SDL2 播放音频(MP4)-编程知识

1.简介

这里引入FFmpeg库，获取音频流数据，然后通过FFmpeg将视频流解码成pcm原始数据，再将pcm数据送入到SDL库中实现音频播放。

2.FFmpeg的操作流程

注册API：av_register_all()
构建输入AVFormatContext上下文：avformat_open_input()
查找音视频流信息：avformat_find_stream_info()
查找解码器：avcodec_find_decoder()
打开解码器：avcodec_open2()
然后通过while循环，不停的读取数据：av_read_frame()
帧解码：avcodec_send_packet()和avcodec_receive_frame()
重采样：swr_convert()

3.SDL音频播放流程

SDL播放音频的流程如下：

初始化音频子系统：SDL_Init()。
设置音频参数：SDL_AudioSpec。
设置回调函数：SDL_AudioCallback。
打开音频设备：SDL_OpenAudio()。
打开pcm文件，读取数据。
开始播放：SDL_PauseAudio()。

4.示例

#include <stdio.h>
#include <SDL.h>
#include <memory>extern "C"
{
#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libswscale/swscale.h"
#include "libavutil/imgutils.h"
#include "libswresample/swresample.h"
};static  Uint8  *audio_chunk;
static  Uint32  audio_len;
static  Uint8  *audio_pos;void  fill_audio(void *udata, Uint8 *stream, int len)
{//SDL 2.0SDL_memset(stream, 0, len);if (audio_len == 0)		/*  Only  play  if  we  have  data  left  */return;len = (len > audio_len ? audio_len : len);	/*  Mix  as  much  data  as  possible  */SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME/2);audio_pos += len;audio_len -= len;	
}AVFrame *recv(AVCodecContext *codecCtx)
{if (!codecCtx){return NULL;}AVFrame *frame = av_frame_alloc();int ret = avcodec_receive_frame(codecCtx, frame);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF){av_frame_free(&frame);return NULL;}else if (ret < 0){av_frame_free(&frame);return NULL;}return frame;
}#undef main
int main(int argc, char* argv[])
{av_register_all();///ffmpegavformat_network_init();AVFormatContext* pFormatCtx = NULL;const char* inputUrl = "./2.mp4";///打开输入的流int ret = avformat_open_input(&pFormatCtx, inputUrl, NULL, NULL);if (ret != 0){printf("Couldn't open input stream.\n");return -1;}//查找流信息if (avformat_find_stream_info(pFormatCtx, NULL) < 0){printf("Couldn't find stream information.\n");return -1;}//找到音频流索引int audio_index = av_find_best_stream(pFormatCtx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);AVStream* st = pFormatCtx->streams[audio_index];AVCodec* codec = nullptr;//找到解码器codec = avcodec_find_decoder(st->codecpar->codec_id);if (!codec){fprintf(stderr, "Codec not found\n");return -1;}//申请AVCodecContextAVCodecContext* pCodecCtx = avcodec_alloc_context3(codec);if (!pCodecCtx){return -1;}avcodec_parameters_to_context(pCodecCtx, pFormatCtx->streams[audio_index]->codecpar);//打开解码器if ((ret = avcodec_open2(pCodecCtx, codec, NULL) < 0)){return -1;}AVPacket* pkt = av_packet_alloc();//------------SDL----------------//Output Info-----------------------------printf("---------------- File Information ---------------\n");av_dump_format(pFormatCtx, 0, inputUrl, 0);printf("-------------------------------------------------\n");SwrContext *swrContext = swr_alloc();if (!swrContext){return -1;}swrContext = swr_alloc_set_opts(NULL,													//ctxAV_CH_LAYOUT_STEREO,																	//输出channel布局AV_SAMPLE_FMT_S16,																		 //输出的采样格式44100,																									//采样率av_get_default_channel_layout(pCodecCtx->channels),						//输入channel布局pCodecCtx->sample_fmt,																	//输入的采样格式pCodecCtx->sample_rate,																	//输入的采样率0, NULL);// 初始化重采样上下文if (swr_init(swrContext) < 0){swr_free(&swrContext);return -1;}if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER)){printf("Could not initialize SDL - %s\n", SDL_GetError());return -1;}//SDL_AudioSpecSDL_AudioSpec wanted_spec;wanted_spec.freq = 44100;wanted_spec.format = AUDIO_S16SYS;wanted_spec.channels = 2;wanted_spec.silence = 0;wanted_spec.samples = 1024;wanted_spec.callback = fill_audio;wanted_spec.userdata = pCodecCtx;if (SDL_OpenAudio(&wanted_spec, NULL) < 0){printf("can't open audio.\n");return -1;}//PlaySDL_PauseAudio(0);// 分配输出音频数据uint8_t	 *out_buffer = nullptr;while (av_read_frame(pFormatCtx, pkt) >= 0){if (pkt->stream_index == audio_index){//一次send 多次recvint ret = avcodec_send_packet(pCodecCtx, pkt);if (ret < 0)continue;//释放资源av_packet_unref(pkt);while (1){AVFrame *frame = recv(pCodecCtx);if (!frame)break;//输入的样本数int in_nb_samples = frame->nb_samples;//1024int out_linesize;int dst_nb_samples = av_rescale_rnd(in_nb_samples, 44100, frame->sample_rate, AV_ROUND_UP);//输出的样本数int out_buffer_size = av_samples_get_buffer_size(NULL, 2, dst_nb_samples, AV_SAMPLE_FMT_S16, 0);if(!out_buffer)out_buffer = (uint8_t *)av_malloc(out_buffer_size);//返回每个通道输出的样本数，错误时为负值int sampleCount = swr_convert(swrContext, &out_buffer, dst_nb_samples,(const uint8_t**)frame->data, in_nb_samples);if (sampleCount <= 0){av_frame_free(&frame);break;}int outSize = sampleCount * 2 * av_get_bytes_per_sample(AV_SAMPLE_FMT_S16);while (audio_len > 0)//Wait until finishSDL_Delay(1);//Set audio buffer (PCM data)audio_chunk = (Uint8 *)out_buffer;//Audio buffer lengthaudio_len = outSize;audio_pos = audio_chunk;av_frame_free(&frame);}}else{//释放资源av_packet_unref(pkt);}}//--------------av_free(out_buffer);av_packet_free(&pkt);swr_close(swrContext);swr_free(&swrContext);avcodec_close(pCodecCtx);avcodec_free_context(&pCodecCtx);avformat_close_input(&pFormatCtx);SDL_CloseAudio();SDL_Quit();
}