ffmpeg重采样中swr_convert和swr_get_out_samples的用法

ffmpeg重采样中swr_convert和swr_get_out_samples的用法

在做mux的时候关于重采样可以用fifo，或者audiofifo做缓存处理，当做demux的时候关于重采样就可以用到上面的swr_convert和swr_get_out_samples做配合处理。先看下这两个函数的注释：

/** Convert audio.
 *
 * in and in_count can be set to 0 to flush the last few samples out at the
 * end.
 *
 * If more input is provided than output space, then the input will be buffered.
 * You can avoid this buffering by using swr_get_out_samples() to retrieve an
 * upper bound on the required number of output samples for the given number of
 * input samples. Conversion will run directly without copying whenever possible.
 *
 * @param s         allocated Swr context, with parameters set
 * @param out       output buffers, only the first one need be set in case of packed audio
 * @param out_count amount of space available for output in samples per channel
 * @param in        input buffers, only the first one need to be set in case of packed audio
 * @param in_count  number of input samples available in one channel
 *
 * @return number of samples output per channel, negative value on error
 */
int swr_convert(struct SwrContext *s, uint8_t **out, int out_count,
                                const uint8_t **in , int in_count);

/**
 * Find an upper bound on the number of samples that the next swr_convert
 * call will output, if called with in_samples of input samples. This
 * depends on the internal state, and anything changing the internal state
 * (like further swr_convert() calls) will may change the number of samples
 * swr_get_out_samples() returns for the same number of input samples.
 *
 * @param in_samples    number of input samples.
 * @note any call to swr_inject_silence(), swr_convert(), swr_next_pts()
 *       or swr_set_compensation() invalidates this limit
 * @note it is recommended to pass the correct available buffer size
 *       to all functions like swr_convert() even if swr_get_out_samples()
 *       indicates that less would be used.
 * @returns an upper bound on the number of samples that the next swr_convert
 *          will output or a negative value to indicate an error
 */
int swr_get_out_samples(struct SwrContext *s, int in_samples);

就说如果传入的nb_samles大于了传出的nb_samplse则SwrContext中会有缓存，会导致内存一直暴涨，解决方法，可以看如下代码：

没有缓存的重采样这么处理：

ret = swr_convert(swrcontext, pOutputFrame->data,pOutputFrame->nb_samples,
		(const uint8_t**)pInputFrame->data,pInputFrame->nb_samples);

有缓存的代码这么处理：

//如果还有缓存在swrcontext中,第二个参数要填写0才能获取到，缓存数据
int fifo_size = swr_get_out_samples(swrcontext,0);
if ( fifo_size >= pOutputFrame->nb_samples)
{
	ret = swr_convert(swrcontext, pOutputFrame->data,pOutputFrame->nb_samples,
	NULL,0);
}

即如果有缓存则先判断是否有缓存在里面，如果有则传入数据为空取出缓存。

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