Use null filter in case no filter is used (#3109)

Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/3109

Change the logic around StreamWriter preprocessing.
Currently, no preprocessing is expressed as `nullptr` to `unique_ptr<FilterGraph>`.

This commit changes it to `[a]null` filter, which is just a pass through.
This makes a code a bit simpler, and serves better preparation for adding
filters for CUDA process.

Reviewed By: xiaohui-zhang

Differential Revision: D43593321

fbshipit-source-id: 9ca71c2c8bf652384a0f56b4c41b32d908f61201

torchaudio/csrc/ffmpeg/stream_writer/audio_output_stream.cpp

@@ -4,23 +4,28 @@ namespace torchaudio::io {
 
 namespace {
 
-std::unique_ptr<FilterGraph> get_audio_filter(
+FilterGraph get_audio_filter(
     AVSampleFormat src_fmt,
     AVCodecContext* codec_ctx) {
+  auto desc = [&]() -> std::string {
     if (src_fmt == codec_ctx->sample_fmt) {
-    return {nullptr};
+      return "anull";
+    } else {
+      std::stringstream ss;
+      ss << "aformat=" << av_get_sample_fmt_name(codec_ctx->sample_fmt);
+      return ss.str();
     }
-  std::stringstream desc;
-  desc << "aformat=" << av_get_sample_fmt_name(codec_ctx->sample_fmt);
-  auto p = std::make_unique<FilterGraph>(AVMEDIA_TYPE_AUDIO);
-  p->add_audio_src(
+  }();
+
+  FilterGraph p{AVMEDIA_TYPE_AUDIO};
+  p.add_audio_src(
       src_fmt,
       codec_ctx->time_base,
       codec_ctx->sample_rate,
       codec_ctx->channel_layout);
-  p->add_sink();
-  p->add_process(desc.str());
-  p->create_filter();
+  p.add_sink();
+  p.add_process(desc);
+  p.create_filter();
   return p;
 }
 

torchaudio/csrc/ffmpeg/stream_writer/output_stream.cpp

@@ -5,7 +5,7 @@ namespace torchaudio::io {
 OutputStream::OutputStream(
     AVFormatContext* format_ctx,
     AVCodecContext* codec_ctx_,
-    std::unique_ptr<FilterGraph>&& filter_)
+    FilterGraph&& filter_)
     : codec_ctx(codec_ctx_),
       encoder(format_ctx, codec_ctx),
       filter(std::move(filter_)),
@@ -13,13 +13,9 @@ OutputStream::OutputStream(
       num_frames(0) {}
 
 void OutputStream::process_frame(AVFrame* src) {
-  if (!filter) {
-    encoder.encode(src);
-    return;
-  }
-  int ret = filter->add_frame(src);
+  int ret = filter.add_frame(src);
   while (ret >= 0) {
-    ret = filter->get_frame(dst_frame);
+    ret = filter.get_frame(dst_frame);
     if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
       if (ret == AVERROR_EOF) {
         encoder.encode(nullptr);

torchaudio/csrc/ffmpeg/stream_writer/output_stream.h

@@ -13,7 +13,7 @@ struct OutputStream {
   // Encoder + Muxer
   Encoder encoder;
   // Filter for additional processing
-  std::unique_ptr<FilterGraph> filter;
+  FilterGraph filter;
   // frame that output from FilterGraph is written
   AVFramePtr dst_frame;
   // The number of samples written so far
@@ -22,7 +22,7 @@ struct OutputStream {
   OutputStream(
       AVFormatContext* format_ctx,
       AVCodecContext* codec_ctx,
-      std::unique_ptr<FilterGraph>&& filter);
+      FilterGraph&& filter);
 
   virtual void write_chunk(const torch::Tensor& input) = 0;
   void process_frame(AVFrame* src);

torchaudio/csrc/ffmpeg/stream_writer/video_output_stream.cpp

@@ -8,26 +8,31 @@ namespace torchaudio::io {
 
 namespace {
 
-std::unique_ptr<FilterGraph> get_video_filter(
+FilterGraph get_video_filter(
     AVPixelFormat src_fmt,
     AVCodecContext* codec_ctx,
     const torch::Device& device) {
-  if (src_fmt == codec_ctx->pix_fmt || device.type() != c10::DeviceType::CPU) {
-    return {nullptr};
+  auto desc = [&]() -> std::string {
+    if (src_fmt == codec_ctx->pix_fmt ||
+        device.type() != c10::DeviceType::CPU) {
+      return "null";
+    } else {
+      std::stringstream ss;
+      ss << "format=" << av_get_pix_fmt_name(codec_ctx->pix_fmt);
+      return ss.str();
     }
-  std::stringstream desc;
-  desc << "format=" << av_get_pix_fmt_name(codec_ctx->pix_fmt);
+  }();
 
-  auto p = std::make_unique<FilterGraph>(AVMEDIA_TYPE_VIDEO);
-  p->add_video_src(
+  FilterGraph p{AVMEDIA_TYPE_VIDEO};
+  p.add_video_src(
       src_fmt,
       codec_ctx->time_base,
       codec_ctx->width,
       codec_ctx->height,
       codec_ctx->sample_aspect_ratio);
-  p->add_sink();
-  p->add_process(desc.str());
-  p->create_filter();
+  p.add_sink();
+  p.add_process(desc);
+  p.create_filter();
   return p;
 }