FFmpeg编程学习笔记一
1、为了学习ffmpeg编程需要单步调试，参照网上的教程用VS2013编译一次成功，之后随便写了个重采样音轨小程序，也就是把一个5.1声道的AC3文件分解成6个WAV文件的简单功能。
2、编译成功执行也正常，但速度奇慢，比同类软件eac3to慢了约5倍。
3、OK上网搜搜咋回事，一天，二天过去了无果。
4、无奈用VS2013的性能与诊断，分析出最占用时间的函数调用具然是：swr_convert参数里的lrintf() lrint() llrint() llrintf() 等几个算法简单的数学函数。
5、问题找到了，下一步就是修改FFMPEG的源文件了，定位到libswresample\audioconvert.c文件，修改如下：
#define lrint_n(f) (f>=0?(int32_t)(f+(double)0.5):(int32_t)(f-(double)0.4)) #define lrintf_n(f) (f>=0?(int32_t)(f+(float)0.5):(int32_t)(f-(float)0.4)) #define llrintf_n(f) (f>=0?(int64_t)(f+(double)0.5):(int64_t)(f-(double)0.4)) #define llrint_n(f) (f>=0?(int64_t)(f+(float)0.5):(int64_t)(f-(float)0.4)) ————————-其它语句————————-//FIXME put things below under ifdefs so we do not waste space for cases no codec will needCONV_FUNC(AV_SAMPLE_FMT_U8 , uint8_t, AV_SAMPLE_FMT_U8 , *(const uint8_t*)pi)CONV_FUNC(AV_SAMPLE_FMT_S16, int16_t, AV_SAMPLE_FMT_U8 , (*(const uint8_t*)pi – 0x80U)<<8)CONV_FUNC(AV_SAMPLE_FMT_S32, int32_t, AV_SAMPLE_FMT_U8 , (*(const uint8_t*)pi – 0x80U)<<24)CONV_FUNC(AV_SAMPLE_FMT_FLT, float , AV_SAMPLE_FMT_U8 , (*(const uint8_t*)pi – 0x80)*(1.0f/ (1<<7)))CONV_FUNC(AV_SAMPLE_FMT_DBL, double , AV_SAMPLE_FMT_U8 , (*(const uint8_t*)pi – 0x80)*(1.0 / (1<<7)))CONV_FUNC(AV_SAMPLE_FMT_U8 , uint8_t, AV_SAMPLE_FMT_S16, (*(const int16_t*)pi>>8) + 0x80)CONV_FUNC(AV_SAMPLE_FMT_S16, int16_t, AV_SAMPLE_FMT_S16, *(const int16_t*)pi)CONV_FUNC(AV_SAMPLE_FMT_S32, int32_t, AV_SAMPLE_FMT_S16, *(const int16_t*)pi<<16)CONV_FUNC(AV_SAMPLE_FMT_FLT, float , AV_SAMPLE_FMT_S16, *(const int16_t*)pi*(1.0f/ (1<<15)))CONV_FUNC(AV_SAMPLE_FMT_DBL, double , AV_SAMPLE_FMT_S16, *(const int16_t*)pi*(1.0 / (1<<15)))CONV_FUNC(AV_SAMPLE_FMT_U8 , uint8_t, AV_SAMPLE_FMT_S32, (*(const int32_t*)pi>>24) + 0x80)CONV_FUNC(AV_SAMPLE_FMT_S16, int16_t, AV_SAMPLE_FMT_S32, *(const int32_t*)pi>>16)CONV_FUNC(AV_SAMPLE_FMT_S32, int32_t, AV_SAMPLE_FMT_S32, *(const int32_t*)pi)CONV_FUNC(AV_SAMPLE_FMT_FLT, float , AV_SAMPLE_FMT_S32, *(const int32_t*)pi*(1.0f/ (1U<<31)))CONV_FUNC(AV_SAMPLE_FMT_DBL, double , AV_SAMPLE_FMT_S32, *(const int32_t*)pi*(1.0 / (1U<<31)))CONV_FUNC(AV_SAMPLE_FMT_U8 , uint8_t, AV_SAMPLE_FMT_FLT, av_clip_uint8( lrintf_n(*(const float*)pi * (1<<7)) + 0x80))CONV_FUNC(AV_SAMPLE_FMT_S16, int16_t, AV_SAMPLE_FMT_FLT, av_clip_int16( lrintf_n(*(const float*)pi * (1<<15))))CONV_FUNC(AV_SAMPLE_FMT_S32, int32_t, AV_SAMPLE_FMT_FLT, av_clipl_int32(llrintf_n(*(const float*)pi * (1U<<31))))CONV_FUNC(AV_SAMPLE_FMT_FLT, float , AV_SAMPLE_FMT_FLT, *(const float*)pi)CONV_FUNC(AV_SAMPLE_FMT_DBL, double , AV_SAMPLE_FMT_FLT, *(const float*)pi)CONV_FUNC(AV_SAMPLE_FMT_U8 , uint8_t, AV_SAMPLE_FMT_DBL, av_clip_uint8( lrint_n(*(const double*)pi * (1<<7)) + 0x80))CONV_FUNC(AV_SAMPLE_FMT_S16, int16_t, AV_SAMPLE_FMT_DBL, av_clip_int16( lrint_n(*(const double*)pi * (1<<15))))CONV_FUNC(AV_SAMPLE_FMT_S32, int32_t, AV_SAMPLE_FMT_DBL, av_clipl_int32(llrint_n(*(const double*)pi * (1U<<31))))CONV_FUNC(AV_SAMPLE_FMT_FLT, float , AV_SAMPLE_FMT_DBL, *(const double*)pi)CONV_FUNC(AV_SAMPLE_FMT_DBL, double , AV_SAMPLE_FMT_DBL, *(const double*)pi)
6、重新编译FFMPEG后问题解决，速度很快，具然超过了eac3to，，呵呵！
闹里有钱，静处安身。
