New method TimestampAligner::TranslateTimestamp

webrtc/base/timestampaligner.h

Index: webrtc/base/timestampaligner.h
diff --git a/webrtc/base/timestampaligner.h b/webrtc/base/timestampaligner.h
index 00431f355b809f9b8bc2881e48ee95f9d7d641a4..590499dab385f816f87425877e1e4c714fbabf6d 100644
--- a/webrtc/base/timestampaligner.h
+++ b/webrtc/base/timestampaligner.h
@@ -13,10 +13,18 @@
 
 #include "webrtc/base/basictypes.h"
 #include "webrtc/base/constructormagic.h"
-#include "webrtc/base/optional.h"
 
 namespace rtc {
 
+// The TimestampAligner class helps translating camera timestamps into
+// the same timescale as is used by rtc::TimeMicros(). Some cameras
+// have built in timestamping which is more accurate than reading the
+// system clock, but using a different epoch and unknown clock drift.
+// Frame timestamps in webrtc should use rtc::TimeMicros (system monotonic
+// time), and this class provides a filter which lets us use the
+// rtc::TimeMicros timescale, and at the same time take advantage of
+// higher accuracy of the camera clock.
+
 // This class is not thread safe, so all calls to it must be synchronized
 // externally.
 class TimestampAligner {
@@ -25,9 +33,23 @@ class TimestampAligner {
   ~TimestampAligner();
 
  public:
+  // Translates camera timestamps to the same timescale as is used by
+  // rtc::TimeMicros(). |camera_time_us| is assumed to be accurate, but
+  // with an unknown epoch and clock drift. |system_time_us| is
+  // time according to rtc::TimeMicros(), preferably read as soon as
+  // possible when the frame is captured. It may have poor accuracy
+  // due to poor resolution or scheduling delays. Returns the
+  // translated timestamp.
+  int64_t TranslateTimestamp(int64_t camera_time_us, int64_t system_time_us);
+
+ protected:
   // Update the estimated offset between camera time and system monotonic time.
   int64_t UpdateOffset(int64_t camera_time_us, int64_t system_time_us);
 
+  // Clip timestamp, return value is always
+  //    <= |system_time_us|, and
+  //    >= min(|prev_translated_time_us_| + |kMinFrameIntervalUs|,
+  //           |system_time_us|).
   int64_t ClipTimestamp(int64_t filtered_time_us, int64_t system_time_us);
 
  private:
@@ -36,11 +58,13 @@ class TimestampAligner {
   // Estimated offset between camera time and system monotonic time.
   int64_t offset_us_;
 
-  // State for timestamp clipping, applied after the filter, to ensure
-  // that translated timestamps are monotonic and not in the future.
-  // Subtracted from the translated timestamps.
+  // State for the ClipTimestamp method, applied after the filter.
+  // A large negative camera clock drift tends to push translated
+  // timestamps into the future. |clip_bias_us_| is subtracted from the
+  // translated timestamps, to get them back from the future.
   int64_t clip_bias_us_;
-  rtc::Optional<int64_t> prev_translated_time_us_;
+  // Used to ensure that translated timestamps are monotonous.
+  int64_t prev_translated_time_us_;
   RTC_DISALLOW_COPY_AND_ASSIGN(TimestampAligner);
 };