Communicate data to per-track meters...

.. To do this correctly, we must duplicate mono buffer in case of pan
differences, and modify each buffer exactly as for gain and pan and microfades
before communicating it

libraries/lib-audio-io/AudioIO.cpp

@@ -2355,7 +2355,7 @@ void AudioIoCallback::CheckSoundActivatedRecordingLevel(
 void AudioIoCallback::AddToOutputChannel(unsigned int chan,
    float * outputMeterFloats,
    float * outputFloats,
-   const float * tempBuf,
+   float * tempBuf,
    bool drop,
    const unsigned long len,
    const PlayableSequence &ps,
@@ -2390,8 +2390,11 @@ void AudioIoCallback::AddToOutputChannel(unsigned int chan,
    // framesPerBuffer, which is influenced by the portAudio implementation in
    // opaque ways
    float deltaGain = (gain - oldGain) / len;
-   for (unsigned i = 0; i < len; i++)
-      outputFloats[numPlaybackChannels*i+chan] += (oldGain + deltaGain * i) *tempBuf[i];
+   for (unsigned i = 0; i < len; ++i) {
+      auto &value = tempBuf[i];
+      value *= (oldGain + deltaGain * i);
+      outputFloats[numPlaybackChannels * i + chan] += value;
+   }
 };
 
 // Limit values to -1.0..+1.0
@@ -2475,11 +2478,6 @@ bool AudioIoCallback::FillOutputBuffers(
       auto vt = mPlaybackSequences[tt].first.get();
       const auto width = vt->NChannels();
 
-      // IF mono THEN clear 'the other' channel.
-      if (width < numPlaybackChannels)
-         // TODO: more-than-two-channels
-         memset(tempBufs[1], 0, framesPerBuffer * sizeof(float));
-
       // Check for asynchronous user changes in mute, solo, pause status
       discardable = drop = SequenceShouldBeSilent(*vt);
 
@@ -2516,6 +2514,13 @@ bool AudioIoCallback::FillOutputBuffers(
          ++iBuffer;
       }
 
+      // IF mono THEN clear 'the other' channel.
+      if (width < numPlaybackChannels)
+         // TODO: more-than-two-channels
+         // Replicate left sample data for the right, before applying pan
+         // differences
+         memcpy(tempBufs[1], tempBufs[0], framesPerBuffer * sizeof(float));
+
       // PRL:  More recent rewrites of SequenceBufferExchange should guarantee a
       // padding out of the ring buffers so that equal lengths are
       // available, so maxLen ought to increase from 0 only once
@@ -2540,13 +2545,15 @@ bool AudioIoCallback::FillOutputBuffers(
          AddToOutputChannel(0, outputMeterFloats, outputFloats,
             tempBufs[0], drop, len, *vt, gains[0]);
 
-         // If one of mPlaybackSequences is mono, this replicates it in both
-         // device channels
-         const auto iBuffer = std::min<size_t>(1, width - 1);
          AddToOutputChannel(1, outputMeterFloats, outputFloats,
-            tempBufs[iBuffer], drop, len, *vt, gains[1]);
+            tempBufs[1], drop, len, *vt, gains[1]);
       }
 
+      for (const auto &wMeter : mPlaybackSequences[tt].second)
+         if (const auto pMeter = wMeter.lock(); pMeter && !pMeter->IsDisabled())
+            pMeter->Update(numPlaybackChannels,
+               framesPerBuffer, tempBuf, false); // non-interleaved
+
       CallbackCheckCompletion(mCallbackReturn, len);
       if (discardable) // no samples to process, they've been discarded
          continue;

libraries/lib-audio-io/AudioIO.h

@@ -280,7 +280,8 @@ class AUDIO_IO_API AudioIoCallback /* not final */
    void AddToOutputChannel( unsigned int chan, // index into gains
       float * outputMeterFloats,
       float * outputFloats,
-      const float * tempBuf,
+      // This buffer is in/out.  Pan, gain, and microfading will be applied
+      float * tempBuf,
       bool drop,
       unsigned long len,
       const PlayableSequence &ps,