fix FfmpegDecoderPlugin to use relative timestamps

NEWS

@@ -3,6 +3,8 @@ ver 0.18.6 (not yet released)
   - cdio_paranoia: support libcdio-paranoia 0.90
 * tags
   - riff: recognize upper-case "ID3" chunk name
+* decoder
+  - ffmpeg: use relative timestamps
 * output
   - openal: fix build failure on Mac OS X
   - osx: fix build failure

src/decoder/FfmpegDecoderPlugin.cxx

@@ -251,13 +251,14 @@ static DecoderCommand
 ffmpeg_send_packet(Decoder &decoder, InputStream &is,
 		   const AVPacket *packet,
 		   AVCodecContext *codec_context,
-		   const AVRational *time_base,
+		   const AVStream *stream,
 		   AVFrame *frame,
 		   uint8_t **buffer, int *buffer_size)
 {
 	if (packet->pts >= 0 && packet->pts != (int64_t)AV_NOPTS_VALUE)
 		decoder_timestamp(decoder,
-				  time_from_ffmpeg(packet->pts, *time_base));
+				  time_from_ffmpeg(packet->pts - stream->start_time,
+				  stream->time_base));
 
 	AVPacket packet2 = *packet;
 
@@ -470,7 +471,7 @@ ffmpeg_decode(Decoder &decoder, InputStream &input)
 		if (packet.stream_index == audio_stream)
 			cmd = ffmpeg_send_packet(decoder, input,
 						 &packet, codec_context,
-						 &av_stream->time_base,
+						 av_stream,
 						 frame,
 						 &interleaved_buffer, &interleaved_buffer_size);
 		else
@@ -481,7 +482,8 @@ ffmpeg_decode(Decoder &decoder, InputStream &input)
 		if (cmd == DecoderCommand::SEEK) {
 			int64_t where =
 				time_to_ffmpeg(decoder_seek_where(decoder),
-					       av_stream->time_base);
+					       av_stream->time_base) +
+				av_stream->start_time;
 
 			if (av_seek_frame(format_context, audio_stream, where,
 					  AV_TIME_BASE) < 0)