Add comments

Author: NicolasHug

src/torchcodec/_core/Encoder.cpp

@@ -179,6 +179,8 @@ void AudioEncoder::initializeEncoder(
 
   desiredNumChannels_ = static_cast<int>(numChannels.value_or(wf_.sizes()[0]));
   validateNumChannels(*avCodec, desiredNumChannels_);
+  // The avCodecContext layout defines the layout of the encoded output, it's
+  // not related to the input sampes.
   setDefaultChannelLayout(avCodecContext_, desiredNumChannels_);
 
   validateSampleRate(*avCodec, sampleRate);
@@ -233,6 +235,8 @@ void AudioEncoder::encode() {
   avFrame->format = AV_SAMPLE_FMT_FLTP;
   avFrame->sample_rate = avCodecContext_->sample_rate;
   avFrame->pts = 0;
+  // We set the channel layout of the frame to the default layout corresponding
+  // to the input samples' number of channels
   setDefaultChannelLayout(avFrame, static_cast<int>(wf_.sizes()[0]));
 
   auto status = av_frame_get_buffer(avFrame.get(), 0);

src/torchcodec/_core/FFMPEGCommon.cpp

@@ -113,6 +113,8 @@ void validateNumChannels(const AVCodec& avCodec, int numChannels) {
       return;
     }
   }
+  // At this point it seems that the encoder doesn't support the requested
+  // number of channels, so we error out.
   std::stringstream supportedNumChannels;
   for (auto i = 0; avCodec.ch_layouts[i].order != AV_CHANNEL_ORDER_UNSPEC;
        ++i) {
@@ -126,6 +128,8 @@ void validateNumChannels(const AVCodec& avCodec, int numChannels) {
     // can't validate, same as above.
     return;
   }
+  // At this point it seems that the encoder doesn't support the requested
+  // number of channels, so we error out.
   for (auto i = 0; avCodec.channel_layouts[i] != 0; ++i) {
     if (numChannels ==
         av_get_channel_layout_nb_channels(avCodec.channel_layouts[i])) {