Bug 1779186 [Linux] Skip non-ref frames after seek when stream is dec…

…oded by VA-API r=alwu

When stream is decoded by VA-API and position is changed we may see decoding artifacts.
MPV project uses AVDISCARD_NONREF frame skip until requested/decoded frames are synced so let's use the same approach.

Depends on D161723

Differential Revision: https://phabricator.services.mozilla.com/D161724

dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp

@@ -831,6 +831,13 @@ MediaResult FFmpegVideoDecoder<LIBAV_VER>::DoDecode(
   packet.flags = aSample->mKeyframe ? AV_PKT_FLAG_KEY : 0;
   packet.pos = aSample->mOffset;
 
+  mCodecContext->skip_frame = mFrameDrop;
+#if MOZ_LOGGING
+  if (mFrameDrop == AVDISCARD_NONREF) {
+    FFMPEG_LOG("Frame skip AVDISCARD_NONREF");
+  }
+#endif
+
 #if LIBAVCODEC_VERSION_MAJOR >= 58
   packet.duration = aSample->mDuration.ToMicroseconds();
   int res = mLib->avcodec_send_packet(mCodecContext, &packet);
@@ -883,6 +890,18 @@ MediaResult FFmpegVideoDecoder<LIBAV_VER>::DoDecode(
           RESULT_DETAIL("avcodec_receive_frame error: %s", errStr));
     }
 
+    if (mFrameDrop == AVDISCARD_NONREF) {
+      FFMPEG_LOG("Requested pts %" PRId64 " decoded frame pts %" PRId64,
+                 packet.pts, GetFramePts(mFrame) + mFrame->pkt_duration);
+      // Switch back to default frame skip policy if we hit correct
+      // decode times. 5 ms treshold is taken from mpv project which
+      // use similar approach after seek (feed_packet() at f_decoder_wrapper.c).
+      if (packet.pts - 5000 <= GetFramePts(mFrame) + mFrame->pkt_duration) {
+        FFMPEG_LOG("Set frame drop to AVDISCARD_DEFAULT.");
+        mFrameDrop = AVDISCARD_DEFAULT;
+      }
+    }
+
     UpdateDecodeTimes(decodeStart);
     decodeStart = TimeStamp::Now();
 
@@ -1213,9 +1232,14 @@ MediaResult FFmpegVideoDecoder<LIBAV_VER>::CreateImageVAAPI(
 
 RefPtr<MediaDataDecoder::FlushPromise>
 FFmpegVideoDecoder<LIBAV_VER>::ProcessFlush() {
+  FFMPEG_LOG("ProcessFlush()");
   MOZ_ASSERT(mTaskQueue->IsOnCurrentThread());
   mPtsContext.Reset();
   mDurationMap.Clear();
+  // Discard non-ref frames on HW accelerated backend to avoid decode artifacts.
+  if (IsHardwareAccelerated()) {
+    mFrameDrop = AVDISCARD_NONREF;
+  }
   return FFmpegDataDecoder::ProcessFlush();
 }
 

dom/media/platforms/ffmpeg/FFmpegVideoDecoder.h

@@ -175,6 +175,7 @@ class FFmpegVideoDecoder<LIBAV_VER>
 
   DurationMap mDurationMap;
   const bool mLowLatency;
+  AVDiscard mFrameDrop = AVDISCARD_DEFAULT;
 
   // True if we're allocating shmem for ffmpeg decode buffer.
   Maybe<Atomic<bool>> mIsUsingShmemBufferForDecode;