QPainter draws the quads for text bitmaps \counterclockwise, while primitives like ellipses and the quads for QPixmap draw clockwise. Result found on QOpenGLWidget.

Is this intentional?

Ex: I want to draw a 3D scene and then try to draw a framerate counter on top of it with QPainter's text functionality.

Justification:

• According to glCullFace, OpenGL face culling defaults to culling the back face, so front faces are shown but back faces are not.
• According to glFrontFace, OpenGL defines a "front face" as counterclockwise polygons.
• Therefore, if face culling is enabled in an OpenGL-running widget, it will default to culling clockwise (back face) polygons and anything that uses them.

Demonstration code:
See the example “2D Painting Example” in Qt Creator (I’m running Qt 5.6.0). This example uses a QPainter object to draw text and rotating elipses on a QOpenGLWidget.

• Add override of initializeGL() to GLwidget.
• In source file, add function:

#include <qopenglfunctions_2_1.h>
void GLWidget::initializeGL()
{
    QOpenGLFunctions_2_1 *f = QOpenGLContext::currentContext()->versionFunctions<QOpenGLFunctions_2_1>();
    f->glEnable(GL_CULL_FACE);
    //f->glCullFace(GL_FRONT);      // uncomment to cull the text and show the rotating elipses
}

• Run the program. The text "Qt" draws but the elipses don't. Uncomment glCullFace(GL_FRONT) to reverse the effect and show the elipses but not the text.
• Now try drawing a pixmap after culling is turned on. The following won’t draw if culling is turned on and it is culling back faces (clockwise polygons).

void GLWidget::paintGL()
{
    QPixmap p(myImageFilePath);
    QPainter painter(this);
    painter.drawPixmap(0, 0, p.scaled(this->size());
}

• To draw a QPixmap when back faces are being culled, you have to do this:

void GLWidget::paintGL()
{
    QPixmap p(myImageFilePath);
    QOpenGLFunctions_2_1 *f = QOpenGLContext::currentContext()->versionFunctions<QOpenGLFunctions_2_1>();
    GLBoolean faceCullingIsOn;
    
    // save current culling state
    f->glGetBooleanv(GL_CULL_FACE, &faceCullingIsOn);
    f->glDisable(GL_CULL_FACE);
    QPainter painter(this);
    painter.drawPixmap(0, 0, p.scaled(this->size());
    if (faceCullingIsOn == GL_TRUE)
    {
        // turn it back on
        f->glEnable(GL_CULL_FACE);
    }
}

Bug or intentional? This isn't a show-stopper for me but it did cause some grief.