rheinhardt]

Andreas Rheinhardt:
> Marton Balint:
>> av_ts_make_time_string() used "%.6g" format in the past, but this format was
>> losing precision even when the timestamp to be printed was not that large. For
>> example for 3 hours (10800) seconds, only 1 decimal digit was printed, which
>> made this format inaccurate when it was used in e.g. the silencedetect filter.
>> Other detection filters printing timestamps had similar issues.
>>
>> So let's change the used format to "%.6f" instead, we have plenty of space in
>> the string buffer, we can somewhat imitate existing behaviour of %g by trimming
>> ending zeroes and the potential decimal point, which can be any non-numeric
>> character. In order not to trim "inf" as well, we assume that the decimal
>> point does not contain the letter "f".
>>
>> We also no longer use scientific representation to make parsing and printing
>> easier, because the theoretical maximum of INT64_MAX*INT32_MAX still fits into
>> the string buffer in normal form.
>>
>> Since the additional processing yields more code, inlineing this function no
>> longer make sense, so this commit also changes the API to actually export the
>> function instead of having it inlinable in the header.
>>
>> Thanks for Allan Cady for bringing up this issue.
>>
>> Signed-off-by: Marton Balint <cus at passwd.hu>
>> ---
>>  doc/APIchanges                               |  3 ++
>>  libavutil/Makefile                           |  1 +
>>  libavutil/timestamp.c                        | 33 ++++++++++++++++++++
>>  libavutil/timestamp.h                        |  8 +----
>>  libavutil/version.h                          |  2 +-
>>  tests/ref/fate/filter-metadata-scdet         | 12 +++----
>>  tests/ref/fate/filter-metadata-silencedetect |  2 +-
>>  7 files changed, 46 insertions(+), 15 deletions(-)
>>  create mode 100644 libavutil/timestamp.c
>>
>> diff --git a/doc/APIchanges b/doc/APIchanges
>> index a44c8e4f10..1afde062a5 100644
>> --- a/doc/APIchanges
>> +++ b/doc/APIchanges
>> @@ -2,6 +2,9 @@ The last version increases of all libraries were on 2024-03-07
>>  
>>  API changes, most recent first:
>>  
>> +2024-03-xx - xxxxxxxxxx - lavu 59.3.100 - timestamp.h
>> +  av_ts_make_time_string() is no longer an inline function. It is now exported.
>> +
>>  2024-03-xx - xxxxxxxxxx - lavu 59.2.100 - channel_layout.h
>>    Add AV_CHANNEL_LAYOUT_RETYPE_FLAG_CANONICAL.
>>  
>> diff --git a/libavutil/Makefile b/libavutil/Makefile
>> index e7709b97d0..5e75aa1855 100644
>> --- a/libavutil/Makefile
>> +++ b/libavutil/Makefile
>> @@ -174,6 +174,7 @@ OBJS = adler32.o                                                        \
>>         threadmessage.o                                                  \
>>         time.o                                                           \
>>         timecode.o                                                       \
>> +       timestamp.o                                                      \
>>         tree.o                                                           \
>>         twofish.o                                                        \
>>         utils.o                                                          \
>> diff --git a/libavutil/timestamp.c b/libavutil/timestamp.c
>> new file mode 100644
>> index 0000000000..06fb47e94c
>> --- /dev/null
>> +++ b/libavutil/timestamp.c
>> @@ -0,0 +1,33 @@
>> +/*
>> + * This file is part of FFmpeg.
>> + *
>> + * FFmpeg is free software; you can redistribute it and/or
>> + * modify it under the terms of the GNU Lesser General Public
>> + * License as published by the Free Software Foundation; either
>> + * version 2.1 of the License, or (at your option) any later version.
>> + *
>> + * FFmpeg is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>> + * Lesser General Public License for more details.
>> + *
>> + * You should have received a copy of the GNU Lesser General Public
>> + * License along with FFmpeg; if not, write to the Free Software
>> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
>> + */
>> +
>> +#include "timestamp.h"
>> +
>> +char *av_ts_make_time_string(char *buf, int64_t ts, const AVRational *tb)
>> +{
>> +    if (ts == AV_NOPTS_VALUE) {
>> +        snprintf(buf, AV_TS_MAX_STRING_SIZE, "NOPTS");
>> +    } else {
>> +        int last = snprintf(buf, AV_TS_MAX_STRING_SIZE, "%.6f", av_q2d(*tb) * ts);
>> +        last = FFMIN(last, AV_TS_MAX_STRING_SIZE - 1) - 1;
>> +        for (; last && buf[last] == '0'; last--);
>> +        for (; last && buf[last] != 'f' && (buf[last] < '0' || buf[0] > '9'); last--);
>> +        buf[last + 1] = '\0';
>> +    }
>> +    return buf;
>> +}
> 
> As has already been said before: Simply using %.6f will discard
> significant digits for small timestamps. E.g. 10^-7 will simply print
> 0.000000, which will then be converted to "0" by your code. The old code
> printed 1.
> 
> Furthermore, there are more problems:
> "A double argument representing an infinity is converted in one of the
> styles [-]inf or [-]infinity — which style is implementation-defined."
> Your code would trim infinity to "inf". It would be even worse with nan:
> "A double argument representing a NaN is converted in one of the styles
> [-]nan or [-]nan(n-char-sequence) — which style, and the meaning of
> any n-char-sequence, is implementation-defined."
> Furthermore, there is no guarantee that the "decimal-point character" is
> actually a single character (it's a char* in struct lcov after all).
> 
> If I am not mistaken, then NANs and infinities can't happen here unless
> the timebase is malformed (denominator zero); and in this case one could
> use the NOPTS codepath.
> 

While just at it: As I already mentioned, this function has a design
defect: It passes the timebase by pointer and not just by value. So if
we add a non-inlined version of this, it should be a new function that
receives the timebase by value and av_ts_make_time_string() should
instead call this new function like
static inline char *av_ts_make_time_string(char *buf, int64_t ts, const
AVRational *tb)
{
    return new_func(buf, ts, *tb);
}

- Andreas