avfilter/vf_fps: fix rounding error accumulation

Fixes Ticket3329
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_fps.c

@@ -45,7 +45,6 @@ typedef struct FPSContext {
 
     /* timestamps in input timebase */
     int64_t first_pts;      ///< pts of the first frame that arrived on this filter
-    int64_t pts;            ///< pts of the first frame currently in the fifo
 
     double start_time;      ///< pts, in seconds, of the expected first frame
 
@@ -83,7 +82,6 @@ static av_cold int init(AVFilterContext *ctx)
     if (!(s->fifo = av_fifo_alloc(2*sizeof(AVFrame*))))
         return AVERROR(ENOMEM);
 
-    s->pts          = AV_NOPTS_VALUE;
     s->first_pts    = AV_NOPTS_VALUE;
 
     av_log(ctx, AV_LOG_VERBOSE, "fps=%d/%d\n", s->framerate.num, s->framerate.den);
@@ -179,7 +177,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
 
     s->frames_in++;
     /* discard frames until we get the first timestamp */
-    if (s->pts == AV_NOPTS_VALUE) {
+    if (s->first_pts == AV_NOPTS_VALUE) {
         if (buf->pts != AV_NOPTS_VALUE) {
             ret = write_to_fifo(s->fifo, buf);
             if (ret < 0)
@@ -188,13 +186,13 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
             if (s->start_time != DBL_MAX && s->start_time != AV_NOPTS_VALUE) {
                 double first_pts = s->start_time * AV_TIME_BASE;
                 first_pts = FFMIN(FFMAX(first_pts, INT64_MIN), INT64_MAX);
-                s->first_pts = s->pts = av_rescale_q(first_pts, AV_TIME_BASE_Q,
+                s->first_pts = av_rescale_q(first_pts, AV_TIME_BASE_Q,
                                                      inlink->time_base);
                 av_log(ctx, AV_LOG_VERBOSE, "Set first pts to (in:%"PRId64" out:%"PRId64")\n",
                        s->first_pts, av_rescale_q(first_pts, AV_TIME_BASE_Q,
                                                   outlink->time_base));
             } else {
-                s->first_pts = s->pts = buf->pts;
+                s->first_pts = buf->pts;
             }
         } else {
             av_log(ctx, AV_LOG_WARNING, "Discarding initial frame(s) with no "
@@ -211,8 +209,8 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
     }
 
     /* number of output frames */
-    delta = av_rescale_q_rnd(buf->pts - s->pts, inlink->time_base,
-                             outlink->time_base, s->rounding);
+    delta = av_rescale_q_rnd(buf->pts - s->first_pts, inlink->time_base,
+                             outlink->time_base, s->rounding) - s->frames_out ;
 
     if (delta < 1) {
         /* drop the frame and everything buffered except the first */
@@ -267,7 +265,6 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
     flush_fifo(s->fifo);
 
     ret = write_to_fifo(s->fifo, buf);
-    s->pts = s->first_pts + av_rescale_q(s->frames_out, outlink->time_base, inlink->time_base);
 
     return ret;
 }