OpenGL glBegin(GL_TRIANGLES) work only first time

Galbarad

Hi all,
first of all I'm completely new in OpenGL and Initiate in Qt )
I try relate Qml and OpenGL, so I take example openglunderqml
as in example I create own class
@class QProject1Main : public QQuickItem

void QProject1Main::handleWindowChanged(QQuickWindow *win) {
if (win) {
connect(win, SIGNAL(beforeRendering()), this, SLOT(paint()), Qt::DirectConnection);
win->setClearBeforeRendering(false);
}
}

void QProject1Main::paint() {
qDebug() << "QProject1Main::paint()";
qreal ratio = window()->devicePixelRatio();
int w = int(ratio * window()->width());
int h = int(ratio * window()->height());
glViewport(0, 0, w, h);

// each iteration bg color change from green to read
glClearColor(((float)++idx)/100, 0.3, 0.3, 1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glLoadIdentity();

// triangle pain only at first iteration
glColor3f(0.5f, 0.5f, 0.5f);
glBegin(GL_TRIANGLES);
glVertex3f( 0.1f, 0.8f, 0.1f);
glVertex3f(-0.8f,-0.8f, 0.1f);
glVertex3f( 0.8f,-0.8f, 0.1f);
glEnd();
}
@

I'm resize window to call paint method and see that background color changed but triangle paint only first time (

Galbarad

I'm add timer to my QML but receive same problem triangle visible only first time

@
Window {
id: wndMain
visible: true
width: 360
height: 360

QProject1Main {
    id: core
    anchors.fill: parent
}

Text {
    text: qsTr("Black rectangle - it's OpenGL baby ;)")
    anchors.centerIn: parent
    color: "red"
}

Timer {
        id: tGameTimer
        interval: 29
        running: true
        repeat: true

        onTriggered: {
            console.log("timer paint call")
            wndMain.update()
        }
}

}
@

Galbarad

in article http://qt-project.org/doc/qt-5/qtquick-scenegraph-openglunderqml-example.html class SquircleRenderer exists but in examples openglunderqml.pro it's not esists

Galbarad

hey all
any body know how to call SwapBuffers?

Cmdr

I've had the same problem. After the first frame, everything I draw is condensed to the lower left corner of the window. I've not come to a definite answer, but my guess here is that QtQuick doesn't cooperate well with the obsolete fixed function pipeline of OpenGL. It seems to tamper with some state variables.

Try using custom shaders.

Galbarad

Try using custom shaders.
are you mean create something like Geometry Shaders and move triangle with sync as in example?
I'll try, thank you

Cmdr

The geometry shader is optional, you'll probably only need vertex and pixel shader.

Although there are other ways, the "OpenGL under QML" example is probably a good place to start from:

http://qt-project.org/doc/qt-5/qtquick-scenegraph-openglunderqml-example.html

Galbarad

yes, I take code from this example, thank you