[FFmpeg-devel] [PATCH] change av_ts_make_time_string precision

[Marton Balint]

On Mon, 11 Mar 2024, Andreas Rheinhardt wrote:

> Allan Cady via ffmpeg-devel:
>> From: "Allan Cady" <allancady at yahoo.com>
>>
>> I propose changing the format to "%.6f", which will
>> give microsecond precision for all timestamps, regardless of
>> offset. Trailing zeros can be trimmed from the fraction, without
>> losing precision. If the length of the fixed-precision formatted
>> timestamp exceeds the length of the allocated buffer
>> (AV_TS_MAX_STRING_SIZE, currently 32, less one for the
>> terminating null), then we can fall back to scientific notation, though
>> this seems almost certain to never occur, because 32 characters would
>> allow a maximum timestamp value of (32 - 1 - 6 - 1) = 24 characters.
>> By my calculation, 10^24 seconds is about six orders of magnitude
>> greater than the age of the universe.
>>
>> The fix proposed here follows the following logic:
>>
>> 1) Try formatting the number of seconds using "%.6f". This will
>> always result in a string with six decimal digits in the fraction,
>> possibly including trailing zeros. (e.g. "897234.73200").
>>
>> 2) Check if that string would overflow the buffer. If it would, then
>> format it using scientific notation ("%.8g").
>>
>> 3) If the original fixed-point format fits, then trim any trailing
>> zeros and decimal point, and return that result.
>>
>> Making this change broke two fate tests, filter-metadata-scdet,
>> and filter-metadata-silencedetect. To correct this, I've modified
>> tests/ref/fate/filter-metadata-scdet and
>> tests/ref/fate/filter-metadata-silencedetect to match the
>> new output.
>>
>> Signed-off-by: Allan Cady <allancady at yahoo.com>
>> ---
>>  libavutil/timestamp.h                        | 53 +++++++++++++++++++-
>>  tests/ref/fate/filter-metadata-scdet         | 12 ++---
>>  tests/ref/fate/filter-metadata-silencedetect |  2 +-
>>  3 files changed, 58 insertions(+), 9 deletions(-)
>>
>> diff --git a/libavutil/timestamp.h b/libavutil/timestamp.h
>> index 2b37781eba..2f04f9bb2b 100644
>> --- a/libavutil/timestamp.h
>> +++ b/libavutil/timestamp.h
>> @@ -25,6 +25,7 @@
>>  #define AVUTIL_TIMESTAMP_H
>>
>>  #include "avutil.h"
>> +#include <stdbool.h>
>>
>>  #if defined(__cplusplus) && !defined(__STDC_FORMAT_MACROS) && !defined(PRId64)
>>  #error missing -D__STDC_FORMAT_MACROS / #define __STDC_FORMAT_MACROS
>> @@ -53,6 +54,32 @@ static inline char *av_ts_make_string(char *buf, int64_t ts)
>>   */
>>  #define av_ts2str(ts) av_ts_make_string((char[AV_TS_MAX_STRING_SIZE]){0}, ts)
>>
>> +/**
>> + * Strip trailing zeros and decimal point from a string. Performed
>> + * in-place on input buffer. For local use only by av_ts_make_time_string.
>> + *
>> + * e.g.:
>> + * "752.378000" -> "752.378"
>> + *        "4.0" -> "4"
>> + *      "97300" -> "97300"
>> + */
>> +static inline void av_ts_strip_trailing_zeros_and_decimal_point(char *str) {
>> +    if (strchr(str, '.'))
>> +    {
>> +        int i;
>> +        for (i = strlen(str) - 1; i >= 0 && str[i] == '0'; i--) {
>> +            str[i] = '\0';
>> +        }
>> +
>> +        // Remove decimal point if it's the last character
>> +        if (i >= 0 && str[i] == '.') {
>> +            str[i] = '\0';
>> +        }
>> +
>> +        // String was modified in place; no need for return value.
>> +    }
>> +}
>> +
>>  /**
>>   * Fill the provided buffer with a string containing a timestamp time
>>   * representation.
>> @@ -65,8 +92,30 @@ static inline char *av_ts_make_string(char *buf, int64_t ts)
>>  static inline 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                      snprintf(buf, AV_TS_MAX_STRING_SIZE, "%.6g", av_q2d(*tb) * ts);
>> +    if (ts == AV_NOPTS_VALUE)
>> +    {
>> +        snprintf(buf, AV_TS_MAX_STRING_SIZE, "NOPTS");
>> +    }
>> +    else
>> +    {
>> +        const int max_fraction_digits = 6;
>> +
>> +        // Convert 64-bit timestamp to double, using rational timebase
>> +        double seconds = av_q2d(*tb) * ts;
>> +
>> +        int length = snprintf(NULL, 0, "%.*f", max_fraction_digits, seconds);
>> +        if (length <= AV_TS_MAX_STRING_SIZE - 1)
>> +        {
>> +            snprintf(buf, AV_TS_MAX_STRING_SIZE, "%.*f", max_fraction_digits, seconds);
>> +            av_ts_strip_trailing_zeros_and_decimal_point(buf);
>> +        }
>> +        else
>> +        {
>> +            snprintf(buf, AV_TS_MAX_STRING_SIZE, "%.8g", seconds);
>> +        }
>> +
>> +    }
>> +
>>      return buf;
>>  }
>>
>
> 1. What makes you believe that all users want the new format and that it
> does not cause undesired behaviour for some (maybe a lot) of them? The
> number of characters written by the earlier code stayed pretty constant
> even when the times became big (in this case, it just printed 8 chars if
> ts>=0), yet your code will really make use of the whole buffer.
> Granted, we could tell our users that they have no right to complain
> about this, given that we always had a "right" to use the full buffer,
> but I consider this a violation of the principle of least surprise. Why
> don't you just change silencedetect or add another function?

I suggested to change av_ts_make_time_string() because this 
problem affect all detection filters, not only silencedetect. So fixing 
it in silencedetect only is wrong.

The API did not make any promises about the format, and as long as it 
provides less chars than AV_TS_MAX_STRING_SIZE, and the string is 
parseable string representation of a floating point number, it is not a 
break.

Sure, it changes behaviour, but that is not unheard of if there is a good 
reason, and in this case, I believe there is. And I think it is more 
likely that some code is broken right now because the function 
loses precision or returns scientific notation for relatively small 
timestamps. Actually, it _was_ a suprise for me, so IMHO the element of 
least suprise is that precision will not fade away for reasonably small 
timestamps.

> 2. For very small timestamps (< 10^-4), the new code will print a lot of
> useless leading zeros (after the decimal point). In fact, it can be so
> many that the new code has less precision than the old code, despite
> using the fill buffer.
> 2. This is way too much code for an inline function.
> 3. Anyway, your placement of {} on their own lines does not match the
> project coding style.

I am OK with any implementation which keeps at least millisecond 
precision for timestamps < 10^10. You are right, it should be as simple as 
possible.

Regards,
Marton