Support start trimming post-decoding. Use it with FFmpegDemuxer.

media/filters/ffmpeg_demuxer.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "media/filters/ffmpeg_demuxer.h"
 
 #include <algorithm>
 #include <string>
 
 #include "base/base64.h"
@@ skipping 32 matching lines @@
     base::Time timeline_offset;
     if (entry != NULL && entry->value != NULL &&
         FFmpegUTCDateToTime(entry->value, &timeline_offset)) {
       return timeline_offset;
     }
   }
 
   return base::Time();
 }
 
+static base::TimeDelta FramesToTimeDelta(int frames, double sample_rate) {
+  return base::TimeDelta::FromMicroseconds(
+      frames * base::Time::kMicrosecondsPerSecond / sample_rate);
+}
+
 //
 // FFmpegDemuxerStream
 //
 FFmpegDemuxerStream::FFmpegDemuxerStream(
     FFmpegDemuxer* demuxer,
     AVStream* stream)
     : demuxer_(demuxer),
       task_runner_(base::MessageLoopProxy::current()),
       stream_(stream),
       type_(UNKNOWN),
@@ skipping 119 matching lines @@
     if (side_data_size > 0) {
       buffer = DecoderBuffer::CopyFrom(packet.get()->data + data_offset,
                                        packet.get()->size - data_offset,
                                        side_data, side_data_size);
     } else {
       buffer = DecoderBuffer::CopyFrom(packet.get()->data + data_offset,
                                        packet.get()->size - data_offset);
     }
 
     int skip_samples_size = 0;
-    uint8* skip_samples = av_packet_get_side_data(packet.get(),
-                                                  AV_PKT_DATA_SKIP_SAMPLES,
-                                                  &skip_samples_size);
+    const uint32* skip_samples_ptr =
+        reinterpret_cast<const uint32*>(av_packet_get_side_data(
+            packet.get(), AV_PKT_DATA_SKIP_SAMPLES, &skip_samples_size));
     const int kSkipSamplesValidSize = 10;
-    const int kSkipSamplesOffset = 4;
+    const int kSkipEndSamplesOffset = 4;

[DaleCurtis, 2014/05/01 01:44:24]

Note to self: This is wrong now, it should be in uint32 bytes, so just 1. 

[DaleCurtis, 2014/05/01 19:21:27]

Done.

     if (skip_samples_size >= kSkipSamplesValidSize) {
-      int discard_padding_samples = base::ByteSwapToLE32(
-          *(reinterpret_cast<const uint32*>(skip_samples +
-                                            kSkipSamplesOffset)));
-      // TODO(vigneshv): Change decoder buffer to use number of samples so that
-      // this conversion can be avoided.
-      buffer->set_discard_padding(base::TimeDelta::FromMicroseconds(
-          discard_padding_samples * 1000000.0 /
-          audio_decoder_config().samples_per_second()));
+      // Because FFmpeg rolls codec delay and skip samples into one we can only
+      // allow front discard padding on the first buffer.  Otherwise the discard
+      // helper can't figure out which data to discard.  See AudioDiscardHelper.
+      int discard_front_samples = base::ByteSwapToLE32(*skip_samples_ptr);
+      if (last_packet_timestamp_ != kNoTimestamp()) {
+        DLOG(ERROR) << "Skip samples are only allowed for the first packet.";
+        discard_front_samples = 0;
+      }
+
+      const int discard_end_samples =
+          base::ByteSwapToLE32(*(skip_samples_ptr + kSkipEndSamplesOffset));
+      const int samples_per_second =
+          audio_decoder_config().samples_per_second();
+      buffer->set_discard_padding(std::make_pair(
+          FramesToTimeDelta(discard_front_samples, samples_per_second),
+          FramesToTimeDelta(discard_end_samples, samples_per_second)));
     }
 
     if (decrypt_config)
       buffer->set_decrypt_config(decrypt_config.Pass());
   }
 
   buffer->set_timestamp(ConvertStreamTimestamp(
       stream_->time_base, packet->pts));
   buffer->set_duration(ConvertStreamTimestamp(
       stream_->time_base, packet->duration));
@@ skipping 800 matching lines @@
   }
   for (size_t i = 0; i < buffered.size(); ++i)
     host_->AddBufferedTimeRange(buffered.start(i), buffered.end(i));
 }
 
 void FFmpegDemuxer::OnDataSourceError() {
   host_->OnDemuxerError(PIPELINE_ERROR_READ);
 }
 
 }  // namespace media