qt - 如何使用 QMediaPlayer 保存帧？

Question

我想从 QMediaPlayer 中保存帧的图像.阅读文档后，我明白我应该使用 QVideoProbe .我正在使用以下代码:

QMediaPlayer *player = new QMediaPlayer();
QVideoProbe *probe   = new QVideoProbe;

connect(probe, SIGNAL(videoFrameProbed(QVideoFrame)), this, SLOT(processFrame(QVideoFrame)));

qDebug()<<probe->setSource(player); // Returns true, hopefully.

player->setVideoOutput(myVideoSurface);
player->setMedia(QUrl::fromLocalFile("observation.mp4"));
player->play(); // Start receving frames as they get presented to myVideoSurface

但不幸的是， probe->setSource(player)总是返回 false对我来说，因此我的插槽 processFrame未触发。

我究竟做错了什么 ？有人有 QVideoProbe 的工作示例吗？ ?

Answer 1

你没有做错任何事。正如@DYangu 指出的那样，您的媒体对象实例不支持监控视频。我遇到了同样的问题( QAudioProbe 也是如此，但我们在这里不感兴趣)。我通过查看 this 找到了解决方案回答和this one .

主要思路是到子类 QAbstractVideoSurface .完成后，它将调用方法 QAbstractVideoSurface::present(const QVideoFrame & frame)您对 QAbstractVideoSurface 的实现并且您将能够处理视频的帧。

据说here ，通常你只需要重新实现两个方法:

supportedPixelFormats以便生产者可以为 QVideoFrame 选择合适的格式

present允许显示框架

但当时我在Qt源码中搜索，开心地找到了 this piece of code这帮助我进行了全面实现。所以， 这是完整的代码 用于使用“视频帧采集器”。

VideoFrameGrabber.cpp:

#include "VideoFrameGrabber.h"

#include <QtWidgets>
#include <qabstractvideosurface.h>
#include <qvideosurfaceformat.h>

VideoFrameGrabber::VideoFrameGrabber(QWidget *widget, QObject *parent)
    : QAbstractVideoSurface(parent)
    , widget(widget)
    , imageFormat(QImage::Format_Invalid)
{
}

QList<QVideoFrame::PixelFormat> VideoFrameGrabber::supportedPixelFormats(QAbstractVideoBuffer::HandleType handleType) const
{
    Q_UNUSED(handleType);
    return QList<QVideoFrame::PixelFormat>()
        << QVideoFrame::Format_ARGB32
        << QVideoFrame::Format_ARGB32_Premultiplied
        << QVideoFrame::Format_RGB32
        << QVideoFrame::Format_RGB24
        << QVideoFrame::Format_RGB565
        << QVideoFrame::Format_RGB555
        << QVideoFrame::Format_ARGB8565_Premultiplied
        << QVideoFrame::Format_BGRA32
        << QVideoFrame::Format_BGRA32_Premultiplied
        << QVideoFrame::Format_BGR32
        << QVideoFrame::Format_BGR24
        << QVideoFrame::Format_BGR565
        << QVideoFrame::Format_BGR555
        << QVideoFrame::Format_BGRA5658_Premultiplied
        << QVideoFrame::Format_AYUV444
        << QVideoFrame::Format_AYUV444_Premultiplied
        << QVideoFrame::Format_YUV444
        << QVideoFrame::Format_YUV420P
        << QVideoFrame::Format_YV12
        << QVideoFrame::Format_UYVY
        << QVideoFrame::Format_YUYV
        << QVideoFrame::Format_NV12
        << QVideoFrame::Format_NV21
        << QVideoFrame::Format_IMC1
        << QVideoFrame::Format_IMC2
        << QVideoFrame::Format_IMC3
        << QVideoFrame::Format_IMC4
        << QVideoFrame::Format_Y8
        << QVideoFrame::Format_Y16
        << QVideoFrame::Format_Jpeg
        << QVideoFrame::Format_CameraRaw
        << QVideoFrame::Format_AdobeDng;
}

bool VideoFrameGrabber::isFormatSupported(const QVideoSurfaceFormat &format) const
{
    const QImage::Format imageFormat = QVideoFrame::imageFormatFromPixelFormat(format.pixelFormat());
    const QSize size = format.frameSize();

    return imageFormat != QImage::Format_Invalid
            && !size.isEmpty()
            && format.handleType() == QAbstractVideoBuffer::NoHandle;
}

bool VideoFrameGrabber::start(const QVideoSurfaceFormat &format)
{
    const QImage::Format imageFormat = QVideoFrame::imageFormatFromPixelFormat(format.pixelFormat());
    const QSize size = format.frameSize();

    if (imageFormat != QImage::Format_Invalid && !size.isEmpty()) {
        this->imageFormat = imageFormat;
        imageSize = size;
        sourceRect = format.viewport();

