How to use QAbstractVideoFilter to process frames from camera

[kolegs]

@theshadowx I've never used openCV so I'm not sure how to use this libraries, but about run() function as far as I know you need to modify the frame that you get(QVideoFrame *input) and simply return it in the same format.

[medyakovvit]

@theshadowx, i don't know how to work with YUV format, but with QVideoFrame::Format_RGB32 you can do something like that:

QVideoFrame ThresholdFilterRunnable::run(QVideoFrame *input, const QVideoSurfaceFormat &surfaceFormat, QVideoFilterRunnable::RunFlags flags)
{
    if (!input->isValid())
        return *input;

    input->map(QAbstractVideoBuffer::ReadWrite);

    cv::Mat mat = cv::Mat(input->height(),input->width(), CV_8UC4, input->bits());

    cv::Mat gray, threshold, color;
    cv::cvtColor(mat, gray, COLOR_RGB2GRAY);
    cv::threshold(gray, threshold, 150.0, 255.0, THRESH_BINARY);
    cv::cvtColor(threshold, color, COLOR_GRAY2RGB);
    this->toVideoFrame(input, color);

    input->unmap();
    return *input;
}

void ThresholdFilterRunnable::toVideoFrame(QVideoFrame *input, cv::Mat &mat)
{
    int aCols = 4*mat.cols;
    int aRows = mat.rows;
    uchar* inputBits = input->bits();

    for (int i=0; i<aRows; i++)
    {
        uchar* p = mat.ptr<uchar>(i);
        for (int j=0; j<aCols; j++)
        {
            int index = i*aCols + j;
            inputBits[index] = p[j];
        }
    }
}

I can't test it right now, because i'm in hospital.

[theshadowx]

@medyakovvit Hi, thanks for responding, I hope everything is fine for you.

if my camera takes video in RGB32 it would be easier as I can create QImage from the edge matrix and then create a QVideoFrame from QImage.:

 ///cv::Mat to QImage
 QImage ImgDest(edges.data, edges.cols, edges.rows, edges.step, QImage::Format_RGB888);
 QVideoFrame *output = new QVideoFrame(ImgDest);

And then I would return the output.

The problem (From what I understood) is that my camera generates images with YUV color scheme. which means that QVideoSurfaceFormat is in YUV, so if I use RGB32 format it won't work.

Unfortunately QImage doesn't support YUV format. :(

[medyakovvit]

@theshadowx So, if correctly understood this YUV420 and YUV format, Y - luma component, U and V - color component. And in single frame Ys go first, then Us and Vs. So if you need grayscale image, you can work with Ys and ignore Us and Vs. Maybe you can try:

QVideoFrame YUVFilterRunnable::run(QVideoFrame *input, const QVideoSurfaceFormat &surfaceFormat, QVideoFilterRunnable::RunFlags flags)
{
    if (!input->isValid())
        return *input;

    input->map(QAbstractVideoBuffer::ReadWrite);

    this->deleteColorComponentFromYUV(input);
    cv::Mat mat(input->height(),input->width(), CV_8UC1, input->bits()); // create grayscale mat with input's Ys only and avoid additional color conversion and copying

    cv::GaussianBlur(mat, mat, Size(7,7), 1.5, 1.5);
    cv::Canny(mat, mat, 0, 30, 3);

    input->unmap();
    return *input;
}

void YUVFilterRunnable::deleteColorComponentFromYUV(QVideoFrame *input)
{
    // Assign 0 to Us and Vs
    int firstU = input->width()*input->height(); // if i correctly understand YUV420
    int lastV = input->width()*input->height() + input->width()*input->height()/4*2;
    uchar* inputBits = input->bits();

    for (int i=firstU; i<lastV; i++)
        inputBits[i] = 0;
}

[theshadowx]

@medyakovvit

Yes that's right, there is one porblem in the code is that with you code I get a greenscale. So to convert to grayscale the solution is that instead of 0 for chroma, it should be 127 (http://stackoverflow.com/a/20609599/2775917) :

void YUVFilterRunnable::deleteColorComponentFromYUV(QVideoFrame *input)
{
    // Assign 0 to Us and Vs
    int firstU = input->width()*input->height(); // if i correctly understand YUV420
    int lastV = input->width()*input->height() + input->width()*input->height()/4*2;
    uchar* inputBits = input->bits();

    for (int i=firstU; i<lastV; i++)
        inputBits[i] = 127;
}

Thanks a lot for your help

[medyakovvit]

@theshadowx I'm glad to help

[cs_a994]

Hello!

Is it possible to use the QVideoFilterRunnuble without QML?

As an example I would like to embed it into Camera example ( http://doc.qt.io/qt-5/qtmultimediawidgets-camera-example.html ) to be able to change data format.

Thank you.

[medyakovvit]

@cs_a994 Don't think so. But you can try to subclass QAbstractVideoSurface.

[cs_a994]

@medyakovvit

Thank you. I know about this way. But use of filters seams to me more neat.

I tryed to use filters with QML ( in the Camera example ) and did not yet get a result.

[medyakovvit]

@cs_a994
OK. I f you have a questions about qml camera and filters, I will try to help you.

[theshadowx]

@cs_a994

I have put a QtQuick app in github using QAbstractVideoFilter with OpenCV.

I hope it helps you.

Cheers

[cs_a994]

@theshadowx

Thank you very much. I'll study it soon. I hope it will help.

[mikeitexpert]

@theshadowx

I get the below error if I try the canny code from git hub. (https://github.com/theshadowx/Qt_OpenCV)

I don't know much about mql but I know QT intermediately.

Thank you

01:45:55: Starting /home/mike/Downloads/Qt_OpenCV-master/QtQuick/build-CannyQml-Desktop_Qt_5_12_0_GCC_64bit-Debug/CannyQml...
QML debugging is enabled. Only use this in a safe environment.

(CannyQml:24471): GStreamer-CRITICAL **: 01:45:57.866: write map requested on non-writable buffer
01:45:57: The program has unexpectedly finished.
01:45:57: The process was ended forcefully.
01:45:58: /home/mike/Downloads/Qt_OpenCV-master/QtQuick/build-CannyQml-Desktop_Qt_5_12_0_GCC_64bit-Debug/CannyQml crashed.

[theshadowx]

@mikeitexpert
Could you add this issue in https://github.com/theshadowx/Qt_OpenCV/issues

Thanks

[mikeitexpert]

here: https://github.com/theshadowx/Qt_OpenCV/issues/5

Is there any way I can convert qml to c++ code just to get an idea how your methods works in pure C++?

Thank you

[mikeitexpert]

Please let me know if there is any update on this.

Mike