[FFmpeg-devel] [PATCH v1] filter select - avoid 2 times rounding

[Ulf Zibis]

Hi,

I have seen, that filter select sometimes doesn't catch a frame
determined by t as cause of floating point rounding. I could experience
less problems after removing 2 times rounding in filter select. I'm
aware that there still is a chance to miss a catch, but I believe it
should be much more rare.

The patch works fine e.g. with:
$ ./ffmpeg in.mp4 -vf
select='eq(t\,10.16)+eq(t\,10.2)+eq(t\,10.24)+eq(t\,10.28)+eq(t\,10.32)+eq(t\,10.36)+eq(t\,10.4)+eq(t\,10.44)+eq(t\,17.52)+eq(t\,47.96)+eq(t\,49.08)+eq(t\,49.2)+eq(t\,55.72)+eq(t\,83.0)'
-vsync vfr out_%02d.png

Without the patch the frame for eq(t\,10.2) was missing.

-Ulf


>From f14142a22d340cba48b3e2352a33026590dec3a5 Mon Sep 17 00:00:00 2001
From: Ulf Zibis <ulf.zi...@cosoco.de>
Date: 19.07.2019, 18:27:51

avfilter/select: avoid twice rounding

diff --git a/libavfilter/f_select.c b/libavfilter/f_select.c
index 1132375..39cc004 100644
--- a/libavfilter/f_select.c
+++ b/libavfilter/f_select.c
@@ -297,6 +297,7 @@
 
 #define D2TS(d)  (isnan(d) ? AV_NOPTS_VALUE : (int64_t)(d))
 #define TS2D(ts) ((ts) == AV_NOPTS_VALUE ? NAN : (double)(ts))
+#define TS2DT(ts, tb) (TS2D((ts) * (tb).num / (double)(tb).den))
 
 static void select_frame(AVFilterContext *ctx, AVFrame *frame)
 {
@@ -307,11 +308,11 @@
     if (isnan(select->var_values[VAR_START_PTS]))
         select->var_values[VAR_START_PTS] = TS2D(frame->pts);
     if (isnan(select->var_values[VAR_START_T]))
-        select->var_values[VAR_START_T] = TS2D(frame->pts) * av_q2d(inlink->time_base);
+        select->var_values[VAR_START_T] = TS2DT(frame->pts, inlink->time_base);
 
     select->var_values[VAR_N  ] = inlink->frame_count_out;
     select->var_values[VAR_PTS] = TS2D(frame->pts);
-    select->var_values[VAR_T  ] = TS2D(frame->pts) * av_q2d(inlink->time_base);
+    select->var_values[VAR_T  ] = TS2DT(frame->pts, inlink->time_base);
     select->var_values[VAR_POS] = frame->pkt_pos == -1 ? NAN : frame->pkt_pos;
     select->var_values[VAR_KEY] = frame->key_frame;
     select->var_values[VAR_CONCATDEC_SELECT] = get_concatdec_select(frame, av_rescale_q(frame->pts, inlink->time_base, AV_TIME_BASE_Q));

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