Refactor audio sample conversion in encoder (#704)

Author: NicolasHug

src/torchcodec/_core/Encoder.cpp

@@ -288,10 +288,13 @@ void AudioEncoder::encode() {
     // encoded frame would contain more samples than necessary and our results
     // wouldn't match the ffmpeg CLI.
     avFrame->nb_samples = numSamplesToEncode;
-    encodeInnerLoop(autoAVPacket, avFrame);
 
-    avFrame->pts += static_cast<int64_t>(numSamplesToEncode);
+    UniqueAVFrame convertedAVFrame = maybeConvertAVFrame(avFrame);
+    encodeInnerLoop(autoAVPacket, convertedAVFrame);
+
     numEncodedSamples += numSamplesToEncode;
+    // TODO-ENCODING set frame pts correctly, and test against it.
+    // avFrame->pts += static_cast<int64_t>(numSamplesToEncode);
   }
   TORCH_CHECK(numEncodedSamples == numSamples, "Hmmmmmm something went wrong.");
 
@@ -304,42 +307,43 @@ void AudioEncoder::encode() {
       getFFMPEGErrorStringFromErrorCode(status));
 }
 
-void AudioEncoder::encodeInnerLoop(
-    AutoAVPacket& autoAVPacket,
-    const UniqueAVFrame& srcAVFrame) {
-  bool mustConvert =
-      (srcAVFrame != nullptr &&
-       (avCodecContext_->sample_fmt != AV_SAMPLE_FMT_FLTP ||
-        getNumChannels(srcAVFrame) != outNumChannels_));
-
-  UniqueAVFrame convertedAVFrame;
-  if (mustConvert) {
-    if (!swrContext_) {
-      swrContext_.reset(createSwrContext(
-          AV_SAMPLE_FMT_FLTP,
-          avCodecContext_->sample_fmt,
-          srcAVFrame->sample_rate, // No sample rate conversion
-          srcAVFrame->sample_rate,
-          srcAVFrame,
-          outNumChannels_));
-    }
-    convertedAVFrame = convertAudioAVFrameSamples(
-        swrContext_,
-        srcAVFrame,
+UniqueAVFrame AudioEncoder::maybeConvertAVFrame(const UniqueAVFrame& avFrame) {
+  if (static_cast<AVSampleFormat>(avFrame->format) ==
+          avCodecContext_->sample_fmt &&
+      getNumChannels(avFrame) == outNumChannels_) {
+    // Note: the clone references the same underlying data, it's a cheap copy.
+    return UniqueAVFrame(av_frame_clone(avFrame.get()));
+  }
+
+  if (!swrContext_) {
+    swrContext_.reset(createSwrContext(
+        static_cast<AVSampleFormat>(avFrame->format),
         avCodecContext_->sample_fmt,
-        srcAVFrame->sample_rate, // No sample rate conversion
-        outNumChannels_);
-    TORCH_CHECK(
-        convertedAVFrame->nb_samples == srcAVFrame->nb_samples,
-        "convertedAVFrame->nb_samples=",
-        convertedAVFrame->nb_samples,
-        " differs from ",
-        "srcAVFrame->nb_samples=",
-        srcAVFrame->nb_samples,
-        "This is unexpected, please report on the TorchCodec bug tracker.");
+        avFrame->sample_rate, // No sample rate conversion
+        avFrame->sample_rate,
+        avFrame,
+        outNumChannels_));
   }
-  const UniqueAVFrame& avFrame = mustConvert ? convertedAVFrame : srcAVFrame;
+  UniqueAVFrame convertedAVFrame = convertAudioAVFrameSamples(
+      swrContext_,
+      avFrame,
+      avCodecContext_->sample_fmt,
+      avFrame->sample_rate, // No sample rate conversion
+      outNumChannels_);
+  TORCH_CHECK(
+      convertedAVFrame->nb_samples == avFrame->nb_samples,
+      "convertedAVFrame->nb_samples=",
+      convertedAVFrame->nb_samples,
+      " differs from ",
+      "avFrame->nb_samples=",
+      avFrame->nb_samples,
+      "This is unexpected, please report on the TorchCodec bug tracker.");
+  return convertedAVFrame;
+}
 
+void AudioEncoder::encodeInnerLoop(
+    AutoAVPacket& autoAVPacket,
+    const UniqueAVFrame& avFrame) {
   auto status = avcodec_send_frame(avCodecContext_.get(), avFrame.get());
   TORCH_CHECK(
       status == AVSUCCESS,

src/torchcodec/_core/Encoder.h

@@ -38,6 +38,7 @@ class AudioEncoder {
   void initializeEncoder(
       int sampleRate,
       const AudioStreamOptions& audioStreamOptions);
+  UniqueAVFrame maybeConvertAVFrame(const UniqueAVFrame& avFrame);
   void encodeInnerLoop(
       AutoAVPacket& autoAVPacket,
       const UniqueAVFrame& srcAVFrame);