OpenGL shows a black screen

Question

I am new at OpenGL, I try this example: https://stackoverflow.com/a/31524956/4564882

but I get only a black widget. The code is exactly the same. this is 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. My Graphic card Driver is updated. What is the problem of such a behaviour?

Is resizeGL(int w, int h) called if you don't ever resize the widget? Throw a breakpoint in there and find out. If not, you should manually call it at the end of your initializeGL() implementation. — Foggzie
– Foggzie, Commented Dec 22, 2015 at 22:08

ProEns08 · Accepted Answer · 2015-12-23 08:50:36Z

After a great search, the problem is solved. 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

nmake

I link after that my application to the new binairies, and my code works well!

8Observer8 · Accepted Answer · 2020-06-05 20:06:19Z

0

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();
}
```

answered Jun 5, 2020 at 20:06

8Observer8

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