Qt + GStreamer：如何在播放实时视频流时拍摄快照

时间：2016-03-14 09:27:01

标签： qt video-streaming gstreamer qtgstreamer

我开发了基于Qt和QtGstreamer的视频播放器。它用于播放直播（RTSP）。我必须添加用户在播放实时流时拍摄快照而不会影响视频播放的可能性。

这里是我所做的管道图：

                                -->queue-->autovideosink
uridecodebin-->videoflip-->tee--|
            |                   -->queue->videoconvert-->pngenc-->filesink
            |
            |->audioconvert-->autoaudiosink

我使用来自pad-added的{​​{1}}信号来动态添加和链接我的元素到管道，接收上限的功能。

uridecodebin

拍摄快照的方法：

void Player::onPadAdded(const QGst::PadPtr &pad)
{
    QGst::CapsPtr caps = pad->currentCaps();
    if (caps->toString().startsWith("video/x-raw")) {
        qDebug("Received 'video/x-raw' caps");
        handleNewVideoPad(pad);
    }
    else if (caps->toString().startsWith("audio/x-raw")) {
        qDebug("Received 'audio/x-raw' caps");
        if (!m_audioEnabled) {
            qDebug("Audio is disabled in the player. Ignoring...");
            return;
        }
        handleNewAudioPad(pad);
    }
    else {
        qWarning("Unsuported caps, arborting ...!");
        return;
    }
}

[...]

void Player::handleNewVideoPad(QGst::PadPtr pad)
{
    m_player->videoTeeVideoSrcPad = m_player->videoTee->getRequestPad("src_%u");

    // Add video elements
    m_player->pipeline->add(m_player->videoFlip);
    m_player->pipeline->add(m_player->videoTee);
    m_player->pipeline->add(m_player->videoQueue);
    m_player->pipeline->add(m_player->videoSink);

    // Add snap elements
    m_player->pipeline->add(m_player->snapQueue);
    m_player->pipeline->add(m_player->snapConverter);
    m_player->pipeline->add(m_player->snapEncoder);
    m_player->pipeline->add(m_player->snapSink);

    // Link video elements
    m_player->videoFlip->link(m_player->videoTee);
    m_player->videoQueue->link(m_player->videoSink);

    // Link snap elements
    m_player->snapQueue->link(m_player->snapConverter);
    m_player->snapConverter->link(m_player->snapEncoder);
    m_player->snapEncoder->link(m_player->snapSink);

    // Lock snap elements
    m_player->snapQueue->setStateLocked(true);
    m_player->snapConverter->setStateLocked(true);
    m_player->snapEncoder->setStateLocked(true);
    m_player->snapSink->setStateLocked(true);

    m_player->videoFlip->setState(QGst::StatePlaying);
    m_player->videoTee->setState(QGst::StatePlaying);
    m_player->videoQueue->setState(QGst::StatePlaying);
    m_player->videoSink->setState(QGst::StatePlaying);

    // Link pads
    m_player->videoTeeVideoSrcPad->link(m_player->videoQueue->getStaticPad("sink"));
    pad->link(m_player->videoSinkPad);

    m_player->videoLinked = true;
}

总线消息回调：

void Player::takeSnapshot()
{
    QDateTime dateTime = QDateTime::currentDateTime();
    QString snapLocation = QString("/%1/snap_%2.png").arg(m_snapDir).arg(dateTime.toString(Qt::ISODate));

    m_player->inSnapshotCaputre = true;

    if (m_player->videoTeeSnapSrcPad) {
        m_player->videoTee->releaseRequestPad(m_player->videoTeeSnapSrcPad);
        m_player->videoTeeSnapSrcPad.clear();
    }
    m_player->videoTeeSnapSrcPad = m_player->videoTee->getRequestPad("src_%u");

    // Stop the snapshot branch
    m_player->snapQueue->setState(QGst::StateNull);
    m_player->snapConverter->setState(QGst::StateNull);
    m_player->snapEncoder->setState(QGst::StateNull);
    m_player->snapSink->setState(QGst::StateNull);

    // Link Tee src pad to snap queue sink pad
    m_player->videoTeeSnapSrcPad->link(m_player->snapQueue->getStaticPad("sink"));

    // Set the snapshot location property
    m_player->snapSink->setProperty("location", snapLocation);

    // Unlock snapshot branch
    m_player->snapQueue->setStateLocked(false);
    m_player->snapConverter->setStateLocked(false);
    m_player->snapEncoder->setStateLocked(false);
    m_player->snapSink->setStateLocked(false);
    m_player->videoTeeSnapSrcPad->setActive(true);

