libavfilter/decimate: Add pts of first frame to all frames.

Fixes ticket #3019

Reviewed-by: Clément Bœsch

libavfilter/vf_decimate.c

@@ -43,6 +43,7 @@ typedef struct {
     AVFrame **clean_src;    ///< frame queue for the clean source
     int got_frame[2];       ///< frame request flag for each input stream
     double ts_unit;         ///< timestamp units for the output frames
+    int64_t start_pts;      ///< base for output timestamps
     uint32_t eof;           ///< bitmask for end of stream
     int hsub, vsub;         ///< chroma subsampling values
     int depth;
@@ -210,11 +211,14 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
             av_frame_free(&dm->queue[i].frame);
         } else {
             AVFrame *frame = dm->queue[i].frame;
+            if (frame->pts != AV_NOPTS_VALUE && dm->start_pts == AV_NOPTS_VALUE)
+                dm->start_pts = frame->pts;
             if (dm->ppsrc) {
                 av_frame_free(&frame);
                 frame = dm->clean_src[i];
             }
-            frame->pts = outlink->frame_count * dm->ts_unit;
+            frame->pts = outlink->frame_count * dm->ts_unit +
+                         (dm->start_pts == AV_NOPTS_VALUE ? 0 : dm->start_pts);
             ret = ff_filter_frame(outlink, frame);
             if (ret < 0)
                 break;
@@ -259,7 +263,7 @@ static int config_input(AVFilterLink *inlink)
 
 static av_cold int decimate_init(AVFilterContext *ctx)
 {
-    const DecimateContext *dm = ctx->priv;
+    DecimateContext *dm = ctx->priv;
     AVFilterPad pad = {
         .name         = av_strdup("main"),
         .type         = AVMEDIA_TYPE_VIDEO,
@@ -285,6 +289,8 @@ static av_cold int decimate_init(AVFilterContext *ctx)
         return AVERROR(EINVAL);
     }
 
+    dm->start_pts = AV_NOPTS_VALUE;
+
     return 0;
 }