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transcode_aac流程：

init_resampler重采样初始化av_audio_fifo_alloc(output_codec_context->sample_fmt,output_codec_context->channels, 1)	//	只要一帧Sample bufferavformat_write_header(output_format_context, NULL))while (1) {
         /* Use the encoder's desired frame size for processing. */      const int output_frame_size = output_codec_context->frame_size;      int finished                = 0;      /* Make sure that there is one frame worth of samples in the FIFO       * buffer so that the encoder can do its work.       * Since the decoder's and the encoder's frame size may differ, we       * need to FIFO buffer to store as many frames worth of input samples       * that they make up at least one frame worth of output samples. */      while (av_audio_fifo_size(fifo) < output_frame_size) {
             /* Decode one frame worth of audio samples, convert it to the           * output sample format and put it into the FIFO buffer. */          if (read_decode_convert_and_store(fifo, input_format_context,                                            input_codec_context,                                            output_codec_context,                                            resample_context, &finished))              goto cleanup;          /* If we are at the end of the input file, we continue           * encoding the remaining audio samples to the output file. */          if (finished)              break;      }      /* If we have enough samples for the encoder, we encode them.       * At the end of the file, we pass the remaining samples to       * the encoder. */      while (av_audio_fifo_size(fifo) >= output_frame_size ||             (finished && av_audio_fifo_size(fifo) > 0))          /* Take one frame worth of audio samples from the FIFO buffer,           * encode it and write it to the output file. */          if (load_encode_and_write(fifo, output_format_context,                                    output_codec_context))              goto cleanup;      /* If we are at the end of the input file and have encoded       * all remaining samples, we can exit this loop and finish. */      if (finished) {
             int data_written;          /* Flush the encoder as it may have delayed frames. */          do {
                 data_written = 0;              if (encode_audio_frame(NULL, output_format_context,                                     output_codec_context, &data_written))                  goto cleanup;          } while (data_written);          break;      }  }    /* Write the trailer of the output file container. */

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