Add an example of AudioPostProcessor to mix the audio of 2 data sources.

lib/src/main/java/com/otaliastudios/transcoder/postprocessor/MixerSourceAudioPostProcessor.java

@@ -0,0 +1,28 @@
+package com.otaliastudios.transcoder.postprocessor;
+
+import androidx.annotation.NonNull;
+
+import java.nio.ShortBuffer;
+import java.util.ArrayDeque;
+import java.util.Queue;
+
+public class MixerSourceAudioPostProcessor implements AudioPostProcessor {
+
+    Queue<ShortBuffer> mBuffers = new ArrayDeque<>();
+
+    @Override
+    public long calculateNewDurationUs(long durationUs) {
+        return 0;
+    }
+
+    @Override
+    public long postProcess(@NonNull ShortBuffer inputBuffer, @NonNull ShortBuffer outputBuffer, long bufferDurationUs) {
+        if (inputBuffer.remaining() > 0) {
+            ShortBuffer sourceBuffer = ShortBuffer.allocate(inputBuffer.limit());
+            sourceBuffer.put(inputBuffer);
+            sourceBuffer.rewind();
+            mBuffers.add(sourceBuffer);
+        }
+        return 0;
+    }
+}

lib/src/main/java/com/otaliastudios/transcoder/postprocessor/MixerTargetAudioPostProcessor.java

@@ -0,0 +1,54 @@
+package com.otaliastudios.transcoder.postprocessor;
+
+import androidx.annotation.NonNull;
+
+import java.nio.ShortBuffer;
+
+import static java.lang.Math.min;
+
+public class MixerTargetAudioPostProcessor implements AudioPostProcessor {
+    private MixerSourceAudioPostProcessor mSource;
+    private float mSourceVolume;
+    private float mTargetVolume;
+
+    @Override
+    public long calculateNewDurationUs(long durationUs) {
+        return durationUs;
+    }
+
+    public MixerTargetAudioPostProcessor(MixerSourceAudioPostProcessor source, float sourceVolume, float targetVolume)
+    {
+        mSource = source;
+        mSourceVolume = sourceVolume;
+        mTargetVolume = targetVolume;
+    }
+
+    private short mixSamples(short sourceSample, short targetSample) {
+        float mixedSample = (sourceSample * mSourceVolume)  + (targetSample * mTargetVolume);
+        if (mixedSample < Short.MIN_VALUE)
+            mixedSample = Short.MIN_VALUE;
+        else if (mixedSample > Short.MAX_VALUE)
+            mixedSample = Short.MAX_VALUE;
+        return (short)mixedSample;
+    }
+
+    @Override
+    public long postProcess(@NonNull ShortBuffer inputBuffer, @NonNull ShortBuffer outputBuffer, long bufferDurationUs) {
+        ShortBuffer sourceBuffer = mSource.mBuffers.peek();
+        int inputRemaining = inputBuffer.remaining();
+        while (sourceBuffer != null && inputRemaining > 0) {
+            int sourceRemaining = sourceBuffer.remaining();
+            int remaining = min(inputRemaining, sourceRemaining);
+            for (int i=0; i<remaining; i++) {
+                outputBuffer.put(mixSamples(sourceBuffer.get(), inputBuffer.get()));
+            }
+            inputRemaining -= sourceRemaining;
+            if (inputRemaining >= 0) {
+                mSource.mBuffers.remove();
+                sourceBuffer = mSource.mBuffers.peek();
+            }
+        }
+        outputBuffer.put(inputBuffer);
+        return bufferDurationUs;
+    }
+}