qt - Integrating OpenCV with larger programs

Question

Can anyone recommend a how-to guide or provide a brief overview of what's involved with integrating OpenCV with larger GUI-based programs? What are the popular ways to do it?

Particularly, processing video with OpenCV while doing video capture/preview without using HighGUI seems especially arcane. I hope someone can demystify this.

My particular configuration is with either Juce or Qt depending on what can be done. The cross platform thing is not critical -- if there is an awesome way of doing this in Windows, I might be convinced. The availability of community support is important.

I have heard that HighGUI is entirely for testing and unsuitable for real applications. Someone recommended the VideoInput library, but it is experimental.

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Key points from answers:

• Use Qt (because Qt is great and has a big community).
• Open a new thread to run cv::VideoCapture in a loop and emit signal after frame capture. Use Qt's msleep mechanism, not OpenCV. So, we are still using OpenCV highgui for capture.

• Convert cv::Mat to QtImage:

  QImage qtFrame(cvFrame.data, cvFrame.size().width, cvFrame.size().height, cvFrame.step, QImage::Format_RGB888);

  qtFrame = qtFrame.rgbSwapped();

• Optional: Render with GLWidget. Convert QtImage to GLFormat with Qt built-in method:

  m_GLFrame = QGLWidget::convertToGLFormat(frame);

  this->updateGL();

Answer 1

Here is how I am doing it with Qt. You are welcome to use whatever may be useful to you :)

/// OpenCV_GLWidget.h
#ifndef OPENCV_GLWIDGET_H_
#define OPENCV_GLWIDGET_H_

#include <qgl.h>
#include <QImage>

class OpenCV_GLWidget: public QGLWidget {
public:
    OpenCV_GLWidget(QWidget * parent = 0, const QGLWidget * shareWidget = 0, Qt::WindowFlags f = 0);
    virtual ~OpenCV_GLWidget();

    void renderImage(const QImage& frame);
protected:
    virtual void paintGL();
    virtual void resizeGL(int width, int height);

private:
    QImage m_GLFrame;
};

#endif /* OPENCV_GLWIDGET_H_ */

/// OpenCV_GLWidget.cpp
#include "OpenCV_GLWidget.h"

OpenCV_GLWidget::OpenCV_GLWidget(QWidget* parent, const QGLWidget* shareWidget, Qt::WindowFlags f) :
QGLWidget(parent, shareWidget, f)
{
    // TODO Auto-generated constructor stub

}

OpenCV_GLWidget::~OpenCV_GLWidget() {
    // TODO Auto-generated destructor stub
}

void OpenCV_GLWidget::renderImage(const QImage& frame)
{
    m_GLFrame = QGLWidget::convertToGLFormat(frame);
    this->updateGL();
}

void OpenCV_GLWidget::resizeGL(int width, int height)
{
    // Setup our viewport to be the entire size of the window
    glViewport(0, 0, width, height);

    // Change to the projection matrix and set orthogonal projection
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0, width, height, 0, 0, 1);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}

void OpenCV_GLWidget::paintGL() {
    glClear (GL_COLOR_BUFFER_BIT);
    glClearColor (0.0, 0.0, 0.0, 1.0);
    if (!m_GLFrame.isNull()) {
        m_GLFrame = m_GLFrame.scaled(this->size(), Qt::IgnoreAspectRatio, Qt::SmoothTransformation);

        glEnable(GL_TEXTURE_2D);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, m_GLFrame.width(), m_GLFrame.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, m_GLFrame.bits() );
        glBegin(GL_QUADS);
        glTexCoord2f(0, 0); glVertex2f(0, m_GLFrame.height());
        glTexCoord2f(0, 1); glVertex2f(0, 0);
        glTexCoord2f(1, 1); glVertex2f(m_GLFrame.width(), 0);
        glTexCoord2f(1, 0); glVertex2f(m_GLFrame.width(), m_GLFrame.height());
        glEnd();
        glDisable(GL_TEXTURE_2D);

        glFlush();
    }
}

This class handles the rendering of the image onto a promoted QWidget. Next, I created a thread to feed the widget. (I cheated using the Qt signal-slot architecture here because it was easy...may not be the best performer in the book, but it should get you started).

void VideoThread::run()
{
    cv::VideoCapture video(0);

    while(!m_AbortCapture)
    {
        cv::Mat cvFrame;
        video >> cvFrame;

        cv::Mat gray(cvFrame.size(), CV_8UC1);
        cv::GaussianBlur(cvFrame, cvFrame, cv::Size(5, 5), 9.0, 3.0, cv::BORDER_REPLICATE);
        cv::cvtColor(cvFrame, gray, CV_RGB2GRAY);

        m_ThresholdLock.lock();
        double localThreshold = m_Threshold;
        m_ThresholdLock.unlock();

        if(localThreshold > 0.0)
        {
            qDebug() << "Threshold = " << localThreshold;
            cv::threshold(gray, gray, localThreshold, 255.0,  cv::THRESH_BINARY);
        }

        cv::cvtColor(gray, cvFrame, CV_GRAY2BGR);

        // convert the Mat to a QImage
        QImage qtFrame(cvFrame.data, cvFrame.size().width, cvFrame.size().height, cvFrame.step, QImage::Format_RGB888);
        qtFrame = qtFrame.rgbSwapped();

        // queue the image to the gui
        emit sendImage(qtFrame);
        msleep(20);
    }
}

Took me a bit to figure that out, so hopefully it will help you and others save some time :D