Sub-sample accurate start of OscillatorNode

third_party/WebKit/Source/modules/webaudio/AudioScheduledSourceNode.cpp

 /*
  * Copyright (C) 2012, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 32 matching lines @@
                                                          float sampleRate)
     : AudioHandler(nodeType, node, sampleRate),
       m_startTime(0),
       m_endTime(UnknownTime),
       m_playbackState(UNSCHEDULED_STATE) {}
 
 void AudioScheduledSourceHandler::updateSchedulingInfo(
     size_t quantumFrameSize,
     AudioBus* outputBus,
     size_t& quantumFrameOffset,
-    size_t& nonSilentFramesToProcess) {
+    size_t& nonSilentFramesToProcess,
+    double& startFrameOffset) {
   DCHECK(outputBus);
   if (!outputBus)
     return;
 
   DCHECK_EQ(quantumFrameSize,
             static_cast<size_t>(AudioUtilities::kRenderQuantumFrames));
   if (quantumFrameSize != AudioUtilities::kRenderQuantumFrames)
     return;
 
   double sampleRate = this->sampleRate();
@@ skipping 23 matching lines @@
     outputBus->zero();
     nonSilentFramesToProcess = 0;
     return;
   }
 
   // Check if it's time to start playing.
   if (state == SCHEDULED_STATE) {
     // Increment the active source count only if we're transitioning from
     // SCHEDULED_STATE to PLAYING_STATE.
     setPlaybackState(PLAYING_STATE);
+    // Determine the offset of the true start time from the starting frame.
+    startFrameOffset = m_startTime * sampleRate - startFrame;
+  } else {
+    startFrameOffset = 0;
   }
 
   quantumFrameOffset =
       startFrame > quantumStartFrame ? startFrame - quantumStartFrame : 0;
   quantumFrameOffset =
       std::min(quantumFrameOffset, quantumFrameSize);  // clamp to valid range
   nonSilentFramesToProcess = quantumFrameSize - quantumFrameOffset;
 
   if (!nonSilentFramesToProcess) {
     // Output silence.
@@ skipping 168 matching lines @@
   // playback state if the context is closed.
   if (context()->isContextClosed())
     return false;
 
   // If a node is scheduled or playing, do not collect the node prematurely
   // even its reference is out of scope. Then fire onended event if assigned.
   return audioScheduledSourceHandler().isPlayingOrScheduled();
 }
 
 }  // namespace blink