Question

我正在尝试使用ffmpeg从视频文件中捕获帧，但我甚至无法获得视频的持续时间。每次当我尝试用pFormatCtx->duration访问它时我都会得到0.我知道指针已经初始化并包含正确的持续时间，因为如果我使用av_dump_format(pFormatCtx, 0, videoName, 0);那么我实际上得到的持续时间数据以及其他信息该视频。这是我使用av_dump_format(pFormatCtx, 0, videoName, 0);时得到的：

输入＃0，avi，来自'futurama.avi'：

持续时间：00：21：36.28，开始：0.000000，比特率：1135 kb / s

Stream＃0.0：视频：mpeg4（高级简单配置文件），yuv420p，512x384

[PAR 1：1 DAR 4：3]，25 tbr，25 tbn，25 tbc

流＃0.1：音频：ac3,48000 Hz，立体声，s16,192 kb / s

我不明白为什么av_dum_format可以显示持续时间而我不能。我查看了函数定义，为了显示持续时间，该函数还使用了pFormatCtx-＆gt; duration。当我在main.cpp中调用它们时，其他成员变量也不会显示正确的数据

这是我的main.cpp：

extern "C" {
    #include<libavcodec/avcodec.h>
    #include<libavformat/avformat.h>
    #include<libswscale/swscale.h>
}


int main(int argc, char *argv[]) {
    AVFormatContext *pFormatCtx = NULL;

    const char videoName[] = "futurama.avi";

    // Register all formats and codecs.
    av_register_all();
    cout << "Opening the video file";
    // Open video file
    int ret = avformat_open_input(&pFormatCtx, videoName, NULL, NULL) != 0;
    if (ret != 0) {
        cout << "Couldn't open the video file." << ret ;
        return -1;
    }
    if(avformat_find_stream_info(pFormatCtx, 0) < 0) {
        cout << "problem with stream info";
        return -1;
    }

    av_dump_format(pFormatCtx, 0, videoName, 0);
    cout << pFormatCtx->bit_rate << endl; // different value each time, not initialized properly.
    cout << pFormatCtx->duration << endl; // 0
    return 0;
}

我不知道它是否有帮助但是，我在Ubuntu上使用QtCreator并静态链接这些库。

感谢您的帮助。

Answer 1

持续时间属性为time_base单位，而不是毫秒或秒。转换为毫秒非常简单，

double time_base =  (double)video_stream->time_base.num / (double)video_stream->time_base.den;
double duration = (double)video_stream->duration * time_base * 1000.0;

持续时间现在以毫秒为单位，只需占用一层或每小时即可获得整数毫秒，无论你喜欢什么。

Answer 2

如何从ffmpeg获取持续时间信息（等等）

前一段时间我和ffmpeg搞砸了，发现学习曲线相当陡峭。因此，即使OP在几个月前提出这个问题，我也会发布一些代码，以防其他人在SO上寻找类似的东西。下面的Open（）函数已经完成，但有许多断言，缺乏正确的错误处理方式。

马上，我看到的一个直接差异是我使用了 av_open_input_file 而不是 avformat_open_input 。我也没有使用 av_dump_format 。

计算持续时间可能很棘手，尤其是对于H.264和MPEG-2;看看 durationSec 的计算方法如下。

注意：此示例还使用JUCE C++ Utility Library。

注意2：此代码是ffmpeg tutorial的修改版本。

void VideoCanvas::Open(const char* videoFileName)
{       
    Logger::writeToLog(String(L"Opening video file ") + videoFileName);
    Close();

    AVCodec *pCodec;

    // register all formats and codecs
    av_register_all();  

    // open video file
    int ret = av_open_input_file(&pFormatCtx, videoFileName, NULL, 0, NULL);
    if (ret != 0) {
        Logger::writeToLog("Unable to open video file: " + String(videoFileName));
        Close();
        return;
    }

    // Retrieve stream information
    ret = av_find_stream_info(pFormatCtx);
    jassert(ret >= 0);

    // Find the first video stream
    videoStream = -1;
    audioStream = -1;
    for(int i=0; i<pFormatCtx->nb_streams; i++) {
        if (pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO && videoStream < 0) {
            videoStream = i;            
        }
        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO && audioStream < 0) {
            audioStream = i;
        }
    } // end for i
    jassert(videoStream != -1);
    jassert(audioStream != -1);

    // Get a pointer to the codec context for the video stream
    pCodecCtx=pFormatCtx->streams[videoStream]->codec;
    jassert(pCodecCtx != nullptr);

    /**
      * This is the fundamental unit of time (in seconds) in terms
      * of which frame timestamps are represented. For fixed-fps content,
      * timebase should be 1/framerate and timestamp increments should be
      * identically 1.
      * - encoding: MUST be set by user.
      * - decoding: Set by libavcodec.
      */
    AVRational avr = pCodecCtx->time_base;
    Logger::writeToLog("time_base = " + String(avr.num) + "/" + String(avr.den));

