How to use QOpenGLFunctions_2_1 properly

[jshafferman]

I am trying to use raw OpenGL with Qt and I am trying to strictly use the Qt API to do so. I am using OpenGL 2.1 because that is our hardware limitation. I am able to build without any errors but the first instance of a gl* fails. The first opengl function is glGetIntergv(GL_ENUM, int*) and through the debugger I am able to see that is where I crash first. I am just curious if anyone can see what I am missing because my guess is I have to do something special to initialize these functions (which I thought initializeOpenGLFunctions() accomplished). Any help or direction would be great!!

I am subclassing with QOpenGLFunctions_2_1 like so:


class GLWidget : public QGLWidget, protected QOpenGLFunctions_2_1


These are the relevant source file functions


GLWidget::GLWidget(QWidget *parent) :
QGLWidget(QGLFormat(QGL::DoubleBuffer | QGL::Rgba | QGL::DepthBuffer),parent)
{
createActions();
createMenus();
}

void GLWidget::initializeGL()
{
initializeOpenGLFunctions();

pixels[0] = pixels[1] = pixels[2] = NULL;
frameGood[0] = frameGood[1] = frameGood[2] = GL_FALSE;

qDebug() << "Pixel Buffer Object Demo";

QGLFunctions funcs(QGLContext::currentContext());
bool isNonPowerTwo = funcs.hasOpenGLFeature(QGLFunctions::NPOTTextures) ;

// // Make sure required functionality is available!
// if (!GLEE_VERSION_2_1 && !GLEE_ARB_pixel_buffer_object)
// {
// qDebug() << "PBO extension is not available!";
//// fprintf(stderr, "PBO extension is not available!\n");
// Sleep(2000);
// exit(0);
// }

if (isNonPowerTwo)
{
npotTexturesAvailable = GL_TRUE;
}
else
{
qDebug() << "GL_ARB_texture_non_power_of_two extension is not available!";
qDebug() << "Only portion of window will be used for rendering";
}

// Check for minimum resources
GLint intVal = 0;
glGetIntegerv(GL_MAX_TEXTURE_UNITS, &intVal);

if (intVal < 3)
{
qDebug() << "Fewer than 3 texture units available!";
Sleep(2000);
exit(0);
}
glGetIntegerv(GL_MAX_TEXTURE_SIZE, &intVal);
if (intVal < 1024)
{
qDebug() << "1024x1024 texture not supported!";
Sleep(2000);
exit(0);
}

qDebug() << "Controls:";
qDebug() << "\tRight-click for menu";
qDebug() << "\tb\tToggle motion blur";
qDebug() << "\tp\tToggle PBO usage";
qDebug() << "\tarrows\t+/- rotation speed";
qDebug() << "\tq\t\tExit demo";

// Colors
glClearColor(0.0f, 0.0f, 0.0f, 1.0f );
glColor4f(0.0f, 0.0f, 0.0f, 1.0f );

// Texture matrix is the only one we'll change
glMatrixMode(GL_TEXTURE);

// Set up textures
SetupTextures();
}


And as stated above the first call to glGetIntergv is what fails :(. Any ideas?

[ChrisW67]

Has initializeOpenGLFunctions() returned true or failed (false)?

[jshafferman]

I haven't had a chance to check it yet, but if it is returning false why would that happen?

[jshafferman]

So it is turning out that my initializeOpenGLFunctions() is returning false. What would cause this to happen? I have already double checked to make sure that my graphics card is capable of supporting OpenGL 2.1 so I know that it isn't hardware related. Any ideas?

[ChrisW67]

Well, the QOpenGLFunctions example sets a surface type and creates a GL context in the class constructor and makes that current before calling initializeOpenGLFunctions.

[jshafferman]

Isn't the SetSurfaceType specific to a QWindow not a QWidget? I have seen that example but I didn't think it applied to the QGLWidget.

[ChrisW67]

This is where my broken link should have been pointing http://qt-project.org/doc/qt-5.0/qtgui/qopenglfunctions.html#details

Yes it is QWindow but there could be a clue in the setup to use the inherited functions. I do not know from first hand use of it.

[jshafferman]

I investigated trying to use the QWindow version and I don't see how that can be applied to the QGLWidget. The QOpenGLContext takes a QSurfaceType (if I remember right) which I don't know if it makes sense to create a QSurfaceType inside of QGLWidget. Anyhow I don't see how I can piggy back off of that example. Do you have any ideas? According to the documentation for QGLWidget context should be set when calling the paintgl and initializeGL functions.

[ChrisW67]

OK. This code works for me (Linux 64-bit, Qt 5.2.0, nVidia proprietary drivers):


#include <QtWidgets>
#include <QtOpenGL>
#include <QOpenGLFunctions_2_1>

class GLWidget : public QGLWidget, protected QOpenGLFunctions_2_1
{
Q_OBJECT
public:
GLWidget(QWidget *parent = 0) :
QGLWidget(QGLFormat(QGL::DoubleBuffer | QGL::Rgba | QGL::DepthBuffer),parent)
{
}
protected:
void initializeGL()
{
bool ok = initializeOpenGLFunctions();

// Check for minimum resources
GLint intVal = 0;
glGetIntegerv(GL_MAX_TEXTURE_UNITS, &intVal);
qDebug() << ok << intVal; // true, 4 on my machine
}
};

int main(int argc, char **argv) {
QApplication app(argc, argv);
GLWidget w;
w.show();
return app.exec();
}
#include "main.moc"


Your initializeOpenGLFunctions() returning false would indicate that there is no OpenGL support on your system.

[jshafferman]

Well I'll be... It appears that the laptop I am using to work with 'says' it has OpenGL support for 2.1 however if I change the QOpenGLFunction_2_1 to 2_0 or 1_* it appears to start working! So I am glad to learn that if initializeOpenGLFunctions fails it is because that particular version of OpenGL is not supported on your system. I wish the Qt doucmentation just said that :(, however thank you very much for your help! I am sure I will have more questions about Qt and OpenGL as get more involved with the process. Thanks!

[ChrisW67]

It is not ideal documentation, but see the second last paragraph in the detailed description of QAbstractOpenGLFunctions.