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之前完成了PCM音频的播放，这次实现的是FFmpeg+SDL2播放任意视频中的音频流。
整体的流程和视频流播放类似，需要了解下的就是 SwrContext 重采样结构体
重采样结构体,就是改变音频的采样率、sample format、声道数等参数，使之按照我们期望的参数输出,当然是原有的音频参数不满足我们的需求，比如在FFMPEG解码音频的时候，不同的音源有不同的格式，采样率等，在解码后的数据中的这些参数也会不一致，如果我们接下来需要使用解码后的音频数据做其他操作，而这些参数的不一致导致会有很多额外工作，此时直接对其进行重采样，获取我们制定的音频参数，这样就会方便很多。
通过重采样，我们可以对 sample rate(采样率)、sample format(采样格式)、channel layout(通道布局，可以通过此参数获取声道数)进行调节。
SwrContext常用函数
swr_alloc
// 用于申请一个SwrContext结构体
struct SwrContext *swr_alloc(void);
swr_init
// 当设置好相关的参数后，使用此函数来初始化SwrContext结构体
int swr_init(struct SwrContext *s);
swr_alloc_set_opts
//分配SwrContext并设置/重置常用的参数。参数包含了输入输出参数中sample rate(采样率)、sample format(采样格式)、channel layout等参数
函数原型：struct SwrContext *swr_alloc_set_opts(struct SwrContext *s,
int64_t out_ch_layout,
enum AVSampleFormat out_sample_fmt,
int out_sample_rate,
int64_t in_ch_layout,
enum AVSampleFormat in_sample_fmt,
int in_sample_rate,
int log_offset,
void *log_ctx);
swr_convert
// 将输入的音频按照定义的参数进行转换，并输出
int swr_convert(struct SwrContext *s, uint8_t **out, int out_count,
const uint8_t **in , int in_count);
swr_free
// 释放掉SwrContext结构体并将此结构体置为NULL;
void swr_free(struct SwrContext **s);
示例代码
//
// Created by 刘伟 on 2019/4/26.
//
#include <stdio.h>
#include <SDL_types.h>
#include "SDL.h"
#include "libswresample/swresample.h"
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.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) {
//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;
}
int WinMain(int argc, char *argv[]) {
char *file = "C:\\Users\\lenovo\\Desktop\\1080p.mov";
AVFormatContext *pFormatCtx = NULL;
int i, audioStream = -1;
AVCodecParameters *pCodecParameters = NULL;
AVCodecContext *pCodecCtx = NULL;
AVCodec *pCodec = NULL;
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
}
// 音频流参数
pCodecParameters = pFormatCtx->streams[audioStream]->codecpar;
// 获取解码器
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;
}
