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：
< code_snippet_id="2015779" snippet_file_name="blog_20161129_1_8930207" name="code" class="cpp">/** 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);
< code_snippet_id="2015779" snippet_file_name="blog_20161129_2_4393658" name="code" class="cpp">/** * 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：
< code_snippet_id="2015779" snippet_file_name="blog_20161129_3_1361233" name="code" class="cpp">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：
< code_snippet_id="2015779" snippet_file_name="blog_20161129_4_9649365" name="code" class="cpp">//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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