Change duration computation to use time_base instead of TB/2.

Patch by Ivan Schreter, schreter gmx net

Originally committed as revision 17570 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -3098,12 +3098,9 @@ typedef struct AVCodecParserContext {
      * It signals, how much longer the frame duration of the current frame
      * is compared to normal frame duration.
      *
-     * frame_duration = (2 + repeat_pict) / (2*fps)
+     * frame_duration = (1 + repeat_pict) * time_base
      *
      * It is used by codecs like H.264 to display telecined material.
-     *
-     * @note This field can also be set to -1 for half-frame duration in case
-     *       of field pictures.
      */
     int repeat_pict; /* XXX: Put it back in AVCodecContext. */
     int64_t pts;     /* pts of the current frame */

libavformat/utils.c

@@ -680,10 +680,7 @@ static void compute_frame_duration(int *pnum, int *pden, AVStream *st,
             *pnum = st->codec->time_base.num;
             *pden = st->codec->time_base.den;
             if (pc && pc->repeat_pict) {
-                // NOTE: repeat_pict can be also -1 for half-frame durations,
-                // e.g., in H.264 interlaced field picture stream
-                *pden *= 2;
-                *pnum = (*pnum) * (2 + pc->repeat_pict);
+                *pnum = (*pnum) * (1 + pc->repeat_pict);
             }
         }
         break;