ffmpeg: add option not to offset seek position by start time

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg.texi

@@ -1158,6 +1158,13 @@ This option enables or disables accurate seeking in input files with the
 transcoding. Use @option{-noaccurate_seek} to disable it, which may be useful
 e.g. when copying some streams and transcoding the others.
 
+@item -seek_timestamp (@emph{input})
+This option enables or disables seeking by timestamp in input files with the
+@option{-ss} option. It is disabled by default. If enabled, the argument
+to the @option{-ss} option is considered an actual timestamp, and is not
+offset by the start time of the file. This matters only for files which do
+not start from timestamp 0, such as transport streams.
+
 @item -thread_queue_size @var{size} (@emph{input})
 This option sets the maximum number of queued packets when reading from the
 file or device. With low latency / high rate live streams, packets may be

ffmpeg.h

@@ -92,6 +92,7 @@ typedef struct OptionsContext {
 
     /* input/output options */
     int64_t start_time;
+    int seek_timestamp;
     const char *format;
 
     SpecifierOpt *codec_names;
@@ -339,6 +340,7 @@ typedef struct InputFile {
     int64_t ts_offset;
     int64_t last_ts;
     int64_t start_time;   /* user-specified start time in AV_TIME_BASE or AV_NOPTS_VALUE */
+    int seek_timestamp;
     int64_t recording_time;
     int nb_streams;       /* number of stream that ffmpeg is aware of; may be different
                              from ctx.nb_streams if new streams appear during av_read_frame() */

ffmpeg_opt.c

@@ -919,7 +919,7 @@ static int open_input_file(OptionsContext *o, const char *filename)
 
     timestamp = (o->start_time == AV_NOPTS_VALUE) ? 0 : o->start_time;
     /* add the stream start time */
-    if (ic->start_time != AV_NOPTS_VALUE)
+    if (!o->seek_timestamp && ic->start_time != AV_NOPTS_VALUE)
         timestamp += ic->start_time;
 
     /* if seeking requested, we execute it */
@@ -2865,6 +2865,9 @@ const OptionDef options[] = {
     { "ss",             HAS_ARG | OPT_TIME | OPT_OFFSET |
                         OPT_INPUT | OPT_OUTPUT,                      { .off = OFFSET(start_time) },
         "set the start time offset", "time_off" },
+    { "seek_timestamp", HAS_ARG | OPT_INT | OPT_OFFSET |
+                        OPT_INPUT,                                   { .off = OFFSET(seek_timestamp) },
+        "enable/disable seeking by timestamp with -ss" },
     { "accurate_seek",  OPT_BOOL | OPT_OFFSET | OPT_EXPERT |
                         OPT_INPUT,                                   { .off = OFFSET(accurate_seek) },
         "enable/disable accurate seeking with -ss" },