    // Synch snapshot branch state with parent
    m_player->snapQueue->syncStateWithParent();
    m_player->snapConverter->syncStateWithParent();
    m_player->snapEncoder->syncStateWithParent();
    m_player->snapSink->syncStateWithParent();
}

问题是：

  • 当我将void Player::onBusMessage(const QGst::MessagePtr & message) { QGst::ElementPtr source = message->source().staticCast<QGst::Element>(); switch (message->type()) { case QGst::MessageEos: { //End of stream. We reached the end of the file. qDebug("Message End Off Stream"); if (m_player->inSnapshotCaputre) { blockSignals(true); pause(); play(); blockSignals(false); m_player->inSnapshotCaputre = false; } else { m_eos = true; stop(); } break; } [...] } 属性设置为snapshot元素的true时，我收到了阻止我的管道的pngenc事件，因此我需要重新启动它，冻结视频播放约半秒钟，这在我的情况下是不可接受的。
  • 当我将EOS属性设置为snapshot元素的false时，我没有管道扰动，但是我的png文件一直在增长，直到我再次调用pngenc方法

我哪里错了？有没有更好的方法呢？ 我尝试为Player::takeSnapshot()分支创建QGst::Bin元素失败。垫探针怎么样？

先谢谢

2 个答案:

答案 0 :(得分：3)

您可以在任何接收器上获取最后样本属性，例如你的视频接收器。它包含一个GstSample，它有一个带有最新视频帧的缓冲区。您可以将其作为快照，例如使用gst_video_convert_sample（）或它的异步变体，将其转换为PNG / JPG /无论如何。

请参阅https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer-libs/html/GstBaseSink.html#GstBaseSink--last-sample和https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gst-plugins-base-libs-gstvideo.html#gst-video-convert-sample

或者，您必须在第一帧之后关闭filesink快照管道。例如，通过使用pad探测器来了解第一帧发生的时间，然后注入EOS事件以防止将更多PNG帧附加到同一文件中。

答案 1 :(得分：2)

感谢@ sebastian-droge的回答，我找到了解决方案，使用gst_video_convert_sample和我的视频接收器的last-sample属性。

我实施的解决方案是：

void Player::takeSnapshot()
{
    QDateTime currentDate = QDateTime::currentDateTime();
    QString location = QString("%1/snap_%2.png").arg(QDir::homePath()).arg(currentDate.toString(Qt::ISODate));
    QImage snapShot;
    QImage::Format snapFormat;
    QGlib::Value val = m_videoSink->property("last-sample");
    GstSample *videoSample = (GstSample *)g_value_get_boxed(val);
    QGst::SamplePtr sample = QGst::SamplePtr::wrap(videoSample);
    QGst::SamplePtr convertedSample;
    QGst::BufferPtr buffer;
    QGst::CapsPtr caps = sample->caps();
    QGst::MapInfo mapInfo;
    GError *err = NULL;
    GstCaps * capsTo = NULL;
    const QGst::StructurePtr structure = caps->internalStructure(0);
    int width, height;

    width = structure.data()->value("width").get<int>();
    height = structure.data()->value("height").get<int>();

    qDebug() << "Sample caps:" << structure.data()->toString();

    /*
     * { QImage::Format_RGBX8888, GST_VIDEO_FORMAT_RGBx  },
     * { QImage::Format_RGBA8888, GST_VIDEO_FORMAT_RGBA  },
     * { QImage::Format_RGB888  , GST_VIDEO_FORMAT_RGB   },
     * { QImage::Format_RGB16   , GST_VIDEO_FORMAT_RGB16 }
     */
    snapFormat = QImage::Format_RGB888;
    capsTo = gst_caps_new_simple("video/x-raw",
                                 "format", G_TYPE_STRING, "RGB",
                                 "width", G_TYPE_INT, width,
                                 "height", G_TYPE_INT, height,
                                 NULL);

    convertedSample = QGst::SamplePtr::wrap(gst_video_convert_sample(videoSample, capsTo, GST_SECOND, &err));
    if (convertedSample.isNull()) {
        qWarning() << "gst_video_convert_sample Failed:" << err->message;
    }
    else {
        qDebug() << "Converted sample caps:" << convertedSample->caps()->toString();

        buffer = convertedSample->buffer();
        buffer->map(mapInfo, QGst::MapRead);

        snapShot = QImage((const uchar *)mapInfo.data(),
                          width,
                          height,
                          snapFormat);

        qDebug() << "Saving snap to" << location;
        snapShot.save(location);

        buffer->unmap(mapInfo);
    }

    val.clear();
    sample.clear();
    convertedSample.clear();
    buffer.clear();
    caps.clear();
    g_clear_error(&err);
    if (capsTo)
        gst_caps_unref(capsTo);
}

我创建了一个简单的测试应用程序，它实现了这个解决方案。该代码可在我的Github

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