lavfi/drawtext: Add %N for drawing fractions of a second

Suggested-By: ffmpeg@fb.com

doc/filters.texi

@@ -11431,10 +11431,14 @@ It can be used to add padding with zeros from the left.
 @item gmtime
 The time at which the filter is running, expressed in UTC.
 It can accept an argument: a strftime() format string.
+The format string is extended to support the variable @var{%[1-6]N}
+which prints fractions of the second with optionally specified number of digits.
 
 @item localtime
 The time at which the filter is running, expressed in the local time zone.
 It can accept an argument: a strftime() format string.
+The format string is extended to support the variable @var{%[1-6]N}
+which prints fractions of the second with optionally specified number of digits.
 
 @item metadata
 Frame metadata. Takes one or two arguments.

libavfilter/vf_drawtext.c

@@ -51,6 +51,7 @@
 #include "libavutil/opt.h"
 #include "libavutil/random_seed.h"
 #include "libavutil/parseutils.h"
+#include "libavutil/time.h"
 #include "libavutil/timecode.h"
 #include "libavutil/time_internal.h"
 #include "libavutil/tree.h"
@@ -1045,15 +1046,78 @@ static int func_strftime(AVFilterContext *ctx, AVBPrint *bp,
                          char *fct, unsigned argc, char **argv, int tag)
 {
     const char *fmt = argc ? argv[0] : "%Y-%m-%d %H:%M:%S";
+    const char *fmt_begin = fmt;
+    int64_t unow;
     time_t now;
     struct tm tm;
+    const char *begin;
+    const char *tmp;
+    int len;
+    int div;
+    AVBPrint fmt_bp;
 
-    time(&now);
+    av_bprint_init(&fmt_bp, 0, AV_BPRINT_SIZE_UNLIMITED);
-    if (tag == 'L')
+
+    unow = av_gettime();
+    now  = unow / 1000000;
+    if (tag == 'L' || tag == 'm')
         localtime_r(&now, &tm);
     else
         tm = *gmtime_r(&now, &tm);
-    av_bprint_strftime(bp, fmt, &tm);
+
+    // manually parse format for %N (fractional seconds)
+    begin = fmt;
+    while ((begin = strchr(begin, '%'))) {
+        tmp = begin + 1;
+        len = 0;
+
+        // skip escaped "%%"
+        if (*tmp == '%') {
+            begin = tmp + 1;
+            continue;
+        }
+
+        // count digits between % and possible N
+        while (*tmp != '\0' && av_isdigit((int)*tmp)) {
+            len++;
+            tmp++;
+        }
+
+        // N encountered, insert time
+        if (*tmp == 'N') {
+            int num_digits = 3; // default show millisecond [1,6]
+
+            // if digit given, expect [1,6], warn & clamp otherwise
+            if (len == 1) {
+                num_digits = av_clip(*(begin + 1) - '0', 1, 6);
+            } else if (len > 1) {
+                av_log(ctx, AV_LOG_WARNING, "Invalid number of decimals for %%N, using default of %i\n", num_digits);
+            }
+
+            len += 2; // add % and N to get length of string part
+
+            div = pow(10, 6 - num_digits);
+
+            av_bprintf(&fmt_bp, "%.*s%0*d", (int)(begin - fmt_begin), fmt_begin, num_digits, (int)(unow % 1000000) / div);
+
+            begin += len;
+            fmt_begin = begin;
+
+            continue;
+        }
+
+        begin = tmp;
+    }
+
+    av_bprintf(&fmt_bp, "%s", fmt_begin);
+    if (!av_bprint_is_complete(&fmt_bp)) {
+        av_log(ctx, AV_LOG_WARNING, "Format string truncated at %u/%u.", fmt_bp.size, fmt_bp.len);
+    }
+
+    av_bprint_strftime(bp, fmt_bp.str, &tm);
+
+    av_bprint_finalize(&fmt_bp, NULL);
+
     return 0;
 }