[android] Refine video render algorithm

1. Improve the precision of timestamp conversion.
2. Set default playback rate to 0 in AudioFrameTracker.

b/332858454

starboard/android/apk/app/src/main/java/dev/cobalt/media/VideoFrameReleaseTimeHelper.java

@@ -158,7 +158,8 @@ public long adjustReleaseTime(
           adjustedFrameTimeNs = candidateAdjustedFrameTimeNs;
           adjustedReleaseTimeNs =
               syncUnadjustedReleaseTimeNs
-                  + (long) ((adjustedFrameTimeNs - syncFramePresentationTimeNs) / playbackRate);
+                  + adjustFrameTimeForPlaybackRate(
+                      adjustedFrameTimeNs - syncFramePresentationTimeNs, playbackRate);
         }
       } else {
         // We're synced but haven't waited the required number of frames to apply an adjustment.
@@ -199,10 +200,20 @@ protected void onSynced() {
   private boolean isDriftTooLarge(long frameTimeNs, long releaseTimeNs, double playbackRate) {
     long elapsedFrameTimeNs = frameTimeNs - syncFramePresentationTimeNs;
     long elapsedReleaseTimeNs = releaseTimeNs - syncUnadjustedReleaseTimeNs;
-    return Math.abs(elapsedReleaseTimeNs - elapsedFrameTimeNs / playbackRate)
+    return Math.abs(
+            elapsedReleaseTimeNs - adjustFrameTimeForPlaybackRate(elapsedFrameTimeNs, playbackRate))
         > MAX_ALLOWED_DRIFT_NS;
   }
 
+  private long adjustFrameTimeForPlaybackRate(long frameTimeNs, double playbackRate) {
+    // We support playback rate of {0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2}. So, we use a multiplier
+    // of 100
+    // here to keep precision of the timestamp conversion.
+    final long PLAYBACK_RATE_ADJUSTMENT_MULTIPLIER = 100;
+    long multipliedPlaybackRate = (long) (playbackRate * PLAYBACK_RATE_ADJUSTMENT_MULTIPLIER);
+    return frameTimeNs / multipliedPlaybackRate * PLAYBACK_RATE_ADJUSTMENT_MULTIPLIER;
+  }
+
   private static long closestVsync(long releaseTime, long sampledVsyncTime, long vsyncDuration) {
     long vsyncCount = (releaseTime - sampledVsyncTime) / vsyncDuration;
     long snappedTimeNs = sampledVsyncTime + (vsyncDuration * vsyncCount);

starboard/android/shared/video_render_algorithm.cc

@@ -66,7 +66,7 @@ void VideoRenderAlgorithm::Render(
     double playback_rate;
     int64_t playback_time = media_time_provider->GetCurrentMediaTime(
         &is_audio_playing, &is_audio_eos_played, &is_underflow, &playback_rate);
-    if (!is_audio_playing) {
+    if (!is_audio_playing || playback_rate == 0.0) {
       break;
     }
     if (playback_rate != playback_rate_) {
@@ -91,8 +91,8 @@ void VideoRenderAlgorithm::Render(
       }
     }
 
-    jlong early_us = (frames->front()->timestamp() - playback_time) /
-                     (playback_rate != 0 ? playback_rate : 1);
+    jlong early_us =
+        (frames->front()->timestamp() - playback_time) / playback_rate;
 
     auto system_time_ns = GetSystemNanoTime();
     auto unadjusted_frame_release_time_ns = system_time_ns + (early_us * 1000);
@@ -101,7 +101,6 @@ void VideoRenderAlgorithm::Render(
         video_frame_release_time_helper_.AdjustReleaseTime(
             frames->front()->timestamp(), unadjusted_frame_release_time_ns,
             playback_rate);
-
     early_us = (adjusted_release_time_ns - system_time_ns) / 1000;
 
     if (early_us < kBufferTooLateThreshold) {

starboard/shared/starboard/player/filter/audio_frame_tracker.h

@@ -54,7 +54,7 @@ class AudioFrameTracker {
   // Usually there are very few elements, so std::vector<> is efficient enough.
   std::vector<FrameRecord> frame_records_;
   int64_t frames_played_adjusted_to_playback_rate_ = 0;
-  double last_playback_rate_ = 1.0;
+  double last_playback_rate_ = 0.0;
 };
 
 }  // namespace filter

starboard/shared/starboard/player/player_internal.h

@@ -60,7 +60,10 @@ struct SbPlayerPrivate {
 #else   // SB_API_VERSION >= 15
   void GetInfo(SbPlayerInfo2* out_player_info);
 #endif  // SB_API_VERSION >= 15
+
+  // TODO: this function is not used.
   void SetPause(bool pause);
+
   void SetPlaybackRate(double playback_rate);
   void SetVolume(double volume);