    /**
     * For some codecs, the time base is closer to the field rate than the frame rate.
     * Most notably, H.264 and MPEG-2 specify time_base as half of frame duration
     * if no telecine is used ...
     *
     * Set to time_base ticks per frame. Default 1, e.g., H.264/MPEG-2 set it to 2.
     */
    ticksPerFrame = pCodecCtx->ticks_per_frame;
    Logger::writeToLog("ticks_per_frame = " + String(pCodecCtx->ticks_per_frame));

    durationSec = static_cast<double>(pFormatCtx->streams[videoStream]->duration) * static_cast<double>(ticksPerFrame) / static_cast<double>(avr.den);
    double fH = durationSec / 3600.;
    int     H = static_cast<int>(fH);
    double fM = (fH - H) * 60.;
    int     M = static_cast<int>(fM);
    double fS = (fM - M) * 60.;
    int     S = static_cast<int>(fS);

    Logger::writeToLog("Video stream duration = " + String(H) + "H " + String(M) + "M " + String(fS, 3) + "S");

    // calculate frame rate based on time_base and ticks_per_frame
    frameRate = static_cast<double>(avr.den) / static_cast<double>(avr.num * pCodecCtx->ticks_per_frame);
    Logger::writeToLog("Frame rate = " + String(frameRate) );

    // audio codec context
    if (audioStream != -1) {
        aCodecCtx = pFormatCtx->streams[audioStream]->codec;

        Logger::writeToLog("Audio sample rate = " + String(aCodecCtx->sample_rate));
        Logger::writeToLog("Audio channels    = " + String(aCodecCtx->channels));       
    }
    jassert(aCodecCtx != nullptr);

    // format:
    // The "S" in "S16SYS" stands for "signed", the 16 says that each sample is 16 bits long, 
    // and "SYS" means that the endian-order will depend on the system you are on. This is the
    // format that avcodec_decode_audio2 will give us the audio in.

    // open the audio codec
    if (audioStream != -1) {
        aCodec = avcodec_find_decoder(aCodecCtx->codec_id);
        if (!aCodec) {
            Logger::writeToLog(L"Unsupported codec ID = " + String(aCodecCtx->codec_id) );
            Close();
            return;  // TODO: should we just play video if audio codec doesn't work?
        }
        avcodec_open(aCodecCtx, aCodec);
    }


    // Find the decoder for the video stream
    pCodec=avcodec_find_decoder(pCodecCtx->codec_id);
    if(pCodec == nullptr) {
        jassert(false);
        // fprintf(stderr, "Unsupported codec!\n");
        //return -1; // Codec not found
    }

    // Open video codec
    ret = avcodec_open(pCodecCtx, pCodec);
    jassert(ret >= 0);

    // Allocate video frame
    pFrame=avcodec_alloc_frame();
    jassert(pFrame != nullptr);

    // Allocate an AVFrame structure
    pFrameRGB=avcodec_alloc_frame();
    jassert(pFrameRGB != nullptr);

    int numBytes = avpicture_get_size(PIX_FMT_RGB32, pCodecCtx->width, pCodecCtx->height);
    jassert(numBytes != 0);
    buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));
    jassert(buffer != nullptr);

    // note: the pixel format here is RGB, but sws_getContext() needs to be PIX_FMT_BGR24 to match (BGR)
    // this might have to do w/ endian-ness....make sure this is platform independent
    if (m_image != nullptr) delete m_image;
    m_image = new Image(Image::ARGB, pCodecCtx->width, pCodecCtx->height, true);

    int dstW = pCodecCtx->width; // don't rescale
    int dstH = pCodecCtx->height;
    Logger::writeToLog(L"Video width = " + String(dstW));
    Logger::writeToLog(L"Video height = " + String(dstH));

    // this should only have to be done once
    img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, dstW, dstH, PIX_FMT_RGB32, SWS_FAST_BILINEAR, NULL, NULL, NULL);
    jassert(img_convert_ctx != nullptr);  

    setSize(pCodecCtx->width, pCodecCtx->height);

} // Open()

Answer 3

av_open_input_file()和avformat_open_input()之间的差异可能是后者不读取流信息 - 因此duration未初始化。致电avformat_find_stream_info()为我解决了这个问题。

我从http://ffmpeg.org/doxygen/trunk/dump_8c_source.html#l00480获取了计算/显示的代码段（请注意，行号可能并且可能会在较新版本中更改）。并添加了一些初始化代码，“它适合我”。希望它有所帮助。

#include <libavutil/avutil.h>
#include <libavformat/avformat.h>

int main()
{
    const char const* file = "sample.mpg";
    AVFormatContext* formatContext = NULL;

    av_register_all();

    // Open video file
    avformat_open_input(&formatContext, file, NULL, NULL);
    avformat_find_stream_info(formatContext, NULL);

    // Lower log level since av_log() prints at AV_LOG_ERROR by default
    av_log_set_level(AV_LOG_INFO);

    av_log(NULL, AV_LOG_INFO, "  Duration: ");
    if (formatContext->duration != AV_NOPTS_VALUE) {
        int hours, mins, secs, us;
        int64_t duration = formatContext->duration + 5000;
        secs  = duration / AV_TIME_BASE;
        us    = duration % AV_TIME_BASE;
        mins  = secs / 60;   
        secs %= 60;          
        hours = mins / 60;   
        mins %= 60;
        av_log(NULL, AV_LOG_INFO, "%02d:%02d:%02d.%02d\n", hours, mins, secs, (100 * us) / AV_TIME_BASE);
    } 

    return 0;
}

要编译，

gcc -o duration -lavutil -lavformat duration.c

FFMPEG无法显示视频的持续时间

3 个答案: