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[FFmpeg-devel] libavutil/timestamp.h: Fix loss of precision in timestamps

Message ID 1149006136.243626.1590899918044@mail.yahoo.com
State New
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Series [FFmpeg-devel] libavutil/timestamp.h: Fix loss of precision in timestamps
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andriy/make_fate success Make fate finished

Commit Message

Allan Cady May 31, 2020, 4:38 a.m. UTC
I prepared this patch a year ago but never followed through to get it accepted. Thought I would give it another try. I've been using a local copy with these changes, and for me it makes the difference between silencedetect being usable or not. 

Brief description: The existing implementation of the silencedetect filter uses the format specifier "%.6g", which keeps a fixed-width, and so loses precision as the time offsets get larger. I propose changing to "%.6f", which keeps six decimal digits precision no matter how large the offset.
Further explanation in the patch comments.
New patch attached, and here is the last correspondence I had with y'all last year.
https://ffmpeg.org/pipermail/ffmpeg-devel/2019-April/242233.html
Subject: [PATCH] libavutil/timestamp.h: Fix loss of precision in timestamps
 for silencedetect on long files

When the silencedetect filter is run against long files, the output
timestamps gradually lose precision as the scan proceeds further into
the file. This is because the output is formatted (in
libavutil/timestamp.h) as "%.6g", which limits the total field
length. Eventually, for offsets greater than 100000 seconds (about 28
hours), fractions of a second disappear altogether, and the
timestamps are logged as whole seconds.

I propose changing the format to "%.6f",
which will give microsecond precision for all timestamps, regardless of offset.

The timestamp string length is limited to 32 characters
(AV_TS_MAX_STRING_SIZE), so this should still be plenty long enough
with the increased length (up to 10^25 seconds).

My interest is in fixing this problem for silencedetect, which
formats the timestamps by calling the macro av_ts2timestr, defined in
timestamp.h. Since av_ts2timestr is also used in many other places (I
count 21 c files), I have created a new macro, av_ts2timestr_format,
with a format string added as a parameter, and left the original
macro interface as is for other usages, to limit the scope of this
change. The same or similar change could be made for other cases
where better precision is desired.
---
 libavfilter/af_silencedetect.c               | 14 +++++-----
 libavutil/timestamp.h                        | 27 +++++++++++++++++---
 tests/ref/fate/filter-metadata-silencedetect |  2 +-
 3 files changed, 32 insertions(+), 11 deletions(-)
diff mbox series

Patch

diff --git a/libavfilter/af_silencedetect.c b/libavfilter/af_silencedetect.c
index ff3b219e77..13c6f6f498 100644
--- a/libavfilter/af_silencedetect.c
+++ b/libavfilter/af_silencedetect.c
@@ -32,6 +32,8 @@ 
 #include "avfilter.h"
 #include "internal.h"
 
+const char TS_FMT_SILENCEDETECT[] = "%.6f";
+
 typedef struct SilenceDetectContext {
     const AVClass *class;
     double noise;               ///< noise amplitude ratio
@@ -87,11 +89,11 @@  static av_always_inline void update(SilenceDetectContext *s, AVFrame *insamples,
                 s->start[channel] = insamples->pts + av_rescale_q(current_sample / s->channels + 1 - nb_samples_notify * s->independent_channels / s->channels,
                         (AVRational){ 1, s->last_sample_rate }, time_base);
                 set_meta(insamples, s->mono ? channel + 1 : 0, "silence_start",
-                        av_ts2timestr(s->start[channel], &time_base));
+                        av_ts2timestr_fmt(s->start[channel], &time_base, TS_FMT_SILENCEDETECT));
                 if (s->mono)
                     av_log(s, AV_LOG_INFO, "channel: %d | ", channel);
                 av_log(s, AV_LOG_INFO, "silence_start: %s\n",
-                        av_ts2timestr(s->start[channel], &time_base));
+                        av_ts2timestr_fmt(s->start[channel], &time_base, TS_FMT_SILENCEDETECT));
             }
         }
     } else {
@@ -102,15 +104,15 @@  static av_always_inline void update(SilenceDetectContext *s, AVFrame *insamples,
             int64_t duration_ts = end_pts - s->start[channel];
             if (insamples) {
                 set_meta(insamples, s->mono ? channel + 1 : 0, "silence_end",
-                        av_ts2timestr(end_pts, &time_base));
+                        av_ts2timestr_fmt(end_pts, &time_base, TS_FMT_SILENCEDETECT));
                 set_meta(insamples, s->mono ? channel + 1 : 0, "silence_duration",
-                        av_ts2timestr(duration_ts, &time_base));
+                        av_ts2timestr_fmt(duration_ts, &time_base, TS_FMT_SILENCEDETECT));
             }
             if (s->mono)
                 av_log(s, AV_LOG_INFO, "channel: %d | ", channel);
             av_log(s, AV_LOG_INFO, "silence_end: %s | silence_duration: %s\n",
-                    av_ts2timestr(end_pts, &time_base),
-                    av_ts2timestr(duration_ts, &time_base));
+                    av_ts2timestr_fmt(end_pts, &time_base, TS_FMT_SILENCEDETECT),
+                    av_ts2timestr_fmt(duration_ts, &time_base, TS_FMT_SILENCEDETECT));
         }
         s->nb_null_samples[channel] = 0;
         s->start[channel] = INT64_MIN;
diff --git a/libavutil/timestamp.h b/libavutil/timestamp.h
index e082f01b40..590c8642d8 100644
--- a/libavutil/timestamp.h
+++ b/libavutil/timestamp.h
@@ -32,6 +32,8 @@ 
 
 #define AV_TS_MAX_STRING_SIZE 32
 
+#define AV_TS_FMT_DEFAULT "%.6g"
+
 /**
  * Fill the provided buffer with a string containing a timestamp
  * representation.
@@ -53,6 +55,23 @@  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)
 
+/**
+ * Fill the provided buffer with a string containing a timestamp time
+ * representation, allowing format specification.
+ *
+ * @param buf a buffer with size in bytes of at least AV_TS_MAX_STRING_SIZE
+ * @param ts the timestamp to represent
+ * @param tb the timebase of the timestamp
+ * @param format string for timestamp output, e.g. AV_TS_FMT_DEFAULT.
+ * @return the buffer in input
+ */
+static inline char *av_ts_make_time_string_format(char *buf, int64_t ts, AVRational *tb, const char *format)
+{
+    if (ts == AV_NOPTS_VALUE) snprintf(buf, AV_TS_MAX_STRING_SIZE, "NOPTS");
+    else                      snprintf(buf, AV_TS_MAX_STRING_SIZE, format, av_q2d(*tb) * ts);
+    return buf;
+}
+
 /**
  * Fill the provided buffer with a string containing a timestamp time
  * representation.
@@ -64,15 +83,15 @@  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, 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);
+    av_ts_make_time_string_format(buf, ts, tb, AV_TS_FMT_DEFAULT);
     return buf;
 }
 
 /**
- * Convenience macro, the return value should be used only directly in
+ * Convenience macros, the return value should be used only directly in
  * function arguments but never stand-alone.
  */
-#define av_ts2timestr(ts, tb) av_ts_make_time_string((char[AV_TS_MAX_STRING_SIZE]){0}, ts, tb)
+#define av_ts2timestr_fmt(ts, tb, fmt) av_ts_make_time_string_format((char[AV_TS_MAX_STRING_SIZE]){0}, ts, tb, fmt)
+#define av_ts2timestr(ts, tb) av_ts_make_time_string_format((char[AV_TS_MAX_STRING_SIZE]){0}, ts, tb, AV_TS_FMT_DEFAULT)
 
 #endif /* AVUTIL_TIMESTAMP_H */
diff --git a/tests/ref/fate/filter-metadata-silencedetect b/tests/ref/fate/filter-metadata-silencedetect
index 16a9d07692..5d8063cb71 100644
--- a/tests/ref/fate/filter-metadata-silencedetect
+++ b/tests/ref/fate/filter-metadata-silencedetect
@@ -1,5 +1,5 @@ 
 pkt_pts=0|tag:lavfi.silence_duration=0.523107|tag:lavfi.silence_end=0.690023|tag:lavfi.silence_start=0.736417
-pkt_pts=46080|tag:lavfi.silence_start=1.27626|tag:lavfi.silence_end=1.80751|tag:lavfi.silence_duration=0.531247
+pkt_pts=46080|tag:lavfi.silence_start=1.276259|tag:lavfi.silence_end=1.807506|tag:lavfi.silence_duration=0.531247
 pkt_pts=92160
 pkt_pts=138240
 pkt_pts=184320