FFmpeg编程学习笔记二：音频重採样

ffmpeg实现音频重採样的核心函数swr_convert功能很强大，但是ffmpeg文档对它的凝视太过简单。在应用中往往会出这样那样的问题，事实上在读取数据->重採样->编码数据的循环中在第一次运行swr_convert后还应用swr_convert再作个缓存检測看看是否还有数据。假设有就要把它写到FIFO中去，留在下次再使用，这点在转码和由低向高转换採样率时特别重要。

以下一段简单的代码，摘自我自已写的測试程序.

	const int frame_size = FFMIN(fifo_size, m_Opt->encode_pCodecCtx->frame_size);

	if ((ret = av_audio_fifo_read(m_fifo, (void **)m_fifo_samples_array, frame_size)) < frame_size) {
		fprintf(stderr, "Could not read data from FIFO\n");
		return AVERROR_EXIT;
	}

	int out_samples = av_rescale_rnd(swr_get_delay(m_Opt->out_resample_context, 48000) + 1536, 44100, 48000, AV_ROUND_UP);

	int conver_samples= swr_convert(m_Opt->out_resample_context, m_fifo_conver_samples_array, frame_size,
		(const uint8_t **)m_fifo_samples_array, frame_size);
	ret = av_audio_fifo_size(m_conver_fifo);

	if ((ret = av_audio_fifo_realloc(m_conver_fifo, av_audio_fifo_size(m_conver_fifo) + frame_size)) < 0) {
			fprintf(stderr, "Could not reallocate FIFO\n");
			return ret;
		}

	ret = av_audio_fifo_write(m_conver_fifo, (void **)m_fifo_conver_samples_array, conver_samples);
		fifo_size = av_audio_fifo_size(m_conver_fifo);

		if ((ret = av_audio_fifo_read(m_conver_fifo, (void **)m_Opt->out_samples_array, frame_size)) < frame_size) {
			fprintf(stderr, "Could not read data from FIFO\n");
			return AVERROR_EXIT;
		}

		ret = Encode_audio(ret);

		int ret1 = 0;

		while ((ret1 = swr_convert(m_Opt->out_resample_context, m_fifo_conver_samples_array, frame_size, NULL, 0)) > 0)
	{

		if ((ret = av_audio_fifo_realloc(m_conver_fifo, av_audio_fifo_size(m_conver_fifo) + ret1)) < 0) {
			fprintf(stderr, "Could not reallocate FIFO\n");
			return ret;
		}

		ret = av_audio_fifo_write(m_conver_fifo, (void **)m_fifo_conver_samples_array, ret1);
		fifo_size = av_audio_fifo_size(m_conver_fifo);

		if (fifo_size > m_Opt->encode_pFrame->nb_samples)
		{

			if ((ret = av_audio_fifo_read(m_conver_fifo, (void **)m_Opt->out_samples_array, frame_size)) < frame_size) {
				fprintf(stderr, "Could not read data from FIFO\n");
				return AVERROR_EXIT;
			}

			ret = Encode_audio(ret);

		}

由于是測试代码，有点乱中间也有一些不必要的步骤。主要是为了调试好用。

重点在：

    if ((r = swr_convert(swr_ctx, output, output_nb_samples,(const uint8_t**)input, nb_samples)) < 0)
        return -1;

    while ((r = swr_convert(swr_ctx, output, output_nb_samples, NULL, 0)) > 0) {

    }