ffmpeg golang transcoding

As video becomes more widely used in modern media, many applications require video transcoding between different platforms and devices. In the process, FFmpeg and Golang have become the transcoding tools of choice for many developers. This article will introduce the basic concepts and usage of FFmpeg and Golang, and how to combine them for efficient video transcoding.

FFmpeg Introduction

FFmpeg is an open source cross-platform video and audio codec library that can be used to process various video formats. It provides a command line tool that allows developers to directly use its features such as format conversion, video cutting, real-time transcoding, etc.

Golang Introduction

Golang is a modern programming language, first developed by Google and open source. It is widely regarded as an efficient, simple and secure programming language, especially suitable for use in network and cloud computing applications.

Combining FFmpeg and Golang

Golang can use CGO technology to call C language libraries, which makes it easy to use the functions in FFmpeg. By using FFmpeg's command line tool, we can easily transcode videos to different formats such as mp4, webm, etc.

However, by directly calling the FFmpeg command line tool, you need to fork a child process, and then wait for the child process to exit to obtain the results. This method is inefficient and is not conducive to program expansion and maintenance.

Therefore, Golang provides a tool called cgo to easily allow us to use C code in Golang programs, and then conveniently use the functions of FFmpeg. In the following example, we will show how to encapsulate the functions of FFmpeg through cgo technology.

First, we need to define a structure in Golang to represent the AVFrame type in FFmpeg.

type AVFrame struct {
    data [8]*uint8
    linesize [8]int32
    best_effort_timestamp int64
    pkt_pts int64
}

Next, we need to define some C function interfaces to call FFmpeg functions. For example, we can define a function to open an audio or video file:

// #cgo LDFLAGS: -lavformat -lavcodec -lavutil
// #include <libavformat/avformat.h>
// #include <libavcodec/avcodec.h>
// #include <libavutil/avutil.h>
import "C"

func av_open_input_file(pFormatContext **C.AVFormatContext, filename string, fmt *C.AVInputFormat, buf_size int, pFormatParams **C.AVFormatParameters) int {
    cfilename := C.CString(filename)
    defer C.free(unsafe.Pointer(cfilename))

    result := C.av_open_input_file(pFormatContext, cfilename, fmt, C.int(buf_size), pFormatParams)
    return int(result)
}

In the above code, we use the comment directive #cgo LDFLAGS to tell the Golang compiler that it needs to link the FFmpeg library file. At the same time, we also use the unsafe.Pointer type provided by CGO to pass pointer objects to C code.

Of course, in order to be able to use other functions provided by FFmpeg, other C function interfaces need to be defined. To simplify the example introduction, only a simple interface function is listed here.

Once we define these interface functions, we can easily use these interface functions in Golang code to take advantage of various functions of FFmpeg.

For example, we can use the following code to convert an audio file in WAV format to mp3 format:

func main() {
    var pFormatContext *C.AVFormatContext
    var inputFormat *C.AVInputFormat
    var formatParams *C.AVFormatParameters

    filename := "input.wav"
    if ret := av_open_input_file(&pFormatContext, filename, inputFormat, 0, &formatParams); ret != 0 {
        log.Fatalf("Could not open input file %s, error code=%d
", filename, ret)
    }

    if ret := C.avformat_find_stream_info(pFormatContext, nil); ret < 0 {
        log.Fatalf("Could not find stream info, error code=%d
", ret)
    }

    audioStreamIndex := -1
    for i := 0; i < int(pFormatContext.nb_streams); i++ {
        st := (*C.AVStream)(unsafe.Pointer(uintptr(unsafe.Pointer(pFormatContext.streams)) + uintptr(i)*unsafe.Sizeof(*pFormatContext.streams)))
        if st.codec.codec_type == C.AVMEDIA_TYPE_AUDIO {
            audioStreamIndex = i
            break
        }
    }

    if audioStreamIndex == -1 {
        log.Fatalf("Could not find audio stream
")
    }

    audioStream := (*C.AVStream)(unsafe.Pointer(uintptr(unsafe.Pointer(pFormatContext.streams)) + uintptr(audioStreamIndex)*unsafe.Sizeof(*pFormatContext.streams)))
    audioCodecContext := (*C.AVCodecContext)(unsafe.Pointer(audioStream.codec))
    audioCodec := C.avcodec_find_decoder(audioCodecContext.codec_id)
    if audioCodec == nil {
        log.Fatalf("Unsupported codec type, codec_id=%d
", audioCodecContext.codec_id)
    }

    if ret := C.avcodec_open2(audioCodecContext, audioCodec, nil); ret < 0 {
        log.Fatalf("Could not open audio codec, error code=%d
", ret)
    }

    tempFilePath := "temp.raw"
    tempFile, _ := os.Create(tempFilePath)
    defer tempFile.Close()
    defer os.Remove(tempFilePath)

    packet := (*C.AVPacket)(C.malloc(C.sizeof_AVPacket))
    defer C.free(unsafe.Pointer(packet))

    frame := (*C.AVFrame)(C.avcodec_alloc_frame())
    defer C.av_free(unsafe.Pointer(frame))

    for {
        if ret := C.av_read_frame(pFormatContext, packet); ret < 0 {
            break
        }

        if packet.stream_index == C.int(audioStreamIndex) {
            if ret := C.avcodec_decode_audio4(audioCodecContext, frame, (*C.int)(nil), packet); ret > 0 {
                numSamples := int(frame.nb_samples)
                dataPtr := uintptr(unsafe.Pointer(frame.data[0]))
                dataSlice := (*[1 << 30]byte)(unsafe.Pointer(dataPtr))
                dataSize := numSamples * int(audioCodecContext.channels) * int(C.av_get_bytes_per_sample(audioCodecContext.sample_fmt))

                tempFile.Write(dataSlice[:dataSize])
            }
        }

        C.av_free_packet(packet)
    }

    tempFile.Close()

    outputFilePath := "output.mp3"
    cmd := exec.Command("ffmpeg", "-y", "-f", "s16le", "-ar", strconv.Itoa(int(audioCodecContext.sample_rate)),
        "-ac", strconv.Itoa(int(audioCodecContext.channels)), "-i", tempFilePath, "-f", "mp3", outputFilePath)
    stdout, _ := cmd.StdoutPipe()
    cmd.Start()

    for {
        buf := make([]byte, 1024)
        n, err := stdout.Read(buf)
        if err != nil || n == 0 {
            break
        }
    }

    cmd.Wait()
}

In the above example, we first use the av_open_input_file function to open the audio file, and then use the avformat_find_stream_info function Get audio stream information.

Next, we traverse all streams to find the audio stream and use the avcodec_open2 function to open the audio decoder. After that, we use the av_read_frame function to read the audio data frame by frame and write the audio data to a temporary file.

Finally, we use the FFmpeg command line tool to convert the audio data in the temporary file into an audio file in mp3 format.

Conclusion

By combining Golang and FFmpeg, we can easily implement an efficient video transcoding program and use Golang's elegant syntax and built-in functions. Although using cgo technology may require some knowledge of the C language, it is not difficult to implement and the results are significant. If you need high performance and portability when developing a video transcoding program, combining Golang and FFmpeg may be a good choice.

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