2023-02-23：请用 go 语言调用 ffmpeg，解码 mp4 文件并保存为 YUV420P 格式文件。

go run ./examples/a07.1video_decode_mp42yuv420p/main.go

package main
import (  "fmt"  "os"  "os/exec"  "unsafe"
  "github.com/moonfdd/ffmpeg-go/ffcommon"  "github.com/moonfdd/ffmpeg-go/libavcodec"  "github.com/moonfdd/ffmpeg-go/libavformat"  "github.com/moonfdd/ffmpeg-go/libavutil")
func main() {  os.Setenv("Path", os.Getenv("Path")+";./lib")  ffcommon.SetAvutilPath("./lib/avutil-56.dll")  ffcommon.SetAvcodecPath("./lib/avcodec-58.dll")  ffcommon.SetAvdevicePath("./lib/avdevice-56.dll")  ffcommon.SetAvfilterPath("./lib/avfilter-56.dll")  ffcommon.SetAvformatPath("./lib/avformat-58.dll")  ffcommon.SetAvpostprocPath("./lib/postproc-55.dll")  ffcommon.SetAvswresamplePath("./lib/swresample-3.dll")  ffcommon.SetAvswscalePath("./lib/swscale-5.dll")
  genDir := "./out"  _, err := os.Stat(genDir)  if err != nil {    if os.IsNotExist(err) {      os.Mkdir(genDir, 0777) //  Everyone can read write and execute    }  }
  filePath := "./resources/big_buck_bunny.mp4" //文件地址  videoStreamIndex := -1                       //视频流所在流序列中的索引  ret := int32(0)                              //默认返回值
  //需要的变量名并初始化  var fmtCtx *libavformat.AVFormatContext  var pkt *libavformat.AVPacket  var codecCtx *libavcodec.AVCodecContext  var avCodecPara *libavcodec.AVCodecParameters  var codec *libavcodec.AVCodec  yuvFrame := libavutil.AvFrameAlloc()
  for {    //=========================== 创建AVFormatContext结构体 ===============================//    //分配一个AVFormatContext，FFMPEG所有的操作都要通过这个AVFormatContext来进行    fmtCtx = libavformat.AvformatAllocContext()    //==================================== 打开文件 ======================================//    ret = libavformat.AvformatOpenInput(&fmtCtx, filePath, nil, nil)    if ret != 0 {      fmt.Printf("cannot open video file\n")      break    }
    //=================================== 获取视频流信息 ===================================//    ret = fmtCtx.AvformatFindStreamInfo(nil)    if ret < 0 {      fmt.Printf("cannot retrive video info\n")      break    }
    //循环查找视频中包含的流信息，直到找到视频类型的流    //便将其记录下来 保存到videoStreamIndex变量中    for i := uint32(0); i < fmtCtx.NbStreams; i++ {      if fmtCtx.GetStream(i).Codecpar.CodecType == libavutil.AVMEDIA_TYPE_VIDEO {        videoStreamIndex = int(i)        break //找到视频流就退出      }    }
    //如果videoStream为-1 说明没有找到视频流    if videoStreamIndex == -1 {      fmt.Printf("cannot find video stream\n")      break    }
    //打印输入和输出信息：长度 比特率 流格式等    fmtCtx.AvDumpFormat(0, filePath, 0)
    //=================================  查找解码器 ===================================//    avCodecPara = fmtCtx.GetStream(uint32(videoStreamIndex)).Codecpar    codec = libavcodec.AvcodecFindDecoder(avCodecPara.CodecId)    if codec == nil {      fmt.Printf("cannot find decoder\n")      break    }    //根据解码器参数来创建解码器内容    codecCtx = codec.AvcodecAllocContext3()    codecCtx.AvcodecParametersToContext(avCodecPara)    if codecCtx == nil {      fmt.Printf("Cannot alloc context.")      break    }
    //================================  打开解码器 ===================================//    ret = codecCtx.AvcodecOpen2(codec, nil)    if ret < 0 { // 具体采用什么解码器ffmpeg经过封装 我们无须知道      fmt.Printf("cannot open decoder\n")      break    }
    os.Remove("./out/result.yuv")    file, err := os.OpenFile("./out/result.yuv", os.O_CREATE|os.O_RDWR, 0777)    if err != nil {      fmt.Println("open file failed,err:", err)      return    }
    w := codecCtx.Width    h := codecCtx.Height
    //=========================== 分配AVPacket结构体 ===============================//    i := 0    pkt = libavcodec.AvPacketAlloc()                  //分配一个packet    pkt.AvNewPacket(codecCtx.Width * codecCtx.Height) //调整packet的数据
    //===========================  读取视频信息 ===============================//    for fmtCtx.AvReadFrame(pkt) >= 0 { //读取的是一帧视频  数据存入一个AVPacket的结构中      if pkt.StreamIndex == uint32(videoStreamIndex) {        if codecCtx.AvcodecSendPacket(pkt) == 0 {          for codecCtx.AvcodecReceiveFrame(yuvFrame) == 0 {            i++            bytes := []byte{}            //y            ptr := uintptr(unsafe.Pointer(yuvFrame.Data[0]))            for j := int32(0); j < w*h; j++ {              bytes = append(bytes, *(*byte)(unsafe.Pointer(ptr)))              ptr++            }            //u            ptr = uintptr(unsafe.Pointer(yuvFrame.Data[1]))            for j := int32(0); j < w*h/4; j++ {              bytes = append(bytes, *(*byte)(unsafe.Pointer(ptr)))              ptr++            }            //v            ptr = uintptr(unsafe.Pointer(yuvFrame.Data[2]))            for j := int32(0); j < w*h/4; j++ {              bytes = append(bytes, *(*byte)(unsafe.Pointer(ptr)))              ptr++            }            //写文件            file.Write(bytes)          }        }      }      pkt.AvPacketUnref() //重置pkt的内容    }    fmt.Printf("There are %d frames int total.\n", i)    file.Close()    break  }  // //===========================释放所有指针===============================//  libavcodec.AvPacketFree(&pkt)  codecCtx.AvcodecClose()  libavformat.AvformatCloseInput(&fmtCtx)  fmtCtx.AvformatFreeContext()  libavutil.AvFrameFree(&yuvFrame)
  _, err = exec.Command("./lib/ffplay.exe", "-pixel_format", "yuv420p", "-video_size", "640x360", "./out/result.yuv").Output()  if err != nil {    fmt.Println("play err = ", err)  }}