Render QWidget in QPixmap with system background - c++

Question

When I render a QWidget with a QOpenGLWidget child with transparency objects, I observe a solarization effect when I tried to take a screenshot like the following picture:
The problem comes with the system background because when I remove it, the problem disappear:
.
My question is: What is the solutions to render a QWidget on a QPixmap with the system background?
Here my render code:
QWidget widget;
QPixmap pixmap(widget.size());
pixmap.fill(Qt::transparent);
QPainter painter(&pixmap);
painter.setRenderHint(QPainter::Antialiasing);
widget.render(&painter, QPoint(), QRegion(), QWidget::DrawWindowBackground /* the problem is this render flag */ | QWidget::IgnoreMask | QWidget::DrawChildren);
pixmap.save("screenshot.png");
The property setAutoFillBackground(true); and the method grab(); do the same like the first screenshot.
EDIT: There is no OpenGL problem. When I mix two pixmaps (the first with the background and the second with widget children), the solarization problem is still present.

Answer 1

A solution is to set a QPalette with Window role to black in my QOpenGLWidget.
QPalette currentWidgetPalette{ palette() };
currentWidgetPalette.setColor(QPalette::Window, Qt::black);
setPalette(currentWidgetPalette);
EDIT: I found a solution to paint destination in tansparent and source with my widget to compose the pixmap correctly.
painter.setCompositionMode(QPainter::CompositionMode_Destination);
painter.fillRect(result.rect(), Qt::transparent);
painter.setCompositionMode(QPainter::CompositionMode_Source);
widget.render(&painter, QPoint(), QRegion(), QWidget::DrawWindowBackground | QWidget::IgnoreMask | QWidget::DrawChildren);