参考资料：
FFmpeg和SDL2播放mp4_sdl 播放mp4 声音-CSDN博客
SimplePlayer/SimplePlayer.c at master · David1840/SimplePlayer · GitHub
在前面的学习中，通过获得的AVFrame进行了播放画面，
[FFmpeg学习]初级的SDL播放mp4测试-CSDN博客
播放音频原理类似，也是获取AVFrame的信息，
cpp
extern "C" {
#include <libavutil/log.h>
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswresample/swresample.h>
}
// Simplest FFmpeg Sync Player.cpp : 定义控制台应用程序的入口点。
//
#include <stdio.h>
#include <SDL_types.h>
#include "SDL.h"
static Uint8* audio_chunk;
static Uint32 audio_len;
static Uint8* audio_pos;
#define MAX_AUDIO_FRAME_SIZE 19200
//音频设备需要更多数据的时候会调用该回调函数
void read_audio_data(void* udata, Uint8* stream, int len) {
fprintf(stderr, "stream addr:%p, audio_len:%d, len:%d\n",
stream,
audio_len,
len);
//首先使用SDL_memset()将stream中的数据设置为0
SDL_memset(stream, 0, len);
if (audio_len == 0)
return;
len = (len > audio_len ? audio_len : len);
SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME);
audio_pos += len;
audio_len -= len;
}
#undef main
int main(int argc, char* argv[]) {
const char* file = "test.mp4";
AVFormatContext* pFormatCtx = NULL; //for opening multi-media file
int i, audioStream = -1;
AVCodecParameters* pCodecParameters = NULL; //codec context
AVCodecContext* pCodecCtx = NULL;
const AVCodec* pCodec = NULL; // the codecer
AVFrame* pFrame = NULL;
AVPacket* packet;
uint8_t* out_buffer;
int64_t in_channel_layout;
struct SwrContext* au_convert_ctx;
if (avformat_open_input(&pFormatCtx, file, NULL, NULL) != 0) {
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Failed to open video file!");
return -1; // Couldn't open file
}
audioStream = av_find_best_stream(pFormatCtx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
if (audioStream == -1) {
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Din't find a video stream!");
return -1;// Didn't find a video stream
}
// Get a pointer to the codec context for the video stream
pCodecParameters = pFormatCtx->streams[audioStream]->codecpar;
// Find the decoder for the video stream
pCodec = avcodec_find_decoder(pCodecParameters->codec_id);
if (pCodec == NULL) {
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Unsupported codec!\n");
return -1; // Codec not found
}
// Copy context
pCodecCtx = avcodec_alloc_context3(pCodec);
if (avcodec_parameters_to_context(pCodecCtx, pCodecParameters) != 0) {
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Couldn't copy codec context");
return -1;// Error copying codec context
}
// Open codec
if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0) {
SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Failed to open decoder!\n");
return -1; // Could not open codec
}
packet = (AVPacket*)av_malloc(sizeof(AVPacket));
av_init_packet(packet);
pFrame = av_frame_alloc();
uint64_t out_channel_layout = AV_CH_LAYOUT_STEREO;//输出声道
int out_nb_samples = 1024;
enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;//输出格式S16
int out_sample_rate = 44100;
