Qt 5: disable vsync

Question

I would like to disable vsync from the QOpenGLContext format in order to facilitate the integration of a third party rendering system.

QSurfaceFormat::swapInterval seems to be the only related parameter in Qt.

I tried several ways to implement this, but even if I set the QSurfaceFormat::swapInterval(0) at an early stage (before the QMainWindow construction), then the QOpenGLContext::create() call is restoring it.

// at application startup, before creating the Qt windows (or in MyQWindow constructor)
QSurfaceFormat format;
format.setSwapBehavior(QSurfaceFormat::DoubleBuffer);
format.setRedBufferSize(8);
format.setGreenBufferSize(8);
format.setBlueBufferSize(8);
format.setAlphaBufferSize(8);
format.setDepthBufferSize(24);
format.setStencilBufferSize(8);
format.setSwapInterval(0);
QSurfaceFormat::setDefaultFormat(format);

QMainWindow *w = new QMainWindow;
w->show();

// at QWindow with QOpenGLContext creation, at the first frame update (or at the constructor)
MyQWindow::initialization() {
    WbOpenGLContext *c = new WbOpenGLContext(this);
    c->setFormat(requestedFormat());
    qDebug() << "requested format:" << c->format();
    c->create();
    qDebug() << "actual format:" << c->format();
}

# output
requested format: QSurfaceFormat(
    version 2.0,
    options QFlags(),
    depthBufferSize 24,
    redBufferSize 8,
    greenBufferSize 8,
    blueBufferSize 8,
    alphaBufferSize 8,
    stencilBufferSize 8,
    samples -1,
    swapBehavior 2,
    swapInterval 0,
    profile 0
)
context format: QSurfaceFormat(
    version 3.0,
    options QFlags(0x4),
    depthBufferSize 24,
    redBufferSize 8,
    greenBufferSize 8,
    blueBufferSize 8,
    alphaBufferSize 8,
    stencilBufferSize 8,
    samples 0,
    swapBehavior 2,
    swapInterval 1, # Not what I asked
    profile 0
)

Is there a way to force vsync to be disabled?

Answer 1

As peppe is suggesting in the comments of the question, the QSurfaceFormat format of the QOpenGLContext instance after its creation is unfortunately not matching the actual format used internally.

This means that the code I implemented is probably working, but the second debug statement displays wrong values.