Rendering QImage on QGLWidget of QML plugin

I'm trying to write a QML plugin that reads frames from a video (using a custom widget to do that task, NOT QtMultimedia/Phonon), and each frame is converted to a QImage RGB888, and then displayed on a QGLWidget (for performance reasons). Right now nothing is draw to the screen and the screen stays white all the time.

It's important to state that I already have all of this working without QGLWidget, so I know the issue is setting up and drawing on QGLWidget.

The plugin is being registered with:

qmlRegisterType&lt;Video&gt;(uri,1,0,"Video");

so Video is the main class of the plugin. On it's constructor we have:

Video::Video(QDeclarativeItem* parent)
: QDeclarativeItem(parent), d_ptr(new VideoPrivate(this))
{    
    setFlag(QGraphicsItem::ItemHasNoContents, false);            

    Q_D(Video);
    QDeclarativeView* view = new QDeclarativeView;
    view->setViewport(&d->canvas()); // canvas() returns a reference to my custom OpenGL Widget
}

Before I jump to the canvas object, let me say that I overloaded Video::paint() so it calls canvas.paint() while passing QImage as parameter, I don't know if this is the right way to do it so I would like some advice on this:

void Video::paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget)
{
    Q_UNUSED(painter);
    Q_UNUSED(widget);
    Q_UNUSED(option);

    Q_D(Video);
    // I know for sure at this point "d->image()" is valid, but I'm hiding the code for clarity
    d->canvas().paint(painter, option, d->image());
}

The canvas object is declared as GLWidget canvas; and the header of this class is defined as:

class GLWidget : public QGLWidget
{
    Q_OBJECT
public:        
    explicit GLWidget(QWidget* parent = NULL);
    ~GLWidget();

    void paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QImage* image);
};

Seems pretty simple. Now, the implementation of QGLWidget is the following:

GLWidget::GLWidget(QWidget* parent)
: QGLWidget(QGLFormat(QGL::SampleBuffers), parent)
{
   // Should I do something here?
   // Maybe setAutoFillBackground(false); ???
}

GLWidget::~GLWidget()
{
}

And finally:

void GLWidget::paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QImage* image)
{
   // I ignore painter because it comes from Video, so I create a new one:
   QPainter gl_painter(this);


   // Perform drawing as Qt::KeepAspectRatio

   gl_painter.fillRect(QRectF(QPoint(0, 0), QSize(this->width(), this->height())), Qt::black);

   QImage scaled_img = image->scaled(QSize(this->width(), this->height()), _ar, Qt::FastTransformation);

   gl_painter.drawImage(qRound(this->width()/2)  - qRound(scaled_img.size().width()/2),
                        qRound(this->height()/2) - qRound(scaled_img.size().height()/2),
                        scaled_img); 
}

What am I missing?

I originally asked this question on Qt Forum but got no replies.

回答1:

Solved. The problem was that I was trying to create a new GL context within my plugin when I should be retrieving the GL context from the application that loaded it.

This code was very helpful to understand how to accomplish that.

By the way, I discovered that the stuff was being draw inside view. It's just that I needed to execute view->show(), but that created another window which was not what I was looking for. The link I shared above has the answer.

回答2:

I think that you have to draw on your glwidget using the opengl functions.

One possible way is to override your paintGL() method in GLWidget and then draw your image with glDrawPixels().

glClear(GL_COLOR_BUFFER_BIT);
glDrawPixels(buffer.width(), buffer.height(), GL_RGBA, GL_UNSIGNED_BYTE, buffer.bits());

Where buffer is a QImage object that needs to be converted using QGLWidget::converrtToGLFormat() static method.

Look at this source for reference: https://github.com/nayyden/ZVector/blob/master/src/GLDebugBufferWidget.cpp