int out_channels = av_get_channel_layout_nb_channels(out_channel_layout);
int out_buffer_size = av_samples_get_buffer_size(NULL, out_channels, out_nb_samples, out_sample_fmt, 1);
out_buffer = (uint8_t*)av_malloc(MAX_AUDIO_FRAME_SIZE * 2);
//Init
if (SDL_Init(SDL_INIT_AUDIO | SDL_INIT_TIMER)) {
printf("Could not initialize SDL - %s\n", SDL_GetError());
return -1;
}
SDL_AudioSpec spec;
spec.freq = out_sample_rate;
spec.format = AUDIO_S16SYS;
spec.channels = out_channels;
spec.silence = 0;
spec.samples = out_nb_samples;
spec.callback = read_audio_data;
spec.userdata = pCodecCtx;
if (SDL_OpenAudio(&spec, NULL) < 0) {
printf("can't open audio.\n");
return -1;
}
in_channel_layout = av_get_default_channel_layout(pCodecCtx->channels);
printf("in_channel_layout --->%d\n", in_channel_layout);
au_convert_ctx = swr_alloc();
au_convert_ctx = swr_alloc_set_opts(au_convert_ctx, out_channel_layout, out_sample_fmt, out_sample_rate,
in_channel_layout, pCodecCtx->sample_fmt, pCodecCtx->sample_rate, 0, NULL);
swr_init(au_convert_ctx);
SDL_PauseAudio(0);
while (av_read_frame(pFormatCtx, packet) >= 0) {
if (packet->stream_index == audioStream) {
avcodec_send_packet(pCodecCtx, packet);
while (avcodec_receive_frame(pCodecCtx, pFrame) == 0) {
swr_convert(au_convert_ctx, &out_buffer, MAX_AUDIO_FRAME_SIZE, (const uint8_t**)pFrame->data,
pFrame->nb_samples); // 转换音频
}
audio_chunk = (Uint8*)out_buffer;
audio_len = out_buffer_size;
audio_pos = audio_chunk;
while (audio_len > 0) {
SDL_Delay(1);//延迟播放
}
}
av_packet_unref(packet);
}
swr_free(&au_convert_ctx);
SDL_Quit();
return 0;
}在windows上，如果有方法废弃的错误，
错误 C4996 'AVCodecContext::channels': 被声明为已否决
可以设置SDL检查为否，来解决
代码流程里，需要注意一下audio_callback是怎么回调的，
audio_callback函数是由SDL音频系统在需要更多音频数据以填充音频缓冲区时自动调用的。当SDL音频系统开始播放音频时，它会周期性地调用音频回调函数以获取新的音频数据。音频回调函数的调用频率取决于音频采样率和缓冲区大小。
在main函数中，我们初始化SDL音频系统并设置了音频回调函数audio_callback：
if (SDL_Init(SDL_INIT_AUDIO) < 0) {
fprintf(stderr, "Could not initialize SDL - %s\n", SDL_GetError());
exit(1);
}
// 设置音频回调函数
SDL_AudioSpec desired;
desired.freq = audioCodecContext->sample_rate;
desired.format = AUDIO_S16SYS; // 使用16位有符号小端字节序样本
desired.channels = audioCodecContext->channels;
desired.samples = 4096; // 音频缓冲区大小，可以根据需要调整
desired.callback = audio_callback;
desired.userdata = NULL;
// 打开音频设备
SDL_AudioDeviceID deviceId = SDL_OpenAudioDevice(NULL, 0, &desired, NULL, SDL_AUDIO_ALLOW_ANY_CHANGE);
if (deviceId <= 0) {
fprintf(stderr, "Failed to open audio device: %s\n", SDL_GetError());
return -1;
}
// 开始播放音频
SDL_PauseAudioDevice(deviceId, 0);
当我们调用SDL_OpenAudioDevice函数时，SDL音频系统会注册音频回调函数audio_callback。当我们调用SDL_PauseAudioDevice(deviceId, 0)开始播放音频时，SDL音频系统会根据音频参数（如采样率、通道数和缓冲区大小）定期调用audio_callback函数。
在audio_callback函数中，我们需要根据音频缓冲区的需求提供音频数据。这通常涉及从文件或实时流中解码音频数据，并将其传递给SDL音频系统。在我们的示例中，我们从全局变量audio_buffer中读取音频数据，该变量由swr_convert函数填充。
总之，audio_callback函数是由SDL音频系统在需要更多音频数据以填充音频缓冲区时自动触发的。您无需手动调用此函数，SDL音频系统会负责调用它。只需确保在回调函数中提供正确的音频数据即可。