How do I use Legacy OpenGL calls with QT6?-CodePudding

I'm new to Qt and am trying to import some older C openGL code. I'm currently using Qt 6.4. I've subclassed my OpenGL-using class to QOpenGlFunctions.

Many of the glFoo calls "work" but the class also uses calls like glEnableClientState, glVertexPointer, glNormalPointer, glTexCoordPointer, glDisableClientState, glColor4fv, & glMaterialfv which come up with errors like undefined reference to __imp_glTextCoordPointer. Looking at the documentation these appear to no long be supported by "default" but it looks like they are supported using older versions of QOpenGlFunctions such as QOpenGlFunction_1_4 (https://doc-snapshots.qt.io/qt6-dev/qopenglfunctions-1-4.html).

Trying to change my subclass from QOpenGLFunctions to QOpenGLFunctions_1_4 complains that really only QOpenGLFunctions_1_4_CoreBackend and QOpenGLFunctions_1_4_DeprecatedBackend exist but there appears to be no documentation on those and if I subclass to one of them I start seeing complaints about my constructor...

How do I actually access the functions from these older versions of the class?

CodePudding user response：

This question was answered by Chris Kawa over at the Qt Forums and it worked for me! Here is his answer:

OpenGL 3.1 introduced profiles. Core profile does not support these old functions and Compatibility profile does. So first you have to make sure you have a context in version either lower than 3.1 (which does not support profiles) or 3.1 and up set up to use Compatibility profile. AFAIK Qt does not support OpenGL < 3.1 anymore, so you only have the latter option. If you're not sure what context you have you can simply do qDebug() << your_context and it will print out all the parameters, or you can query individual fields of it e.g. your_conext->format()->profile(). If your context is not set correctly the simplest way is to set it up like this before any OpenGL is initialized in your app:

QSurfaceFormat fmt;
fmt.setVersion(3,1);
fmt.setProfile(QSurfaceFormat::CompatibilityProfile);
fmt.setOptions(QSurfaceFormat::DeprecatedFunctions);
QSurfaceFormat::setDefaultFormat(fmt);

When you have the correct context you can access the deprecated functions like this:

QOpenGLFunctions_1_4* funcs = QOpenGLVersionFunctionsFactory::get<QOpenGLFunctions_1_4>(context());
if(funcs)
{
    //do OpenGL 1.4. stuff, for example
    funcs->glEnableClientState(GL_VERTEX_ARRAY);
}
else
{
    // Not a valid context?
}

And for anyone as amateur as I am, context() comes from QOpenGLWidget::context() which returns a QOpenGLContext*