Qt: glClear does not work with overpainting on OpenGL - c++

Question

Problem
When QPainter is created after glClear the latter has no effect.
Description
I use Qt 5.7.1. I get same results with gcc on Linux and vc++ on Windows.
I have the following in a widget derived from QGLWidget:
void CanvasWidget::initializeGL()
{
qglClearColor(m_backgroundColor);
}
V1:
void CanvasWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT);
QPainter painter(this);
painter.drawLine(0, 0, 1000, 1000);
}
Produces:
V2:
void CanvasWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT);
}
Produces:
What I want is:
Which can be done with a hack:
void CanvasWidget::paintGL()
{
QPainter painter(this);
qglClearColor(m_backgroundColor);
glClear(GL_COLOR_BUFFER_BIT);
painter.drawLine(0, 0, 1000, 1000);
}
Question
What is going on? Why can't glClean and QPainter work together? Why can't I get it with V1?
Minimal Reproducible Example
main.cpp
#include "MainWindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MainWindow mainwindow;
mainwindow.show();
return app.exec();
}
MainWindow.h
#pragma once
#include "CanvasWidget.h"
#include <QMainWindow>
#include <memory>
class MainWindow : public QMainWindow
{
public:
explicit MainWindow(QWidget *parent = 0);
MainWindow(const MainWindow &) = delete;
MainWindow & operator= (const MainWindow &) = delete;
virtual ~MainWindow() = default;
private:
std::unique_ptr<CanvasWidget> m_canvasWidget;
};
MainWindow.cpp
#include "MainWindow.h"
#include <QHBoxLayout>
MainWindow::MainWindow(QWidget* parent)
: QMainWindow(parent)
, m_canvasWidget(new CanvasWidget(parent))
{
setCentralWidget(m_canvasWidget.get());
}
CanvasWidget.h
#pragma once
#include <QGLWidget>
class CanvasWidget : public QGLWidget
{
Q_OBJECT
public:
CanvasWidget(QWidget* parent = 0, const QGLWidget* shareWidget = 0, Qt::WindowFlags f = 0);
private:
virtual void initializeGL() override;
virtual void paintGL() override;
private:
QColor m_backgroundColor;
};
CanvasWidget.cpp
#include "CanvasWidget.h"
#include <QMessageBox>
#include <QWheelEvent>
CanvasWidget::CanvasWidget(
QWidget* parent /*= 0*/,
const QGLWidget* shareWidget /*= 0*/,
Qt::WindowFlags f /*= 0 */)
: QGLWidget(parent, shareWidget, f)
, m_backgroundColor(0, 93, 196)
{}
void CanvasWidget::initializeGL()
{
qglClearColor(m_backgroundColor);
}
void CanvasWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT);
QPainter painter(this);
painter.drawLine(0, 0, 1000, 1000);
}

Answer 1

Adding setAutoFillBackground(false) in CanvasWidget constructor solves the problem.
All the credit goes to #G.M.

Answer 2

In Qt tutorial about overpainting OpenGL with QPainter it is stated:
When overpainting 2D content onto 3D content, we need to use a
QPainter and make OpenGL calls to achieve the desired effect. Since
QPainter itself uses OpenGL calls when used on a QGLWidget subclass,
we need to preserve the state of various OpenGL stacks when we perform
our own calls
So, looking at your code in V1 I suppose that QPainter sets its own clear color state, and if you don't set your glClearColor explicitly, you will get what QPainter object has set.
And I would also suggest you to use a tool like RenderDoc or gDebugger to trace GL commands of your app to see exactly what happens under the hood.