        QAbstractVideoSurface::start(format);

        widget->updateGeometry();
        updateVideoRect();

        return true;
    } else {
        return false;
    }
}

void VideoFrameGrabber::stop()
{
    currentFrame = QVideoFrame();
    targetRect = QRect();

    QAbstractVideoSurface::stop();

    widget->update();
}

bool VideoFrameGrabber::present(const QVideoFrame &frame)
{
    if (frame.isValid()) 
    {
        QVideoFrame cloneFrame(frame);
        cloneFrame.map(QAbstractVideoBuffer::ReadOnly);
        const QImage image(cloneFrame.bits(),
                           cloneFrame.width(),
                           cloneFrame.height(),
                           QVideoFrame::imageFormatFromPixelFormat(cloneFrame .pixelFormat()));
        emit frameAvailable(image); // this is very important
        cloneFrame.unmap();
    }

    if (surfaceFormat().pixelFormat() != frame.pixelFormat()
            || surfaceFormat().frameSize() != frame.size()) {
        setError(IncorrectFormatError);
        stop();

        return false;
    } else {
        currentFrame = frame;

        widget->repaint(targetRect);

        return true;
    }
}

void VideoFrameGrabber::updateVideoRect()
{
    QSize size = surfaceFormat().sizeHint();
    size.scale(widget->size().boundedTo(size), Qt::KeepAspectRatio);

    targetRect = QRect(QPoint(0, 0), size);
    targetRect.moveCenter(widget->rect().center());
}

void VideoFrameGrabber::paint(QPainter *painter)
{
    if (currentFrame.map(QAbstractVideoBuffer::ReadOnly)) {
        const QTransform oldTransform = painter->transform();

        if (surfaceFormat().scanLineDirection() == QVideoSurfaceFormat::BottomToTop) {
           painter->scale(1, -1);
           painter->translate(0, -widget->height());
        }

        QImage image(
                currentFrame.bits(),
                currentFrame.width(),
                currentFrame.height(),
                currentFrame.bytesPerLine(),
                imageFormat);

        painter->drawImage(targetRect, image, sourceRect);

        painter->setTransform(oldTransform);

        currentFrame.unmap();
    }
}

VideoFrameGrabber.h

#ifndef VIDEOFRAMEGRABBER_H
#define VIDEOFRAMEGRABBER_H

#include <QtWidgets>

class VideoFrameGrabber : public QAbstractVideoSurface
{
    Q_OBJECT

public:
    VideoFrameGrabber(QWidget *widget, QObject *parent = 0);

    QList<QVideoFrame::PixelFormat> supportedPixelFormats(
            QAbstractVideoBuffer::HandleType handleType = QAbstractVideoBuffer::NoHandle) const;
    bool isFormatSupported(const QVideoSurfaceFormat &format) const;

    bool start(const QVideoSurfaceFormat &format);
    void stop();

    bool present(const QVideoFrame &frame);

    QRect videoRect() const { return targetRect; }
    void updateVideoRect();

    void paint(QPainter *painter);

private:
    QWidget *widget;
    QImage::Format imageFormat;
    QRect targetRect;
    QSize imageSize;
    QRect sourceRect;
    QVideoFrame currentFrame;

signals:
    void frameAvailable(QImage frame);
};
#endif //VIDEOFRAMEGRABBER_H

备注 : 在 .h 中，你会看到我添加了一个 signal将图像作为参数。 这将允许您处理您的框架 在您的代码中的任何地方。当时，这个信号花了 QImage作为参数，但您当然可以采用 QVideoFrame如果你想。

现在，我们准备使用这个视频帧采集器:

QMediaPlayer* player = new QMediaPlayer(this);
// no more QVideoProbe 
VideoFrameGrabber* grabber = new VideoFrameGrabber(this);
player->setVideoOutput(grabber);

connect(grabber, SIGNAL(frameAvailable(QImage)), this, SLOT(processFrame(QImage)));

现在你只需要声明一个名为 processFrame(QImage image) 的插槽您将收到 QImage每次进入方法 present您的 VideoFrameGrabber .

我希望这会帮助你!