QOpenGLWidget show black screen - c++

Question

I tried the QOpenGLWidget example described here:
https://stackoverflow.com/a/31524956/4564882
but I get only a black widget. The code is exactly the same. this the code associated to the QopenGLWidget:
OGLWidget::OGLWidget(QWidget *parent)
: QOpenGLWidget(parent)
{
}
OGLWidget::~OGLWidget()
{
}
void OGLWidget::initializeGL()
{
glClearColor(0,0,0,1);
glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHT0);
glEnable(GL_LIGHTING);
glColorMaterial(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE);
glEnable(GL_COLOR_MATERIAL);
}
void OGLWidget::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBegin(GL_TRIANGLES);
glColor3f(1.0, 0.0, 0.0);
glVertex3f(-0.5, -0.5, 0);
glColor3f(0.0, 1.0, 0.0);
glVertex3f( 0.5, -0.5, 0);
glColor3f(0.0, 0.0, 1.0);
glVertex3f( 0.0, 0.5, 0);
glEnd();
}
void OGLWidget::resizeGL(int w, int h)
{
glViewport(0,0,w,h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45, (float)w/h, 0.01, 100.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0,0,5,0,0,0,0,1,0);
}
I tried the example here: https://doc.qt.io/archives/qt-5.3/qtopengl-2dpainting-example.html. It works fine (trying the both base class: QGLWidget and QOpenGLWidget. this is the code associated to the Widget:
GLWidget::GLWidget(Helper *helper, QWidget *parent)
: QGLWidget(QGLFormat(QGL::SampleBuffers), parent), helper(helper)
{
elapsed = 0;
setFixedSize(200, 200);
setAutoFillBackground(false);
}
void GLWidget::animate()
{
elapsed = (elapsed + qobject_cast<QTimer*>(sender())->interval()) % 1000;
repaint();
}
void GLWidget::paintEvent(QPaintEvent *event)
{
QPainter painter;
painter.begin(this);
painter.setRenderHint(QPainter::Antialiasing);
helper->paint(&painter, event, elapsed);
painter.end();
}
I use Qt 5.5.1 binairies built on my machine. I let the Build Configuration by default, so it is based on Qt ANGLE not Desktop OpenGL.
What is the problem of such a behaviour?

Answer 1

In my case, my laptop uses NVIDIA external graphics card. So I went to NVIDIA Control Panel -> Manage 3D Settings -> Program Settings, and then selected "high-performance" for the .EXE file. This worked.

Answer 2

The problem was because I use Qt5 binaries built with the default configuration. The default in Qt 5.5 is "dynamic" GL -- both ANGLE (ES2)
ANGLE ((Almost Native Graphics Layer Engine) is an open source project by
Google. Its aim is to map OpenGL ES 2.0 API calls to DirectX 9 API.)
and Desktop backends (Desktop OpenGL)are built, the decision on which one to use is taken at runtime.
The problem is that ANGLE only supports OpenGL>3.x, so the first code that I test is deprecated and not supported by ANGLE. The second is supported, that's why it worked.
So, I rebuild Qt to target Desktop OpenGL only to support my deprecated code, using:
configure -debug-and-release -opensource -opengl desktop -platform win32-msvc2015
and then run nmake, link my application to the new binaries, and my code works well!

Answer 3

I had a black screen on desktop. I solved the problem by adding this line of code:
QCoreApplication::setAttribute(Qt::AA_UseDesktopOpenGL);
For example, put it here:
#include "widget.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_UseDesktopOpenGL);
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}