ffmpeg tutorial 3 教程 结果分析

目标：

关注声音回调函数与 声音解码函数的关系

关注视频帧调用过程

未完成

 

测试案列：

测试平台：

测试代码：

 

  首先来说，为什么可以出声音，我认为是这个函数 SDL_pauseAudio 的行为.

关注声音回调函数与 解码的关系

  用数据来说话，分别在两个位置下断点

  位置一是：audio_callback函数位置

  位置二是：audio_decode_frame解码函数位置

  让我们来看看实际的代码：

 

void audio_callback(void *userdata, Uint8 *stream, int len) {  AVCodecContext *aCodecCtx = (AVCodecContext *)userdata;  int len1, audio_size;  static uint8_t audio_buf[(MAX_AUDIO_FRAME_SIZE * 3) / 2];  static unsigned int audio_buf_size = 0;  static unsigned int audio_buf_index = 0;  while(len > 0) {    if(audio_buf_index >= audio_buf_size) {

      /* We have already sent all our data; get more */      audio_size = audio_decode_frame(aCodecCtx, audio_buf, audio_buf_size);//请特别注意。

      if(audio_size < 0) {/* If error, output silence */audio_buf_size = 1024; // arbitrary?memset(audio_buf, 0, audio_buf_size);      } else {audio_buf_size = audio_size;      }      audio_buf_index = 0;    }

    len1 = audio_buf_size - audio_buf_index;    if(len1 > len)      len1 = len;    memcpy(stream, (uint8_t *)audio_buf + audio_buf_index, len1);    len -= len1;    stream += len1;    audio_buf_index += len1;  }}

 

 结果如下，可以看到至少4次audio_decode_frame后，系统将会调用 回调函数。这样的回调会周期性发生。

 有同学会问，为什么是4次？我想要么是给系统回调的声音buffer放满了，要么是audio_callback 定时调用。

 

Function: audio_callback(void *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_callback(void *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_callback(void *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_callback(void *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_callback(void *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程Function: audio_decode_frame(AVCodecContext *, unsigned char *, int), Thread: 0x81B8 Win32 线程

 

 

关注视频帧调用过程

  之所以贴在这里，想以后再分析，再给出结果。

理论上，12个时刻，应该对应12个不同的帧，但是结果不是，看看以下结果（用notepad++处理后，用于统计个数）。

 

Search "video" (11 hits in 1 file)  new  2 (11 hits)Line 120: read one video frameLine 286: read one video frameLine 291: read one video frameLine 292: read one video frameLine 293: read one video frameLine 294: read one video frameLine 295: read one video frameLine 296: read one video frameLine 297: read one video frameLine 298: read one video frameLine 299: read one video frameSearch "audio" (288 hits in 1 file)

未完成

其他：看看第一次调用audio_callback时，是否遇到返回为空的情形。

其他：请打印出每次解码出的buffer大小，尤其注意那个中间连续调用五次解码的过程。

其他：是否可以结合dump出的文件信息，量化分析过程。