New method TimestampAligner::TranslateTimestamp

webrtc/base/timestampaligner.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 73 matching lines @@
   offset_us_ += error_us / frames_seen_;
   return offset_us_;
 }
 
 int64_t TimestampAligner::ClipTimestamp(int64_t time_us,
                                         int64_t system_time_us) {
   // Make timestamps monotonic.
   if (!prev_translated_time_us_) {
     // Initialize.
     clip_bias_us_ = 0;
-  } else if (time_us < *prev_translated_time_us_) {
-    time_us = *prev_translated_time_us_;
+    // We expect that users always call ClipTimestamp together with
+    // UpdateOffset. Then at reset, we always get the translated
+    // timestamp, time_us, equal to system_time_us. Which also ensures

[perkj_webrtc, 2016/09/08 07:25:23]

nit: |time_us|

+    // that we get monotonous timestamps over a reset.
+    RTC_DCHECK_EQ(time_us, system_time_us);

[nisse-webrtc, 2016/09/08 08:14:09]

On second look, it appears this expectation is not correct. AdaptFrame, both in
cricket::VideoCapturer and AndroidVideoTrackSource, calls UpdateOffset but not
ClipTimestamp, in the case that frame_wanted is false. Which I think makes
sense. 

So back to the drawing board, in particular, it's no longer correct to clear
prev_translated_time at filter reset.

+  } else if (time_us <= *prev_translated_time_us_) {
+    time_us = *prev_translated_time_us_ + 1;
   }
 
-  // Clip to make sure we don't produce time stamps in the future.
+  // Clip to make sure we don't produce timestamps in the future. In
+  // the anomalous case that this function is called multiple times
+  // with exactly the same system_time_us, we may also output the same
+  // timestamp several times, violating strict monotonicity.
   time_us -= clip_bias_us_;
   if (time_us > system_time_us) {
     clip_bias_us_ += time_us - system_time_us;
     time_us = system_time_us;
   }
   prev_translated_time_us_ = rtc::Optional<int64_t>(time_us);
   return time_us;
 }
 
 }  // namespace rtc